How to Build a Latch Circuit with Transistors

Latch is logic circuit of electronics which has two stable states. A latch is a device similar to a transistors. It has 3 terminals which function like a transistor base, collector, and emitter. In transistor when current is fed to base the sufficient current flows from emitter to collector as long as base feeding but in latch when current is fed to base current permanently start flow from collector to emitter even if the current at the base is removed.

so when we have to shut the circuit we have to disconnect the power supply of latch. Latch circuit mostly is mostly used in alarm circuits. when we press alarm button the sirens flash until we shutdown the alarm.

List of components

  • R1=R2=R4 = 10K
  • R3 = 100K.
  • T1 = BC547.
  • T2 = BC556.
  • C1 = 1uF/25V.
  • D1 = 1N4007.
  • C1 = 1uF/25V.
  • Relay = According to input supply.

In this logic circuit we are going to design a latch circuit using transistors. It consists on two transistors are bc 547 NPN and BC557 PNP. We can easily explain it by using the circuit diagram given below.

Transistor Latch Circuit

As shown in the above figure transistor T1 (bc547 npn) and T2 (bc557 pnp) are configured in such a way that BC557 follows BC547 to either start or stop the conduction depending upon the trigger received from T1 Side.

BC557 also works as a buffer it also better response even on very small signals from T1. As a small signal is given at the input (base) of T1 (bc547), it trigger to T2.

The initial current of T2 also pass a small current to the base of T1.

It must be clear that t1 is npn that is way it response with positive and t2 is pnp it response with negative.

Calculating Transistor Biasing

It is the basic formula for both transistor

  • VBE = 0.7V
  • IE = (β + 1)IB ≅ IC
  • IC = βIB



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