NOR Gate using Diode and Transistor (DTL)

This is a Diode-Transistor Logic (DTL) NOR Gate circuit using a general-purpose bipolar junction transistor (BC547), and general-purpose diodes (1N4148). This page shows how to make this circuit, and the implementation is on a breadboard using discrete components.

The working logic is very simple. The emitter-collector junctions of the transistor connect in parallel to the light emitting diode (LED). When the transistor conducts (ON), it bypasses the current because almost all the current passes through the emitter-collector junctions, and therefore the LED goes OFF. The transistor will conduct when either one or both of the inputs receive a logic 1 signal. When both inputs are logic 0, the transistor stops conducting (OFF) and all the current passes through the LED instead, hence the LED lights up.

DTL NOR Gate on Breadboard

This is a very simple NOR gate circuit construction using a pair of diodes and a transistor. The implementation is on a solderless breadboard using discrete components. This diagram shows how to make a simple DTL circuit that has the same function as a NOR gate. The design is very similar to the transistor gates shown in this multi-part article.

Make sure you connect the transistor right way round. You may wish to follow the links in the parts list to see how they are connected. To learn how to use the light emitting diode, please refer to the LED Resistor Calculator article.

We apply the power to the circuit through the red wire, which is positive, and the blue wire, which is negative, and a PP3 9 V dry cell powers this circuit.

The green wires are for the signal input, and the black wire is for the common ground. The input signal is with respect to ground. For practical purposes, you could use a 1.5 V AA size battery for the signal inputs, where the black wire connects to the negative of the battery and the green wire to the positive.

Parts List

Semiconductors
1 × Tr1: BC549, BC548, or BC547
1 × LED: 2.1 V forward voltage drop

Input Diodes
2 × 1N4148

Resistors
2 × 1 kΩ

Misc
Solderless Breadboard
1 × PP3 9 V Dry Cell
1 × AA Dry Cell for signal input

My Breadboard Build

I built this circuit on breadboard and it works perfectly well. This is a very basic circuit used as a teaching aid to show how a two input NOR gate works, it is in no way a substitute for a TTL circuit.

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