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Beiträge vom August, 2011

Digital Inputs

Wednesday, 31. August 2011 1:26

Logic inputs for experimental circuits are easy to produce. Just connect an input to either +5 volts or ground, and you have it. However, sometimes it is desirable to have the input signal sources gathered together in one place so they can be located and monitored easily, and checked as needed. When you’re doing this on a breadboard socket, you can still use a set of jumper wires for the purpose. But on a formal experimental platform, you’ll need a set of logic switches of one kind or another.
Some commercial platforms use standard slide switches in this application. However, even small slide switches take up a significant amount of space and still won’t fit on a breadboard socket. I still don’t like jumper wires for this particular application, so I use a little construct called a dip switch instead.
The DIP Switch
Dip switches are small blocks that look very much like ICs in some ways. The difference, as shown to the right, is that the block contains a series of switches. (They may be either miniature slide switches or rocker switches.) This provides a simple and neat way to mount logic switches on the breadboard socket or experimental platform. The spacing between pin rows is 0.3″, just like a standard IC. I’ve seen them in packages of two, three, four, five, eight, and ten switches, so you can get whatever suits your need.
Schematic Diagram
The logic switch is an extremely simple circuit. We begin with a 1K pull-up resistor to +5 volts. This matches the 1K output resistor we will use in most of our experimental circuits to demonstrate the different logic families.
If the switch is open, the circuit applies a logic 1 to whatever input is connected to that switch. If the switch is closed, it overrides that by directly grounding the connection, thus forcing its output to logic 0. Thus, this circuit can be used to manually control the state of a digital signal.
Parts List
For this circuit, we’ll use an 8-station DIP switch so we can be sure of having all the input signals we’ll need for our experiments. The complete list of parts is:
(8) 1K, ¼-watt resistors. 
(1) 8-station DIP switch. 
Red hookup wire. 
Black hookup wire. 
Constructing the Circuit
You will construct this circuit on your breadboard socket just to the right of your LED indicators but to the left of the center. There’s just enough room here to install your logic switches. You’ll also be installing jumpers to supply power to the right-hand side of the breadboard socket.
Getting Started
We will install the logic switches just to the right of the LED indicator circuits, which are partly shown in the assembly diagram to the right for reference. There is just enough room for this to the left of the center divider. This will leave the entire right-hand side of your breadboard socket available for experimental circuits.
The DIP Switch
Install the DIP switch across the center channel as shown to the right, just to the right of the indicator LEDs already in place. Note especially that the switch block is installed so that the switches will be on or closed when pulled towards you, and off or open when pushed away from you. When your assembly is completed, a switch will connect the corresponding input signal to ground (logic 0) when it is on, but will allow that signal to rise to +5 volts (logic 1) when it is off.
Be sure to place the switch so that its contact pins are aligned with the resistors and jumpers you just installed. When the DIP switch is positioned correctly, gently press it down so its pins enter their appropriate contact holes. Do not force it down. If the DIP switch does not seat easily, you may have a pin out of alignment. Check all pins again and then try once more to seat the switch block. Then, click on its image to continue.
Assembly Complete
This completes the assembly of your logic switches. In your experiments, you will be making various connections to these switches. To identify them accurately, we will designate them as S0 through S7, with S0 being on the right as shown in the assembly diagram.
Now, scroll on down the page to test the operation of your logic switches and verify that they all work correctly.
Testing the Logic Switches
You will test your logic switches by connecting each of them in turn to LED indicator L0 and then manipulating that switch. To accomplish this, cut a 3″ length of orange hookup wire and remove ¼” of insulation from each end. In your experiments, you’ll use several of these to make short connections to the inputs of your experimental circuits. You’ll also make and use some 6″ orange jumpers to make longer input connections. For now, however, you’ll only need one short jumper.
Connect one end of your 3″ orange jumper to the L0 input, and the other end to S0. Turn on power and observe L0. Turn S0 on and off as you continue to observe the LED. You may find it helpful to use a pen or pencil, or a similar object, to manipulate the switch. Determine which position of the switch turns the LED on, and which position turns it off.
Remove the orange jumper from S0 and connect it to S1 instead. Again, manipulate the switch as you observe L0. Does S1 behave in the same way as S0?
Repeat this action for each of the remaining switches in turn as you continue to observe L0. Do all switches behave in the same way?
Turn off power when you have checked all eight switches.
You should have found that all switches behaved in exactly the same way. With the switch in the up (open) position, L0 turned on, indicating the presence of a logic 1 signal. With the switch in the down (closed) position, that input was grounded and L0 turned off.
If the direction of operation of your switches was reversed from this, you need to remove the DIP switch, reverse it, and then re-install it in the same location. If one switch was unable to turn L0 off, remove the DIP switch and check the pins. One of the pins for that switch may have been bent under the body of the DIP switch rather than making contact with the breadboard socket. This can easily happen if you forced the DIP switch into place.
If one of the DIP switches leaves L0 off under all circumstances, try removing and reinserting the jumper, and then the 1K resistor associated with that switch. It is possible that a speck of dirt is preventing a good electrical connection.
When you are sure that all eight switches are working correctly, go on to verify the layout of your breadboard socket prior to performing the upcoming experiments.

