This circuit acts like a two-position switch but is operated using a pushbutton. After power has been applied, the circuit is in the following initial state: the bases of T1 and T2 are at the positive supply potential and the base of T3 is at ground potential. All transistors are cut off. The other contact of the pushbutton is at ground potential. No current flows through the relay coil and the status LED is off.
If the pushbutton is pressed, T2 and (after a slight delay due to the RC network) T3 switch on. The collector of T3 is now nearly at ground potential, so current flows through the relay coil, and the function LED is illuminated. T1 can also switch on. This situation is stable since ground potential can reach the base of T2 via R1, so nothing changes when the pushbutton is released. C1 is charged via R3 to cause a positive potential to be present at the push button. If the pushbutton is now pressed again, it connects a positive potential to the base of T2 instead of the ground potential. This causes everything to toggle back into the initial state.
A similar operation can be obtained using a thyristor circuit, and in fact, T2 and T3 form a sort of thyristor. However, the circuit shown here is largely independent of the voltage and current demands of the connected load. The relay coil should be suitable for the supply voltage (5–12 V) and should not draw more than 250 mA since otherwise, T3 will go up in smoke. With our lab prototype, we measured current consumption of 70 mA in the ‘on’ state and less than 0.1 mA in the ‘off’ state.
