Fun With NOR Gates

See A Vintage Binary Electronic Adder Project for some background.

Another work-in-progress - my own graphics & photos are to be added.

Transistor NOR Gate

I have built and tested one successfully, based on the circuit above, but having:

• a 5V power supply

• an NPN transistor (BC548)

• an LED between collector and ground to indicate the output state

Input resistors at left are 10k ohms, the resistor at upper right, between the collector/output and supply, is 1k ohm.

When no voltage is applied at the base of the transistor, then current flows from emitter to collector and the LED is lit. If a voltage is now applied to the base, that current no longer flows and the LED is unlit. Why a NOR gate? Well, with just 2 inputs, X and Y, say, the output is live/true if neither X nor Y is live/true. 

This circuit negates the logical truth of the input.

Set-Reset Latch

Here is a logical representation of this circuit, using the NOR gate symbol. The output of each NOR gate is fed into the input of the other. Input S to the upper NOR gate is deemed to Set the logical state of the circuit, to ON (deemed when output Q is live, not Q is not live), while input R to the lower NOR gate is deemed to Reset it, i.e. to OFF (deemed when output Q is not live, not Q is live). 

Since Q and not Q are complementary states, this circuit can be used to store a piece or one bit of binary information. As such, only one LED is required to show the state of the circuit and so the one attached to not Q can be dispensed with.

I have built and tested one successfully based on the circuit shown above, with NOR gates constructed as before, but having:

• 1k ohm resistors, not 1k ohm, between collectors and power supply

• an additional input to the base of each transistor, again via a 10k ohm resistor, that is used to set each NOR gate. Note that these inputs cannot both be high at the same time.

Toggle Flip-Flop/Scale-of-Two circuit

A diode pulse routing network has been added to the S-R latch to ensure that each input pulse is sent to the required NOR gate.

As I understand it, the diodes prevent current flowing between the bases of the two transistors and interfering with the NOR gates and also prevent corruption of the input signal. Somehow, the capacitors ensure the input goes to where it is needed.

I used 220 pF capacitors, and added 1k ohm resistors, not 10k ohm, to the S-R latch circuit. 

Output is taken from one of the NOR gates, i.e. from one of the collectors and is at half the rate of the input. By connecting the output of one circuit to the input of another, we can chain these together to form a binary counter.

Tested with manual on/off input - toggling is not perfect, possibly because of poor connections on the breadboard.