The following series of functions to control the relay kutup. With an input signal greater than 0.3V pk-pk (100mV RMS) the positive half of the waveform will switch on transistor Q1, and Q2 and the relay. As the input signal switches to its negative transition, Q1 will switch off, but the base current in using-multimeter-to-measure-transistor.html">transistor Q2 continues to flow via the C2, so Q2 and hence load relays remain on. This will happen for any ac signal within 50 to 1000Hz. R1 prevents excessive base current flowing in transistor Q2, if required a series resistor of 100 ohms can be included with C1 to reduce excessive current flow, though this may decrease sensitivity.
C2 has a dual purpose; as well as smoothing the input signal, it adds a delay to the on / off operation. The delay is dependent on the value of C2 and the coil resistance of the relay. Instead of a relay, a LED and series resistor of 1k could be used instead, however the relay has the advantage of being able to switch large loads on and off. C2 has a dual purpose; as well as smoothing the input signal, it adds a delay to the on / off operation.