Thema: semiconductors | Kommentare (0) | Autor:

Logic Indicators

Wednesday, 31. August 2011 1:21

One of the requirements for any breadboarding and test system is to have signal sources and output indicators of some kind. For digital circuits, we’ll need steady inputs from logic switches, individual pulses from pushbuttons, and a clock generator of some kind to generate a pulse train. It would also be nice if we could choose the clock frequency for some purposes.
On the output side, we need at least some LEDs as logic state indicators. In some cases, we might also want to set up a 7-segment LED display to show a set of digits rather than straight binary, but this is not a general necessity.
All such circuits take up space, or “real estate,” as it is often called. You can fill up an entire large breadboard socket with nothing but switches, pushbuttons, and LED indicators. Then you’d need a second breadboard just to hold your experimental circuit. This is why I strongly urge that you use a breadboarding system that already has the switches, buttons, indicators, power supplies, and assorted signal sources already available. That way you don’t have to spend time or effort constructing such circuits on your breadboard, except to actually test and demonstrate them.
At the same time, you should know what these circuits are and how they work, so you’ll understand and recognize the conditions they require, and those conditions under which these circuits might not work as intended. Therefore, we’ll begin our experimental digital procedures by constructing and demonstrating some basic but quite practical digital input and output circuits.
Schematic DiagramAn easy way to provide an LED indicator is shown in the schematic diagram to the right. Essentially, this is a modified RTL inverter circuit, with a light emitting diode (LED) inserted into the collector circuit. When a logic 1 is applied to the input, the transistor turns on and thus energizes the LED. As a result, the LED reflects the logic state of the signal applied to the transistor.
While this circuit works quite well and can be driven by almost any logic family (ECL won’t work directly), it does have a few drawbacks. We don’t need to worry about the fact that RTL is slow to switch states, at least by modern standards, since the human eye can’t tell the difference in any case. But that 22K resistor in the base lead is almost all the resistance there is between the circuit being tested and ground. That value is small enough to load down some kinds of circuits and change their operating parameters. We want to reduce this problem as much as possible.
We would prefer to have a circuit with a much higher input resistance, so it can be used to monitor digital outputs without loading them significantly. In addition, it would be nice if this circuit would have a switching threshold close to 2.5 volts, for a more balanced distinction between logic 0 and logic 1.
The requirement of high input resistance immediately suggests the CMOS logic family. In addition, the input switching threshold of CMOS gates is nominally 50% of the power supply voltage, although it can range from about 30% to 70% in worst-case conditions. However, CMOS inverters and gates are not designed to provide much output current. Is there one that will be able to properly drive an LED?
As it happens, there is indeed a CMOS IC that will meet our needs. The 4049 IC is designed to provide an interface between CMOS and TTL circuitry. It contains six inverters designed to accept a CMOS input signal of as much as +15 volts, and output a signal capable of driving TTL circuitry, with a logic 1 level of +5 volts. Since it can provide the necessary sink current to drive at least two TTL gates (and typically four TTL gates), we can be assured of sufficient current capacity to drive an LED. Each LED indicator, then, will use the circuit shown to the right.
The 1K resistor in series with the LED limits the LED current to about 3 milliamperes (mA). Since the 4049 inverter can sink at least 3.2 mA and typically 6.4 mA, this is not a problem for the IC. The 100K resistor on the input provides a high-resistance ground reference whenever no input signal is applied to the inverter. This prevents the input from “floating” uncontrolled, and possibly picking up a static charge that it couldn’t tolerate. Where most CMOS ICs include input protection circuits to prevent this, the 4049 and its companion 4050 non-inverting hex buffer are designed to allow input voltages to exceed the supply voltage so they can translate a 0-15 volt CMOS logic signal to 0-5 volt TTL levels. Therefore, care must be taken to prevent the input voltage from exceeding allowable limits. It is a good idea to observe such precautions with all MOS and CMOS devices in any case, but it is especially important here.
There is nothing critical about the 100K value of the input resistor; it could as easily be 10M to provide an even higher input resistance. The 100K value was selected mainly because the ¼-watt resistor package from Radio Shack includes 30 of the 100K resistors, and only ten 10M units. Since we’ll be using six such resistors here, we chose not to deplete the supply of 10M resistors. If you prefer, you can readily substitute 10M resistors without causing any difficulties.
Parts List
To construct and test the LED driver circuitry on your breadboard, you will need the following experimental parts:
(4) 1K, ¼-watt resistors (brown-black-red). 
(6) 100K, ¼-watt resistors (brown-black-yellow). 
(1) 4049 CMOS hex inverter/driver IC. 
(4) Red LEDs. 
Black hookup wire. 
Brown hookup wire. 
Red hookup wire. 
Orange hookup wire. 
Constructing the Circuit
Your logic indicator circuitry will occupy the space immediately to the right of your power supply, on your breadboard socket. When this circuit is in place, the first three sets of bus contacts will be occupied.
This circuit is not the original design for the logic indicators. However, it fits in the same space. If you built the original logic indicator circuitry using the RTL-based transistor design, remove all components (resistors, transistors, LEDs, and jumpers) involved with those logic indicators. Put these components away carefully for use in future experiments. You will need only one of the black jumpers here. You can use the same LEDs if you wish, or you can use smaller LEDs here to avoid crowding.
When you are ready, refer to the assembly instructions and diagram below, and construct your LED driver circuit as indicated. Be sure to follow the indicated component placement exactly, to avoid future placement conflicts.
Circuit Assembly
Starting the Assembly
Your +5 volt power supply should already be in place on the left end of your breadboard socket, as shown here. If you are converting the LED indicators from the original RTL design, you may also have additional circuitry installed to the right of the visible portion of the breadboard socket, in the assembly diagram. This will not be a problem; the new circuitry will fit in the same space. If you had the RTL LED circuits installed, you should have already removed them before beginning this assembly procedure.
Click on the `Start’ button below to begin. If at any time you wish to start this procedure over again from the beginning, click the `Restart’ button that will replace the `Start’ button.
0.3″ Black Jumper
Locate or prepare a 0.3″ black jumper wire (¼” length of black insulation between the two ends) as shown in the pictorial drawing here. Install this jumper in the location shown in the assembly diagram to the right. Note that you will need to install this jumper on a slight diagonal in order to connect to the ground bus strip.
Click on the image of the jumper you just installed to continue.
0.5″ Red Jumper
In the same manner as before, locate or prepare a 0.5″ red jumper wire (7/16″ length of red insulation). Install this jumper in the location indicated in the assembly diagram.
Again, click on the image of the jumper you just installed to continue.
0.3″ Brown Jumper
Locate or prepare a 0.3″ brown jumper, using the same method as before. Install this jumper in the location indicated to the right.
As before, click on the image of the jumper you just installed to continue.
0.2″ Bare Jumper
Locate or prepare a 0.2″ jumper, with no insulation at all. If you have a clipped lead from a previously-installed component, this makes a wonderful bare jumper. If not, simply remove about ¾” of insulation from the end of a length of hookup wire and form the exposed end into the required jumper. Install this jumper in the location indicated in the assembly diagram.
Once more, click on the image of the jumper you just installed to continue.
1K, ¼-Watt Resistor
Locate a 1K, ¼-watt resistor (color code brown-black-red) and form its leads to a spacing of 0.5″. Clip the formed leads to a length of ¼” and install this resistor in the location indicated to the right.
Click on the image of the resistor you just installed to continue.
1K, ¼-Watt Resistor
Locate a second 1K, ¼-watt resistor (brown-black-red) and form its leads to a spacing of 0.5″. Clip the formed leads to a length of ¼” and install this resistor in the location indicated in the assembly diagram.
Again, click on the image of the resistor you just installed to continue.
1K, ¼-Watt Resistor
Locate another 1K, ¼-watt resistor (brown-black-red) and form its leads to a spacing of 0.5″. Clip the formed leads to a length of ¼” and install this resistor in the location indicated to the right.
As before, click on the image of the resistor you just installed to continue.
1K, ¼-Watt Resistor
Locate one more 1K, ¼-watt resistor (brown-black-red) and form its leads to a spacing of 0.5″. Clip the formed leads to a length of ¼” and install this resistor in the location indicated in the assembly diagram.
As usual, click on the image of the resistor you just installed to continue.
