NAND Gate Transistor Logic

This is a Transistor-Transistor Logic (TTL) NAND Gate circuit using bipolar junction transistors. A basic circuit using any general-purpose bipolar transistor such as the BC549, BC548, or BC547, could be used to construct such a gate. In this circuit, we have two bipolar transistors connected in series across the light emitting diode (LED). The current to the diode is bypassed only when both transistors are conducting at the same time. If only one transistor is conducting, then the bypass path remains open, and the LED stays ON. NAND gates are very useful in digital electronics, and you could use them to make a half-adder circuit.

NAND Circuit Built on Breadboard

Here is a simple breadboard construction of the NAND gate. Make sure you connect the transistors right way round, and they both should be the same type. Please 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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