An AC triggered switch for low frequency signals.
This is a basic ac voltage operated switch made from discrete components. Both Q1 and Q2 work as common emitter amplifiers, but the biasing of Q1 is arranged by R3 and R4 so that about 0.5V is applied to its base; so with no input signal both
transistors and the load is off.
With an input signal greater than 0.3V pk-pk (100mV RMS) the positive half of the waveform will switch on Q1, and Q2 and the relay. As the input signal switches to its negative transition, Q1 will switch off, but base current in Q2 continues to flow via C2, so Q2 and hence relay load remain on. This will happen for any ac signal within 50 to 1000Hz.
R1 prevents excessive base current flowing in Q2, if required a series reistor 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.