100K, ¼-Watt Resistor
Locate a 100K, ¼-watt resistor (brown-black-yellow) and form its leads to a spacing of 0.4″. Clip the formed leads to a length of ¼” and install this resistor in the location indicated to the right.
Again, click on the image of the resistor you just installed to continue.
100K, ¼-Watt Resistor
Locate another 100K, ¼-watt resistor (brown-black-yellow) and form its leads to a spacing of 0.4″. Clip the formed leads to a length of ¼” and install this resistor in the location indicated in the assembly diagram.
Once more, click on the image of the resistor you just installed to continue.
100K, ¼-Watt Resistor
Locate a third 100K, ¼-watt resistor (brown-black-yellow) and form its leads to a spacing of 0.4″. Clip the formed leads to a length of ¼” and install this resistor in the location indicated to the right.
As before, click on the image of the resistor you just installed to continue.
100K, ¼-Watt Resistor
Locate another 100K, ¼-watt resistor (brown-black-yellow) and form its leads to a spacing of 0.4″. Clip the formed leads to a length of ¼” and install this resistor in the location indicated in the assembly diagram.
Note that this resistor must be installed on a diagonal in order to reach the ground bus.
Once more, click on the image of the resistor you just installed to continue.
100K, ¼-Watt Resistor
Locate another 100K, ¼-watt resistor (brown-black-yellow) and form its leads to a spacing of 0.4″. Clip the formed leads to a length of ¼” and install this resistor in the location indicated to the right.
Again, click on the image of the resistor you just installed to continue.
100K, ¼-Watt Resistor
Locate one final 100K, ¼-watt resistor (brown-black-yellow) and form its leads to a spacing of 0.4″. Clip the formed leads to a length of ¼” and install this resistor in the location indicated in the assembly diagram.
One more time, click on the image of the resistor you just installed to continue.
4049 CMOS Hex Inverter/Buffer IC
Locate a type 4049 CMOS IC. This IC is housed in a 16-pin DIP package. Make sure all 16 pins are straight and aligned vertically.
Place the IC gently on top of the breadboard socket in the location shown to the right, with the notch that indicates pin 1 oriented to the right. When you are sure that all pins are correctly aligned with their respective contact holes on the breadboard socket, press the IC down into full contact. Be careful that the IC pins do not bend or fold up under the body of the IC as you press it into place.
Click on the image of the IC you just installed to continue.
0.6″ Brown Jumper
Locate or prepare a 0.6″ brown jumper (use a 9/16″ length of insulation). Install this jumper in the location shown in the assembly diagram.
Click on the image of the jumper you just installed to continue.
0.1″ Bare Jumper
Prepare a 0.1″ bare jumper (a clipped resistor lead makes an excellent jumper for this purpose), and install this jumper in the location shown to the right.
Again, click on the image of the jumper you just installed to continue.
0.9″ Red Jumper
Prepare a 0.9″ red jumper (7/8″ length of insulation) in the usual manner. Before installing it, bend it into a right angle so that the horizontal leg will be 0.5″ long. Then install this jumper in the location shown in the assembly diagram.
Once more, click on the image of the jumper you just installed to continue.
1.1″ Black Jumper
Prepare a 1.1″ black jumper (1-1/16″ of insulation). Install this jumper in the location indicated to the right.
Click on the image of the jumper you just installed to continue.
1.1″ Orange Jumper
This next jumper is also 1.1″ long. However, it must also be raised above the level of the breadboard socket to clear the red jumper you installed recently. Therefore, the orange insulation for this jumper needs to be 1-3/8″ long.
Bend the ends as shown in the pictorial, so that the jumper will be 1.1″ long, and raised about 1/8″ above the breadboard socket. Bend the jumper at a right angle so the horizontal leg will be 0.7″ long, and install this jumper in the location shown in the assembly diagram.
Again, click on the image of the jumper you just installed to continue.
1.6″ Black Jumper
This black jumper must also be raised above the breadboard socket, to clear both the red and orange jumpers under it. Therefore cut the black insulation to a length of 2-1/8″, and bend the ends so the main body of the jumper is 1.6″ long. This will leave about ¼” of insulation on each end to hold the jumper up.
Bend this jumper at a right angle as shown to the right, so that the horizontal leg is 1.2″ long. Then install this jumper in the location indicated in the assembly diagram.
One more time, click on the image of the jumper you just installed to continue.
Locate a round red LED. This is the first of four that you will need for this project; they should all be the same for best results. If you use very small LEDs, they will fit easily in front of the 1.1″ black jumper. If you use larger LEDs (of the size depicted here), you will find that the black jumper can be pushed back far enough to allow the LED to rest firmly on the surface of your breadboard socket. You can also choose a different shape or color if you like; it’s your circuitry.
Note that either one LED lead is shorter than the other, or else one side has a flat section (or both). The short lead and/or flat section denotes the cathode connection to the LED. Note which lead is the cathode, and then clip both leads to ¼”. Install the LED on your breadboard socket in the location indicated to the right, with the cathode lead oriented to the left.
Click on the image of the LED you just installed to continue.
Locate a second round red LED. Note which lead is the cathode, and then clip both leads to ¼”. Install the LED on your breadboard socket in the location indicated in the assembly diagram, with the cathode lead oriented to the left.
Again, click on the image of the LED you just installed to continue.
Locate another round red LED. Note which lead is the cathode, and then clip both leads to ¼”. Install the LED on your breadboard socket in the location indicated to the right, with the cathode lead oriented to the left.
As before, click on the image of the LED you just installed to continue.
Locate a final round red LED. Note which lead is the cathode, and then clip both leads to ¼”. Install the LED on your breadboard socket in the location indicated in the assembly diagram, with the cathode lead oriented to the left.
Once more, click on the image of the LED you just installed to continue.
Assembly Complete
This completes the construction of your experimental circuit. Check your assembly carefully against the figure to the right, and correct any errors you might find.
The inputs to your LED indicators are marked in the assembly diagram. We have color-coded them, using the resistor color code, so they will be easy to identify in the future. Each of the four inputs has the appropriately-colored jumper connecting it to one of the inverters in the 4049 IC.
When you are ready, proceed with the experiment on the next part of this page.
Testing the LED Indicators
To test and use your LED indicator circuits, you’ll need a means of connecting them to other circuits on the breadboard socket. To do this, cut a 10″ length of white hookup wire (you can substitute another color, but whatever you use should be designated only for connections to the LED indicator circuits) and remove ¼” of insulation from each end. This will enable you to connect an LED indicator to any circuit on the breadboard socket, and still route the wire behind the experimental circuit, out of the way. Eventually, you will want four of these jumpers to connect all four indicators to experimental circuits.
Without connecting the 10″ jumper anywhere, turn on power and observe the four red LEDs you just installed. Are they on or off?
Leaving power on, connect one end of your white jumper to the ground bus above the LED indicators. (Do not attempt to use the right-hand side of the breadboard socket yet; you haven’t connected power to those bus strips.) Connect the free end of the jumper to the L0 input. Does this LED turn on now?
Disconnect the jumper from L0 and move it to the L1 input connection, observing the L1 LED as you do so. Repeat with L2 and L3, noting the results in each case. Leave the jumper connected to L3 for now.
Remove the other end of your white jumper from its ground connection, and connect it to the +5 volt bus instead. How does the L3 LED respond? Disconnect the jumper from the L3 input and move it to L2, L1, and L0 in turn. In each case, note the responses of the LEDs.
When you have completed these tests, turn off power and remove the white jumper. Put it aside for future use.
When you turned power on, the LED indicators should all have remained off. Those 100K resistors hold the inverter inputs at ground in the absence of a digital signal. This holds the inverter outputs, and therefore the LED cathodes, at +5 volts. The LED anodes are also returned to +5 volts, through the 1K resistors. Therefore, there is no voltage applied to the LEDs and no current through them, and the LEDs cannot turn on.
When you connected the indicator inputs to ground, they remained off. Ground also represents a logic 0, so it is proper for the LEDs to remain off to indicate this. Thus we see that an open input to these indicators is equivalent to a logic 0.
When you connected an input to +5 volts, the corresponding LED turned on. Electronically, the +5 volt input represents a logic 1, so that inverter switches to a logic 0 output. and causes current to flow through the LED. This turns on the LED to correctly indicate a logic 1 input signal.
If you did not get this behavior, check your connections again, and make sure that the IC and the LED for each indicator are oriented correctly. If the indicators all behaved as expected, you are ready to proceed to the next stage.

