The latch is a logic circuit of electronics that has two stable states. A latch is a device similar to transistors. It has 3 terminals which function like a transistor base, collector, and emitter. In the transistor when current is fed to base the sufficient current flows from the emitter to the collector as long as the base is feeding but in the latch when the current is fed to the base current permanently starts flow from the collector to the 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 the latch. Latch circuit mostly is mostly used in alarm circuits. when we press the alarm button the sirens flash until we shut down the alarm.
In this logic circuit, we are going to design a latch circuit using transistors. It consists of two transistors bc 547 NPN and BC557 PNP. We can easily explain it by using the circuit diagram given below. 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.