The small transistor latch circuit can be easily built and used for the relevant purpose of making and breaking a relay and the subsequent load just through the press of a couple puss switches.
Please refer the CIRCUIT DIAGRAM for knowing exactly how to make an electronic latch or a set reset switch circuit.
Cut a small piece of veroboard about ½ by 1.2 inch for fixing the components.
Connect the BC 547 transistor and the BC 557 transistor on the above veroboard. The emitter of the BC 547 will go to the ground.
The emitter of the BC557 will go
The collector of the BC547 is connected to the resistors 4k7 and another 4k7.
One of the 4k7 resistors will go to the positive supply while the other 4k7 resistor will go to the base of the transistor BC 557.
The resistor having a value of 100 k goes to the base of the transistor BC 547and futther terminates to the switch S1. S1 is a push to ON switch.
The collector of the transistor BC 557 goes to the relay and the corresponding flyback diode 1N4007. This diode is kept for protecting the BC557 transistor from the relays back EMFs.
A feedback resistor of 100 K is connected across the collector of the BC557 and the base of the BC547. This feed back resistor is actually responsible for the latching action of the circuit.
The base of the BC 547 is also connedted to another push button S2 which is uaed for breaking the latch while S1 is used for the opposite that is for making the latch.
The circuit functions with voltages from 5 to 15 volts DC.