Thema: Leds, semiconductors | Kommentare (0) | Autor:

Power Supplies

Tuesday, 30. August 2011 1:58

When dealing with electronic circuits, we have to meet the basic requirement of providing electrical power for them to work. Without that power, your circuit is no more useful than a single raindrop in a hurricane.

The basic purpose of a power supply is to provide a fixed voltage to the working circuit, with sufficient current-handling capacity to maintain the operating conditions of the circuit. The power source doesn’t have to be fancy; the typical hand-held transistor radio uses a 9-volt battery as its power source. A flashlight uses cells that are physically much larger, but provide a lower voltage. Major electronic appliances such as television sets, VCRs, and microwave ovens have electronic circuits built in that take power from a wall socket and convert it to the form and voltages required by the other internal circuits of the appliance.

Such an electronic power supply circuit is imperative if you plan on doing long-term experimenting with electronic circuits. The alternative is to spend a considerable amount of money on replacement batteries at regular intervals.

The requirements for a power supply depend primarily on what you intend to do with it. A power supply for digital IC circuitry must have an output voltage as required by the ICs of the logic family you’re using. The most common modern requirement is +5 volts dc, as required by TTL devices and quite suitable for CMOS devices. However, RTL ICs will want +3.6 volts, while ECL ICs are designed to work with a -5.2 volt supply (-5 volts will work, and so will +5 volts if you remain consistent).

On the other hand, a power supply for analog circuits such as radio receivers and audio amplifiers will depend on the design requirements of the circuitry. A pocket transistor radio is specifically designed to operate at 9 volts, so it can be powered by a single 9-volt battery. However, any circuit employing operational amplifiers (op amps) will require both a positive and negative power supply, in the range of ±12 to ±15 volts. Such voltages are also quite satisfactory for experimental analog circuits, and so are practical values to have for a breadboarding system.

As a starting point for the early analog experiments on these pages, you can use a 9-volt battery. This will also work for digital experiments using CMOS ICs (which can operate from any voltage in the range +5 to +15 volts). As you continue with these experiments, however, we will introduce working power supply circuits that you can build and readily use for the rest of the experiments and for your own investigations.

