NAND Gate using Diode Circuit

This is a Diode-Transistor Logic (DTL) NAND Gate circuit using a bipolar junction transistor. A basic circuit using any general-purpose bipolar transistor such as the BC549, BC548, or BC547, could be used to construct the gate. The diodes could be any general-purpose 1N4148 type. In this configuration, the emitter-collector junctions of the bipolar transistor are in parallel across the diode. When the transistor conducts, the current flows through the transistor bypassing the diode, hence the LED switches OFF.

This is a slightly complicated circuit, but not impossible to understand. As a beginner, you may be thinking that logic 0 is the same as no signal at the input. However, that is incorrect; logic 0 is equivalent to zero (ground) potential.

The 10 kΩ resistor keeps the transistor in a conducting mode, however, if any one of the inputs is zero (ground), the current flow will be through the 10 kΩ resistor, through the diode, and to ground. The base current therefore shorts to ground through the diode. When this happens, the transistor stops conducting, hence the LED comes ON.

DTL NAND Gate on Breadboard

Here is a simple breadboard construction of the NAND gate. Make sure you connect the transistors the 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: BC549, BC548, or BC547
LED: 2.1 V forward voltage drop

Input Diodes
2 × 1N4148

Resistors
1 × 10 kΩ
1 × 1 kΩ

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

This Article Continues...