In the circuit in the figure, when a pulse of light falls on the LDR, the circuit arms with the oscillator going into operation. To disarm, the circuit must be reset by touching X3. The reader can modify this circuit by connecting a relay driver stage to the output (pin 4) of the bistable to activate a higher external power circuit. The circuit can be used as a light alarm or for another purpose that requires the detection of light pulses. The sensitivity adjustment is made in P1. Greater directivity and sensitivity can be obtained by placing the LDR in a tube with a lens. The circuit can be powered by batteries and its consumption is very low when the transducer is not emitting sound.