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Test Instruments

Tuesday, 30. August 2011 1:57

When you build an experimental circuit, you will generally need to have some way to determine whether or not it is working correctly, and if so, how well. If it is not working, you’ll need to locate the problem so you can fix it. To accomplish this, a wide range of test instruments has been developed. Some of these apply to many different kinds of circuits, while others are specific to one type or class of circuits.

This page lists the more common types of test instruments that you may want to use with these experiments, with descriptions of what they do and where they are useful. Some of them will be the subjects of experimental circuits themselves, so you can actually experience how they work and what they do. Others are beyond this scope, or are not feasible as experimental circuits.

           Volt-Ohm-Milliammeter (VOM)

The basic test instrument in electronics work. Originally available only as an analog instrument with meter movement, it is now more common as a digital instrument (often called a Digital Multimeter or DMM). As its name suggests, it is able to measure resistance, ac voltage, and dc voltage and current over a wide range of values.

We strongly suggest you obtain and use a DMM for the experiments in general electronics. Inexpensive versions are readily available, although this instrument is too complex to be constructed as an experiment before you will need to use it. For theoretical handling of experimental circuits on these pages, you will use a virtual DMM on the screen.

Wheatstone Bridge

The Wheatstone bridge is a very simple but amazingly accurate device for making measurements. It gets applied in one way or another in a wide range of circumstances.

In the course of the experiments on these pages, you will construct and demonstrate a practical Wheatstone Bridge circuit to measure resistance and capacitance values.

Frequency Counter

As its name suggests, this instrument measures and displays the frequency of its input signal. You might also see it identified as an EPUT meter (Events Per Unit Time). This instrument operates by amplifying the input signal and clipping it into a clean rectangular waveform, and then counting the number of pulses that occur within a precise time interval (usually 1 second). The frequency counter works with any regular signal, analog or digital, over the frequency range for which it is intended.

You will construct and demonstrate a working frequency counter in the course of the experiments on these pages. You will not actually need a commercial frequency counter to perform any experiments, but you may want to obtain or build such an instrument if you plan to do much work in the electronics field.

Audio Function Generator

This device provides one or more waveforms at frequencies ranging from approximately 0.1 Hertz (one cycle every 10 seconds) to perhaps 100 kHz. Typical waveforms are sine wave, triangle wave, sawtooth waveform, square wave, and narrow rectangular pulse. In many cases a dc offset can be imposed on the signal, or the signal can be coupled through a suitable capacitor to eliminate any dc bias. This allows a known signal to be inserted into any analog circuit being tested, so that the progress of the signal through the circuit can be traced.

You will construct a practical, working function generator as part of the experiments on these pages. It will be small enough that you can actually leave it set up on your breadboard socket if you like. Or, you can purchase one if you plan to be working with audio-frequency circuitry. It is a good idea to have a known signal source so you can test and troubleshoot other circuits.

RF Signal Generator

This is a higher-frequency version of the audio function generator. The rf signal generator produces a sine wave at frequencies ranging from 100 kHz to 30 MHz or higher (the higher the top frequency, the more expensive the instrument). It often includes a simple, 1 kHz audio oscillator which can be used to modulate either the frequency or the amplitude of the main signal.

The rf generator is primarily used to test and troubleshoot radio tuners, i-f amplifiers and detector circuits. You will not need one for the experiments on these pages, and will not be constructing such a circuit here.


The oscilloscope provides a two-dimensional visual display of a signal. Most commonly it is used to show signal amplitude versus time, thus displaying the waveform of the signal being monitored. Some oscilloscopes have two or even four inputs, all using a common time base, so you can examine and compare the signal amplitude and shape as it progresses through stages of a circuit under test.

An oscilloscope might provide interesting insight into the operation of your test circuits, but is not required here. Unless you are planning to get into electronics in a major way, you should not shell out serious money for an oscilloscope.

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Sorting Components

Monday, 29. August 2011 2:29

When you’re constructing an experimental circuit, the last thing you need is to go hunting through a pile of assorted components for just the right resistor, capacitor, or coil. To avoid this, you need to have your experimental components organized in some way that makes it easy to locate and select the particular part you want.

There are many different ways of sorting and organizing your components so that later on you can easily find them when you need them. You are welcome to use one of the methods described here, or possibly adapt it to your own preferences and methods. If the methods discussed here do not suit you, you are of course welcome to use your own methods. Whatever method you choose, however, you will need to be able to sort rapidly through your components to find the ones you need for a particular experiment.

One simple method, which is also used when assembling a permanent piece of equipment, is to use strips of corrugated cardboard, such as the cardboard used in shipping cartons. The corrugations are large enough to hold component leads, but will not pass most component bodies. In the case of ¼-watt and smaller resistors, you can put a piece of transparent tape across the edge to make sure the resistor body can’t fall inside.

In any case, the idea is to group components together be value and by type. Thus, one or two cardboard strips might have all your resistors on them, grouped together by value. Another strip would hold your capacitors, again grouped together.

The main drawback with this approach appears when you have a large number of experimental parts which are to be used and then returned to their “home.” It can become difficult and confusing, and occupy a lot of table surface.

An alternate method is to use “parts cabinets” to hold your components. These are metal or plastic boxes with a number of plastic drawers to hold the parts. In most versions, plastic dividers may be used to separate the drawers into smaller compartments.

In my own case, I use a 24-drawer cabinet for ¼-watt resistors, a second one for ½-watt and larger resistors, and a third, larger cabinet for capacitors. The drawers for the resistors are labelled according to the significant digits of allowed resistance values. Thus, these drawers are simply labelled 10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, and 91. The “10″ drawer holds resistors of 1, 10, 100, 1k, 10k, 100k, 1M, and 10M ohms. It is not difficult to check the color codes and locate which of these resistors I want, since all the rest of the color bands will be the same for all resistors in the drawer. (If necessary, you can review the Color Code and even get some practice reading it.)

