SCR With Transistors

The SCR in circuit A produces a 'LATCH.' When the button is pressed, the LED remains illuminated. The SCR can be replaced with two transistors as shown in circuit B. To turn off circuit A, the current through the SCR is reduced to zero by the action of the OFF button. In circuit B the OFF button removes the voltage on the base of the BC547. The OFF button could be placed across the two transistors and the circuit will turn off.