TRANSISTORIZED SOFT SWITCH

Now a days soft on/off switches have become quite popular. Normally these are made around CMOS ICs. Here we are giving a simple transistorized circuit which is built by using only three transistors and controlled by two small push to on switches. Here transistors Tl and T2 form a flip flop while the third transistor T3 has been used to drive the relay. When switch SI is pressed then positive volts are made available to the base of transistor Tl (BC08) and it starts conducting  At this stage the collector volts of Tl register a drop and transistor T2(BC108) is cut off and its collector voltage becomes high. This high voltage is received at the base of transistor T3(SL100) through the resistance R7(10K) and this transistor becomes ON to give negative supply to the relay. Thus, the relay is turned ON. A 6V-12V100E relay has been used.

To maintain this ON Position of the relay, positive supply is received at the base of transistor T1(BC108) through the resistance R6(100K). How, when the switch S2 is pressed then negative supply reaches the base of transistor T1(BC108) through the resistance R4(10K) and Tl stops conducting. At this stage its collector volts go high and these are received at the base of T2 through R5(100K). This makes T2 ON and its collector volts drop. In this stage not positive supply is received at the base of T3 and it remains OFF. Thus, no negative supply reaches the relay and it also remains off. Diode D1(IN4007) is connected in reverse bias in the circuit to protect the relay from the back EMF generated when it switches OFF.