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Two-Input DL OR Gate

Introduction

Diode Logic (DL) circuits are easy to build and require few components. Used judiciously, they can be designed to perform their tasks in a very small amount of space. However, there are practical limits on how they can be used and how far they can go. In this group of experiments we will construct and demonstrate some DL circuits, and in the process learn exactly what the limits are.



2-Input Diode Logic OR Gate

Schematic Diagram

As shown to the right, a Diode Logic OR gate consists of nothing more than diodes (one for each input signal) and a resistor. There's nothing magical about the 10K value we'll be using here; this is simply a convenient value that serves to provide a ground reference for the output signal. If there are no input signals connected to the diodes, the output will be ground, or logic 0. Thus, an open input is equivalent to a logic 0 input, and will have no effect on the operation of the rest of the circuit.

It is possible to add any number of input diodes to this circuit, each with its separate input signal. However, two inputs are quite sufficient to demonstrate the operation of the circuit.



Parts List

To construct and test the two-input DL OR gate on your breadboard socket, you will need the following components:



Constructing the Circuit

Select an area on your breadboard socket that is clear of other circuits. Just to the right of the center works well for this circuit. Then, install your experimental components as shown below.



Circuit Assembly

Start assembly procedure



Starting the Assembly

The assembly diagram to the right shows two of the logic switches as well as the jumpers across the center divider, which you should already have in place on your breadboard socket. This will serve as a reference to help you place the various components of your experimental circuit.

Click on the `Start' button below to begin assembling your experimental circuit.

10K ¼-Watt Resistor

Locate a 10K, ¼-watt resistor (color code brown-black-orange) and form the leads to a spacing of 0.3" as shown. Clip the leads to ¼" length and insert the resistor on your breadboard socket to the right of the center space, as shown in the assembly diagram to the right.

Click on the resistor image to continue with your assembly.

1N914 Silicon Diode

The body of the 1N914 diode is made of glass, so it must be handled with reasonable care. Even more than resistors, it is important not to bend the wire leads right at their junction with the diode body.

Locate a 1N914 diode and form its leads to a spacing of 0.3". Clip the leads to a length of ½" so the diode body will be held up above the surface of the breadboard socket. Note that there is a band of ink (probably black) drawn around the body of the diode, near one end. This band denotes the cathode end of the diode, and for this experiment must be oriented towards the 10K resistor you already installed.

When you have installed the diode correctly, click on its image to the right to continue.

1N914 Silicon Diode

Locate a second 1N914 diode and form its leads to a spacing of 0.4". Clip the leads to ½" length as before, and install the diode next to the first one, as shown to the right. Be sure to observe the orientation of the diode.

Again, click on the image of the diode you just installed to continue.

3" Orange Jumper

You should have a 3" orange jumper left over from testing the logic switches you installed earlier. If not, cut a 3" length of orange hookup wire (use a different color if you prefer, but be sure you can tell these jumpers from others easily). Remove ¼" of insulation from each end of each jumper, and install this jumper as indicated to the right.

This jumper is of course longer than needed for this particular experiment. You will be saving it and more like it, and using them as input jumper wires in a wide range of different experiments in the future.

As usual, click on the image of the jumper to move on to the next step.

3" Orange Jumper

If necessary, cut a second 3" length of orange hookup wire and remove ¼" of insulation from each end. Install this jumper as indicated to the right.

As before, click on the image of the jumper to move on to the next step.

10" White Jumper

Locate the 10" white jumper you prepared when testing the LED indicators, and connect one end of this jumper as shown in the figure to the right. Connect the other end to the L0 LED input point.

Once more, click on the image of the jumper to move on to the next step.

Assembly Complete

This concludes the assembly of your experimental circuit. Check your circuit one last time to make sure it matches the assembly diagram to the right, and then scroll down to the next part of this page to perform the experiment.

Restart assembly procedure


Performing the Experiment

To perform this experiment, turn on power to your experimental circuit, and then observe L0 as you set S0 and S1, to all four possible combinations of two switches. For each input combination, note the logical output state of the circuit as indicated by L0 (on = 1; off = 0), and record that result in the form to the right. Does your result match the normal behavior of an OR gate?

As an additional test, use your voltmeter to measure the output voltage of this circuit for each combination of inputs. If we assume that any output above 2.5 volts is a logic 1 while any output below 2.5 volts is a logic 0, are these output voltages legal? Do you see any potential problem in view of your results?

When you have completed your determinations, turn off the power to your experimental circuit.

Inputs Output
S1 S0 L0 Voltage
0 0
0 1
1 0
1 1


The DL OR gate with logic switches connected

Discussion

You should have found that the L0 remains off when both switches send a logic 0 to the gate, and turns on when either or both S0 and S1 send a logic 1 to the gate. This is normal OR gate behavior.

With both inputs at logic 0, the output voltage was 0 volts, which is certainly a logic 0. With one input at logic 1, the output rose to about +3.77 volts (remember, our power supply measures 4.85 volts; yours may be slightly different). This is a valid logic 1, and seems reasonable in view of the fact that there is a voltage drop across the diode, and the two resistors involved (10K in the gate itself, 1K in the logic switch) form a voltage divider, so that the output voltage is 10/11 of the supply voltage after the diode drop has been subtracted. The complete circuit is shown to the right.

When both inputs were at logic 1, the output voltage rose to about +4 volts. Since we now have two 1K resistors in parallel pulling the output voltage up, their equivalent combined resistance is only 0.5K, and the output rises to 10.5/11.5 of the supply voltage, after accounting for the diode voltage drop. This is still a logic 1, so our output levels are all still within range.

However, the fact that the output voltage changes for different numbers of logic 1 inputs suggests a potential problem if DL gates should be cascaded. Gate behavior may change according to the number of inputs in use, and that cannot be allowed. We will investigate this possibility as we continue to experiment with Diode Logic.

When you have completed this experiment, remove your experimental components and jumpers and set them aside for use in later experiments. This is one of the useful points about breadboarding circuits to test them: you can re-use the same parts in many different test circuits.


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