Capacitors require more drawers because there are different types that cover different ranges. A 24-drawer cabinet would easily hold ceramic, polystyrene, and similar smaller types for general use. In this case, however, I find it practical to use dividers to separate capacitance ranges from each other. Most modern capacitors have a shape that allows the capacitance value to be readily printed on the body of the capacitor, although the numbers may be small. Therefore they seldom use color codes, and are harder to distinguish on sight.

In addition, there is another category of capacitors, known as electrolytic capacitors. These use a different construction method in order to get large values of capacitance in relatively small containers. These capacitors do not always follow the standard numerical values of the smaller capacitors and resistors, and are often physically larger as well, so they are easier to store and find in separate drawers.

Coils (inductors) and other parts need to go in appropriate drawers, bins, or boxes according to their size and purpose.

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Experimental Components

Monday, 29. August 2011 2:27


Before you can begin experimenting with electronics, you’ll need a basic set of components and tools. In the USA, all necessary components and tools can be obtained inexpensively at Radio Shack. I don’t know what alternatives are available in other countries around the world, but you’ll need some source of components in order to perform these experiments in person. In any case, Radio Shack is my source for the specific items listed on this page.

Where possible, I’ve obtained items in bulk packages. They are common components and are quite satisfactory for all of these experiments. The components used in these experiments include:

500-piece carbon-film resistor assortment. This package includes at least 5 of each of 65 different resistance values in the range of 1 ohm to 10 megohms. All resistors are ¼ watt, 5% components. 

100-piece assortment of ceramic disc capacitors. This package does not contain any specific guaranteed values, but will have components suitable here. 

Signal Diodes
Package of 50 1N914 silicon signal diodes. These are suitable for switching as well as for analog signals. 

Rectifier Diodes
Package of 20 rectifier diodes. These are good for 1 Ampere of forward current, and will serve in all required power supplies. 

Package of 20 NPN Silicon transistors (similar to 2N3904), and a package of 20 2N3906-style PNP silicon transistors. If you need alternatives, the 2N4124 and 2N4126 have very similar characteristics but lower voltage ratings. The voltage ratings will not be a problem here. 

Package of 20 assorted LEDs. This will primarily be a mix of red, yellow, and green LEDs in round and rectangular packages. If possible, locate a package with at least four each of red, yellow, and green LEDs in the basic 5 millimeter round package. 

Integrated Circuits
These depend on which experiments you want to run. Operational amplifiers will be type 741 ICs or type 1458 (or 5558) dual-741 ICs. Digital ICs may be of the 7400 TTL series, the 4000 CMOS series, or the 74HCT00 series, which are CMOS devices with 7400-series pin configurations. Specific digital IC requirements will be provided with those experiments, along with appropriate pin configuration diagrams. 

Hookup Wire
You will need some wire to make connections between some components. For easy identification, you should have wire with many different colors of insulation. For breadboarding purposes, you should use either AWG (American Wire Gauge) #22 or #24 solid hookup wire. Numbers higher than 24 indicate wire that may be too thin to provide reliable connections, while numbers lower than 22 indicate wire that is thick enough to damage the breadboard socket contacts. 

If you’re planning to build electronic power supplies for your experiments, you will also need the components listed on the pages for these power supplies. Refer to these separate pages for the required components and for assembly instructions of the power supplies you will need.

Note: Remember that it is better to have the power supplies and support circuitry separate from the breadboard socket, so they don’t use up valuable breadboarding space.

If you’re planning to build power supplies for multiple categories of experiments, select a transformer with the capacity to handle all of the power requirements. At a minimum, such a transformer might have a secondary winding rated at 25.2 volts center tapped (or 12.6-0-12.6), with at least a 1.5 Ampere capacity. That way, you can use the same breadboarding system for all of your experimental circuits.

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The Unijunction Transistor

Friday, 26. August 2011 2:09

One of the oddest semiconductor devices in use is the unijunction transistor (UJT). As its name suggests, this is a three-terminal device which nevertheless has only one PN junction. It cannot amplify an applied signal, but it nevertheless can be used as the active element in an oscillator circuit.

The figure to the right shows the physical construction of a typical UJT. It consists of a bar of N-type silicon with electrical connections at either end, plus an aluminum wire bonded to a point along the length of the silicon bar. At the bonding point, the aluminum creates a P-type region in the silicon bar, thus forming a PN junction.

Because there’s only one junction, it’s not reasonable to use the terms anode or cathode, so designations are taken instead from transistor notations. The P-type connection is known as the “Emitter,” while the two N-type connections are designated “Base 1″ and “Base 2.” For this reason, the device is sometimes called a “double-base diode.”

The schematic symbol for the UJT appears to the right. The symbol actually represents the construction of the UJT, as shown above, quite well.In use, an appropriate bias voltage is applied between the two bases, with B2 made positive with respect to B1. Because the N-type bar is resistive, a relatively small current will flow through it, and the applied voltage will be distributed evenly along its length. If we start with the Emitter grounded, the junction will be reverse biased and there will be no emitter current. As the emitter voltage increases, there is no change until the junction suddenly becomes forward biased.

At this point, the emitter injects holes into the silicon bar, greatly reducing the effective resistance of the bar between E and B1. This will lower the emitter voltage required to keep the junction forward biased, and will sustain a heavy emitter current. This condition will continue as long as the circuit connected to the emitter can sustain the heavier current flow. The UJT thus behaves like a variation of the SCR, as suggested by the equivalent circuit shown to the right.

It is also possible to build a UJT with a P-type silicon bar and an N-type emitter. It isn’t used as commonly, but has been constructed. It is known as a complementary UJT (CUJT); its schematic symbol is shown to the right. The CUJT behaves the same as the UJT, but with reversed voltage polarities.

