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Two-Input DL AND Gate


The Diode Logic (DL) AND gate is just as simple as the DL OR gate, although it is configured a bit differently.

Diode Logic AND Gate

Schematic Diagram

As shown to the right, a Diode Logic AND gate consists of nothing more than diodes (one for each input signal) and a resistor. As with the DL OR gate, the 10K resistor provides a reference connection. Unlike the OR gate, however, this is a reference to +5 volts, rather than to ground. If there are no input signals connected to the diodes, the output will be +5 volts, or logic 1. Thus, an open input will not affect the rest of the circuit, which will continue to operate normally.

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 AND gate on your breadboard socket, you will need the following components. Most of them can be the components left over from your experiment with the DL OR gate:

Constructing the Circuit

Your DL AND gate will be constructed in the same place as your DL OR gate from the previous experiment. You can use the same diodes and jumpers, but we strongly urge you to prepare a new 10K resistor, since the lead spacing is different for this experiment.

Circuit Assembly

Start assembly procedure

Starting the Assembly

Your experimental AND gate will be located in the same place as the OR gate you explored in your previous experiment. You should still have the parts you used for the OR gate available, and will re-use most of them here.

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

10K, ¼-Watt Resistor

Locate a new 10K, ¼-watt resistor (color code brown-black-orange) and form the leads for a spacing of 0.5". 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.

Click on the resistor image to continue with your assembly.

1N914 Silicon Diode

You should have two 1N914 silicon diodes left over from the previous experiment. One of them already has its leads formed to a spacing of 0.3" in accordance with the illustration above. Install this diode in the location shown in the assembly diagram. Note that the cathode, as indicated by the color band around one end, must be oriented to the left this time.

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

1N914 Silicon Diode

You should also have a diode available with its leads already formed to a spacing of 0.4". Install this diode next to the first one, as shown to the right. Be sure to observe the orientation of the diode.

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

3" Orange Jumper

You should also have two 3" orange jumpers left over from the previous experiment. Install one as shown to the right. If necessary, 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) and remove ¼" of insulation from each end. Then install the jumper as indicated to the right.

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

3" Orange Jumper

Locate the second 3" orange jumper (or make another one as before), and install it as shown in the assembly diagram.

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

10" White Jumper

You should also have a 10" white jumper save from the previous experiment. If not, cut a 10" length of white hookup wire and remove ¼" of insulation from each end. Connect this jumper from your experimental circuit to L0 as shown in the assembly diagram.

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 the 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 the L0 and record that result in the form to the right. Does your result match the normal behavior of an OR gate?

Also measure the output voltage for each combination and record that voltage in the text field for that combination of inputs. Are the measured voltages legal logic signals? Are they what you expected?

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

Inputs Output
S2 S1 L0 Voltage
0 0
0 1
1 0
1 1

The DL AND gate with logic switches connected.


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

However, your measured voltages may have seemed a bit strange. A logic 0 output measured about 0.6-0.7 volt, which is the normal diode voltage drop you would expect. However, the logic 1 output voltage was only about 4.5 volts. In view of the fact that all resistors pull the output up to +5 volts as shown in the schematic diagram, this seems odd. What's causing this discrepancy?

The answer is the LED driver circuit for L0. Remember that the gate output is connected to L0, so that circuit constitutes a load on this gate. It consists of a 100K resistor (plus a CMOS gate input) connected to ground. The resistor forms a voltage divider with the 10K resistor in the AND gate. This will pull the output voltage down somewhat, causing the result you obtained. Note that if we used the RTL-based LED driver circuit here, the load would be a 22K resistor plus a forward-biased diode. This would have a far greater effect on the output voltage of this gate.

Although the error caused by the input circuit to L0 is small enough to allow the circuit to still operate within legal voltages for both logic levels, it does point up a potential problem: we must allow for possible interactions between logic gates, and make sure that the signal does not get degraded too far.

Make sure power is turned off, but leave your experimental circuit in place. The next experiment will expand on this circuit to continue our work with Diode Logic.

Prev: Two-Input DL OR Gate Next: Two, Two-Input DL AND-OR Gate

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