How does the low current relay works?
When the power is turned ON, the C1 charges via D1 and the relay coil and this current activates the relay. D1 ensures that the base of T1 is always more positive than its emitter and because of this T1 and T2 are always blocked.
Once the power is turned OFF, the emitter of T1 is coupled to the charge voltage at the positive pole of C1. Its base and the relay coil on the other hand are coupled to the negative pole of C1 and now T1 and T2 conduct, C1 can discharge through T2 and relay. The current flows to the relay coil but in reverse order so is activated to its other state.
It has the advantage of consuming little current, around 150 µA. For a reliable operation, select the relay’s operating voltage as 2/3 to 3/4 of the main power supply. For example is using a 12V power supply select a 9V relay.
T1 = 2SA499 = BC557, BC558
T2 = 2SC734 = BC547, BC548