The 555 monostable has a 1 second period, If pin 2 gets low trigger pulses of a frequency more than 1 Hz, the output pin 3 will remain high, if the trigger pulses came in 10 second delays then output will go on and off, so also the relay and the electrical load on relay contacts. Like an integrator, it will not operate relay very fast. Even in a fridge stabilizer they put something like this else the compressor motor is damaged on frequent black-brown outs.
Now for the LM567 part, The clock at pin 5 flashes the IR Transmitter LED LD1 at a frequency set by C7-R5. The IR Photo Sensitive Diode or Transistor LD2 conducts when light is got from the IR LED. As the lights is in pulses, so also the impedance of LD2 goes low and high at same frequency. This develops an AC waveform across R3 over a DC representing Ambient light. If direct sunlight falls on this arrangement, the Photo-diode saturates, then this circuit or any other circuit won't work. But when ample protection from ambient light is made, some pulses good enough for LM567 tone detection, 100mV rms, is obtained and it operates the relay via a LM555.
Now for the LM567 part, The clock at pin 5 flashes the IR Transmitter LED LD1 at a frequency set by C7-R5. The IR Photo Sensitive Diode or Transistor LD2 conducts when light is got from the IR LED. As the lights is in pulses, so also the impedance of LD2 goes low and high at same frequency. This develops an AC waveform across R3 over a DC representing Ambient light. If direct sunlight falls on this arrangement, the Photo-diode saturates, then this circuit or any other circuit won't work. But when ample protection from ambient light is made, some pulses good enough for LM567 tone detection, 100mV rms, is obtained and it operates the relay via a LM555.
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