Based on NE555 this circuit turns on and off the IC output by a momentary switch. In other words it works as a mechanical latching relay, but the circuit backs to the start condition when you switch off the power supply. This feature is often required in automotive devices. No relay contacts are used, infact I connected the output to a led. Momentary Switch Circuit Schematic Once the momentary switch circuit is supplied the output (pin 3) keeps off because pin 2 and 6 are at half voltage. When the button is pressed Q1 turns on within a fraction of second because of the capacitor, while Q2 keeps off. By switching on Q1 leads pin 2 and 6 to low voltage, then the output gets high. When the button is pressed again Q2 switches on and leads pin 4 to low voltage (I used a pull up resistor), then the circuit takes the start condition.