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Logic Indicators

Introduction

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 Diagram

Schematic diagram of a basic LED logic indicator circuit.

An 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?


Schematic diagram of a CMOS LED driver circuit.

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:



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

Start assembly procedure































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.

Red LED

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.

Red LED

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.

Red LED

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.

Red LED

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.

Restart assembly procedure
Continue assembly procedure


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.



Discussion

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.


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