Supplied with 12 Volt DC this basic circuit drives a relay by a momentary button. The coil takes about 2,5 Volt when the button is pressed and 5 Volt when the button is released and the transistor conducts. In other words the circuit uses the coil hysteresis.
By pressing the button both transistors conduct but the coil keeps off due to the low voltage.
With released button Q2 transistor keeps on for a short time, while the relay switches on.
By pressing the button again Q1 conducts, discharges the capacitor and causes a power fall on the base of Q2, while the relay keeps on.
Once the button is released Q2 would switch on with a very small delay, because the capacitor has to charge. Since the relay is faster the circuit backs to the start condition.
Unlike the mechanical bistable relay this circuit backs to the start condition when you cut off the power supply.
N.1 relay 5V DC - 125 ohm
N.5 diodes 1N4007
N. 3 resirtors 8,2K
N.1 resistor 18K
N.1 resistor 82
N.1 resistor 180 (0,5W)
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