Using some electronic components can be built a simple relay driver to drive a relay. This circuit change state of a standard relay each time a pulse is applied (or press a button).
When the relay is disengaged, the transistor T1 is blocked because the base and emitter or are maintained at the same potential by R2. Capacitor C1is charged through R4, R3 and relay coil. When applying a pulse (logic zero, less than 0.5 s) or press the S1, T1 conducts and act relay. Relay changes state, so T2 delivers current to relay coil. Capacitor C1 is discharged until the voltage across it is equal to the base-emitter voltage of T2.
When the pulse disappears, or is released, T1 locks again, but the relay remains operated by T2.
To disable relay is required a slightly longer pulse: should be long enough to allow full discharging of C1 through R3.
Input of short pulses have no effect when the relay is activated.
If applied impulses are long enough to relay in idle state circuit works as a standard relay which is activated almost instantly by impulse and is disabled immediately pulse stops.
Maintaining voltage relay should not exceed half the supply voltage.