How to design a Latch circuit using BJT Transistors

Introduction

Latch circuit is a circuit that retains the state if the output even when the input is removed. The output signal will remain until a reset signal is applied to reset the signal to its default state.

Background

BJT transistors have 4 modes of operation:

• Saturation mode, the transistor acts like short circuit, current flows freely from collector to emitter

• Cut-off mode, The transistor acts like an open circuit, no current flows from collector to emitter

• Active mode,

The current form the collector to the emitter is proportional to the current at the base

• Reverse Active,

Like Active mode, but the current flows in the reverse direction

Circuit Diagram

Working Explanation

The circuit uses 2 transistors, one is an NPN transistor named T1, the other is PNP named T2.

When a high level signal is applied at the base of T1, T1 is activated and then connects the base of T2 to ground, as a result T2 is also activated.

a feedback voltage is connected between the collector of T2 to the base of T1, Which will make T1 active all the time, until the power is removed or a low level signal is applied at the base of T1.

Area of Application

Burglar Alarm

Fire Alarm