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Specialized Diodes

Friday, 26. August 2011 2:08

By adjusting the doping levels and gradients as well as the geometry of a semiconductor crystal, we can modify the behavior of the device. This page lists a wide range of diodes whose properties have been deliberately controlled to produce specific capabilities.
Each of these specialized diodes has its own schematic symbol, shown to the right of its description below. The symbols are all specific variations on the basic diode symbol, so that the nature and function of the device is clear on a schematic diagram.
Light Emitting Diode (LED) 
One of the questions semiconductor manufacturers asked themselves was, “What happens if we increase the doping levels in the silicon crystal?” Trying this gave rise, among other things, to the tunnel diode. Then they took the process even further, to the point where they skipped the silicon completely, and produced what is called a “III-V” device, named after the fact that P-type dopants are from column III of the Periodic Table (aluminum, gallium, indium) and N-type dopants are from column V (phosphorus, arsenic).
The resulting Gallium Arsenide (GaAs) crystal had the interesting property of radiating significant amounts of infrared radiation from the junction. By adding Phosphorus to the equation, they shortened the wavelength of the emitted radiation until it became visible red light. Further refinements have given us yellow and green LEDs. More recently, blue LEDs have been produced, by putting nitrogen into the crystal structure. This makes full-color flat-screen LED displays possible.
The mechanism of emitting light is interesting. The atomic structure of the LED is carefully designed so that as free electrons cross the junction from the N-type side to the P-type side, the amount of energy each electron releases as it drops into a nearby hole corresponds to the energy of a photon of some particular color. Therefore, that photon is released as a visible photon of that color.
P-I-N Diode 
The p-i-n diode doesn’t actually have a junction at all. Rather, the middle part of the silicon crystal is left undoped. Hence the name for this device: p-intrinsic-n, or p-i-n. Because this device has an intrinsic middle section, it has a wide forbidden zone when unbiased. However, when a forward bias is applied, current carriers from the p- and n-type ends become available and conduct current even through the intrisic center region. The end regions are heavily doped to provide more current carriers.
The p-i-n diode is highly useful as a switch for very high frequencies. They are commonly used as microwave switches and limiters.
Tunnel Diode 
As we mentioned in our discussion of semiconductor physics, the addition of either P-type or N-type impurities causes the Fermi level in the silicon crystal to shift towards the valence band (P-type impurities) or the conduction band (N-type impurities). The higher the doping level, the greater the shift. In the tunnel diode, the doping levels are so high that the Fermi levels in both halves of the crystal have been pushed completely out of the forbidden zone and into the valence and conduction bands.
As a result, at very low forward voltages, electrons don’t have to gain energy to get over the Fermi level or into the conduction band; they can simply “tunnel through” the junction and appear at the other side. Furthermore, as the forward bias increases, the applied voltage shifts the levels apart, and gradually back to the more usual diode energy pattern. Over this applied forward voltage range, diode current actually decreases as applied voltage increases. Thus, over part of its operating range, the tunnel diode exhibits a negative resistance effect. This makes it useful in very high frequency oscillators and related circuitry.
Varactor Diode 
One characteristic of any PN junction is an inherent capacitance. When the junction is reverse biased, increasing the applied voltage will cause the depletion region to widen, thus increasing the effective distance between the two “plates” of the capacitor and decreasing the effective capacitance.
By adjusting the doping gradient and junction width, we can control the capacitance range and the way capacitance changes with applied reverse voltage. A four-to-one capacitance range is no problem; a typical varactor diode (sometimes called a “varicap diode”) might vary from 60 picofarads (pf) at zero bias down to 15 pf at 20 volts. Very careful manufacturing can get a capacitance range of up to ten-to-one, although this seems at present to be a practical limit.
Varactor diodes are used in electronic tuning systems, to eliminate the use of and need for moving parts.
Zener Diode 
When the reverse voltage applied to a diode exceeds the capability of the diode to withstand it, one of two things will happen, yielding essentially the same result in either case. If the junction is wide, a process called avalanche breakdown occurs, whereby the current through the diode increases as much as the external circuit will permit. A narrow junction will experience Zener breakdown, which is a different mechanism but has the same effect.
The useful feature here is that the voltage across the diode remains nearly constant even with large changes in current through the diode. In addition, manufacturing techniques allow diodes to be accurately manufactured with breakdown voltages ranging from a few volts up to several hundred volts. Such diodes find wide use in electronic circuits as voltage regulators.
Schottky Barrier Diode 
When we get into high-speed applications for electronic circuits, one of the problems exhibited by semiconductor devices is a phenomenon called charge storage. This term refers to the fact that both free electrons and holes tend to accumulate inside a semiconductor crystal while it is conducting, and must be removed before the semiconductor device will turn off. This is not a major problem with free electrons, as they have high mobility and will rapidly leave the semiconductor device. However, holes are another story. They must be filled more gradually by electrons jumping from bond to bond. Thus, it takes time for a semiconductor device to completely stop conducting. This problem is even worse for a transistor in saturation, since then by definition the base region has an excess of minority carriers, which tend to promote conduction even when the external drive is removed.
The solution is to design a semiconductor diode with no P-type semiconductor region, and therefore no holes as current carriers. Such a diode, known as a Schottky Barrier Diode, places a rectifying metal contact on one side on an N-type semiconductor block. For example, an aluminum contact will act as the P-type connection, without requiring a significant P-type semiconductor region.
This diode construction has two advantages in certain types of circuits. First, they can operate at very high frequencies, because they can turn off as fast as they can turn on. Second, they have a very low forward voltage drop. This is used to advantage in a number of ways, including as an addition to TTL ICs. When a Schottky diode is placed across the collector-base junction of a transistor as shown to the right, it prevents the transistor from becoming saturated, by bypassing the excess base current around the transistor. Therefore, the transistor can turn off faster, thus increasing the switching speed of the IC. The full power versions of these TTL ICs are the 74S00 series, and have switching speeds similar to ECL, and similar power requirements. The low power versions, the 74LS00 series, have switching times comparable to standard TTL, but with a much lower power requirement.
Experimentation is always in progress, and new applications are invented regularly. As new diode types come to my attention, I will add them to the list above. If you should hear of a diode type not yet in the list, please contact the webmaster and let me know there. I will research the device and add it as quickly as possible. Thanks.

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A Touch of Physics

Thursday, 25. August 2011 1:58

In order to understand how diodes can have very different properties according to how they are manufactured, it is necessary to first delve a bit into the physics involved. Don’t worry; we won’t get any deeper in than we have to, but we have to use this method to understand the differences between different kinds of diodes, and why they can do the odd (and useful) things they do.

