This circuit consists of a IGMF NBPF and a ac rectifier. When the input frequency is 1KHz which is the BPF of the filter will go to a comparator which convert a sine wave into a PWM signal. The duty cycle is controlled by the variable resistor. The diode will rectified the PWM signal to a DC for turning on Q1. Relay will ON when Q1 is ON
If the input frequency is deviated from 1KHz, say 2KHz, the duty cycle of the PWM will be too small to generate sufficient DC to turn on the Q1. Hence, Q1 will ON and the relay will turn ON also.
