NOR Gate Transistor Logic

NOR Gate TTL Circuit

This is a Transistor-Transistor Logic (TTL) NOR Gate circuit using bipolar junction transistors. A basic circuit using any general-purpose transistor such as the BC549, BC548, or BC547, could be used to construct the gate.

In this configuration, there are two transistors whose emitter collector junctions are connected in parallel across the diode. This is similar to two switches connected in parallel across the diode. Either of these transistors is capable of bypassing the LED. If Tr1 conducts then all the current passes through it and the LED turns OFF. If Tr2 conducts then all the current passes through it, and again the LED will turn OFF. The LED lights when both transistors are not conducting and OFF.

NOR Circuit Built on Breadboard

NOR Gate Circuit on Breadboard

Here is a simple breadboard construction of the NOR gate. Make sure you connect the transistors right way round. 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.

The power to the circuit is applied through the red wire, which is positive, and the blue wire, which is negative. A PP3 9 V dry cell is used to power this circuit.

The green wires are for the signal input, and the black wire is for the 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.

Parts List

Semiconductors
Tr1, Tr2: BC549, BC548, or BC547
LED: 2.1 V forward voltage drop

Resistors
2 × 100 kΩ
3 × 1 kΩ

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

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Please Note: These circuits are for educational purposes to show basic gate operation. These circuits cannot take the place of real TTL 74LSxx integrated circuits, and are not to 74LS specifications.

Basic Logic Gates, Truth Tables, and Functions Explained
Transistor Logic NOT Gate - Inverter
Transistor Logic AND Gate
Transistor Logic OR Gate
NOR Gate Transistor Logic
NAND Gate Transistor Logic
NAND Gate using Diode Circuit
NOR Gate using Diode and Transistor (DTL)