Introduction:- A latch is a device that is like a transistor. It has 3 terminals just like a transistor which function as a base, collector, and emitter. The only difference between a transistor and a latch is that once sufficient current is fed into the base terminal, current permanently flows from the collector to emitter. In a transistor, current flows only across the collector to emitter when sufficient current is fed into the base. The latch is different. When sufficient current is fed once into the base, it permanently conducts current from the collector to emitter, even if the current at the base is removed. It’s a latch. The only way to shut off the circuit and reset it is by removing the supply voltage.
• BC547 NPN transistor
• BC557 PNP transistor
• 2 2.2KΩ resistors
• 1KΩ resistor
• 330Ω resistor
• Power source
Working:- This is how the latch circuit operates.
When 0.65V is fed into the base of the BC547 transistor, it turns on. Once it turns on, current flows from VCC down to the base of the BC557 transistor. This, in turn, turns on the BC557 transistor. Current now flows from the collector of the BC557 in 2 directions. Some of the current flows to turn on the load, which in this case is the LED. And some of the current flows into the base of the BC547 transistor. This is really how the latch is formed. Once we turn on the BC547, it, in turn, turns on the BC557. And then the BC557 provides continuous current to the base of the BC547, so that it remains on, even if we turn off the input voltage supply, VIN. The only way to turn off the circuit and reset it, then, is to turn off the supply voltage VCC. Turning off the input voltage, VIN will have no effect. Once we turn it on the input voltage once to the threshold voltage of 0.65V, which is the voltage needed to turn on the BC547, it’s latched on.