Consider the diagram to the right. This figure shows the important range of electron energy in four different kinds of materials. To understand this diagram, let’s define a few terms.

Conduction Band
That range of electron energy where electrical conduction is possible. Electrons with this much energy are free of their parent atoms, and can move through the medium in which they exist.
Valence Band
That range of electron energy where electrical conduction is not possible. Electrons with this much energy are bound into the atomic structure of the material, and are unavailable to conduct an electrical current.
Forbidden Zone
That energy range between the valence band and the conduction band. Electrons cannot remain within this range of energy; they must either gain or lose energy so as to attain either the conduction band or the valence band.
Fermi Level
The highest energy level in the crystal that can remain populated by electrons at a temperature of Absolute Zero. Electrons with greater energy than this may be available for conduction; electrons with less energy are bound to the crystal structure.

Diagram A above represents a good conductor, such as copper or silver. Here, at temperatures above Absolute Zero, electrons are always available to conduct electrical current, even with no applied energy. In metals, the valence and conduction bands actually overlap.

Diagram B shows a typical insulator, such as glass. All electrons are pretty much locked into the atomic structure, and are unavailable as current carriers. It will take a lot of energy to break any electrons loose for conduction. It’s not impossible (a lightning bolt can go through almost anything), but it takes a lot of applied energy.

Diagram C represents a crystal of N-type silicon (or germanium). The forbidden zone is still present, but much smaller than for an insulator. That’s why this type of material is called a “semiconductor.” With the crystal doped with N-type impurities, there are lots of electrons around with almost enough energy to roam freely, so the Fermi level gets pushed up close to the conduction band. If the doping level is heavy enough (large dosage of impurities), the Fermi level can actually enter the conduction band.

Diagram D represents a P-type semiconductor crystal. Here, the p-type impurities have left holes in the atomic structure, which tend to attract and hold free electrons. This pulls the Fermi level down until it gets close to the valence band. Similar to the highly-doped N-type crystal, a highly-doped P-type crystal will have its Fermi level within the valence band instead of just above it.

There are two important factors regarding the Fermi level in semiconductors. First, since the Fermi level is close to one of the working energy levels, it requires very little energy to push an electron over the edge and make it available for conduction. In an N-type crystal, only a very small applied voltage will kick the free electrons up into the conduction band to carry a current. In a P-type crystal, a small amount of energy will kick a bound electron just over the top of the valence band into the forbidden zone. This doesn’t make the electron available for bulk conduction, but does allow the applied voltage to push the electron over into a hole, causing it to leave another hole behind it. In this way, a series of electrons can “hop” from bound state to bound state in a new location, allowing the hole to appear to move in the opposite direction. This is another way to think of hole conduction in semiconductor crystals.

The second factor to remember is that when a PN junction is formed in a single silicon or germanium crystal, the entire crystal as a whole has one Fermi level (see Diagram E above). The conduction and valence bands have differing energy levels across the crystal. As a result, the N-type conduction band is very close in energy to the P-type valence band. The transition region corresponds to the depletion region within the crystal. This is a major factor in the operation of all semiconductor devices, and helps to explain how we can get specific properties from a given device, according to just how we manufacture the semiconductor crystal.

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The Diac and Triac

Thursday, 25. August 2011 1:57

One of the drawbacks of all of the four-layer diodes is that they all require a dc voltage of the correct polarity in order to operate. It would be nice if we could have some sort of SCR that works for either polarity, so it can be used with an applied ac voltage.

Now, we created the four-layer devices by essentially connecting two transistors back to back in a single silicon crystal. Can we extend this concept and connect two SCRs back to back?

The diagram to the right shows the resulting five-layer device, which is known as a diac. At first glance, it seems unreasonable or even impossible, considering that each connection to the semiconductor crystal overlaps a pn junction. However, the device does work, and indeed works well.

The terms anode and cathode no longer apply, so the connections are simply named terminal 1 (T1) and terminal 2 (T2). Each terminal can serve as either anode or cathode, according to the polarity of the applied voltage.

That same applied polarity also determines which of the end junctions is active, and which one is bypassed. Thus, if T1 is positive with respect to T2, T1′s N-type region is ignored (electrons are pulled away from that junction) and its P-type region serves as the anode. At the same time, the relative negative voltage at T2 pulls holes from the P-type region towards the terminal (removing them from the next junction), but tends to push electrons from its N-type region across that junction into the P-type region, thus making them available for conduction.

The diac, like the four-layer diode, remains non-conducting until its breakover voltage is reached, at which point it turns on fully and remains on until the applied voltage or circuit current are reduced below the holding values at which conduction can be maintained. Since the diac is normally used in ac circuits, operating as part of the control circuit for devices powered from a household wall socket or similar source, this is not a problem. In such applications, the diac is triggered each half-cycle of ac power, and then turns off at the end of the half-cycle when the line voltage reverses polarity.

The drawback of the diac is the same as it was for the four-layer diode: it cannot be triggered at just any point in the ac power cycle; it triggers at its preset breakover voltage only. If we could add a gate to the diac, we could have variable control of the trigger point, and therefore a greater degree of control over just how much power will be applied to the line-powered device.

The figure to the right shows the result. This device is known as a triac. Here, the main connections are simply named main terminal 1 (MT1) and main terminal 2 (MT2). The gate designation still applies, and is still used as it was with the SCR.

The useful feature of the triac is that it not only carries current in either direction, but the gate trigger pulse can be either polarity regardless of the polarity of the main applied voltage. The gate can inject either free electrons or holes into the body of the triac to trigger conduction either way. For this reason, you may see the triac referred to as a “four-quadrant” device.

As with the diac, the triac is used in an ac environment, so it will always turn off when the applied voltage reaches zero at the end of the current half-cycle. If we apply a turn-on pulse at some controllable point after the start of each half cycle, we can directly control what percentage of that half-cycle gets applied to the load, which is typically connected in series with MT2. This makes the triac an ideal candidate for light dimmer controls and motor speed controls. This is a common application for triacs.

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