A transistor latch circuit is a device through which an electrical load can be latched into operation with a single touch of a switch. The benefit of this type of electronic switch is that mechanical types of operations are totally avoided. We will learn here how to make a simple transistor latch circuit.
Referring to the diagram we see the entire circuit is configured around some easy wiring of a couple of transistors and some resistors.
You will need the following electronic parts for teh construction of the above discussed circuit:
Transistor BC547 - 1 no.
Transistor BC557 - 1 no.
Resistors 4k7 -2no.
Resistor 22K - 1no.
Resistor 10K - 1 no.
Resistor 1K - 1 no.
LED - 1no.
Relay - 1no. (optional).
Veroboard, cut into suitable size.
All the parts needs to be fixed over the veroboard by soldering as explained below:
First insert the two transistors so that their leads are held well wide of each. Keep ample space in between them also.
Another 4K7 resistoris fixed across the collector of BC547 and the base of BC557.
A feed back resistor of 22K is connected across the collector of BC557 and the base of BC547. This resistor is actually responsible for making the circuit latch, through single triggers.
An input trigger resistor is connected at the base of BC547.
Finally the LED is connected across the collector of BC557 and ground with a series resistor of 1K.
The supply of 3 to 12 volts DC is enough to make the circuit work.
Now if you just touch the free end of the 10K resistor, you will find the LED illuminates and remains ON eben after the finger is removed from the resistor 10K.
The circuit has latched. Switch OFF the power to break the latch.