This is a
design circuit for low
current relay circuit is designed to be used in battery operated electronic
devices. Its operating current is in micro amperes (µA). This is done by using
a bistable relay and adding some components to force the relay to behave like a
mono stable relay. A bistable relay stays at its last state when the power is
turned off but consume at least 50mA trigger current. A mono stable relay
switches back to its original state when the power is turned off. Here’s the
figure of the circuit;
How this circuit can operate? 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.
