Analog light wave Transmitter / Receiver
Électronique / Electronic
555

R1 can be any sensor having variable resistance when R1 is a cadmium sulfite (cds) photoresistor, the frequency of signal rise with light level.

This circuit pulse an infrared-emitting diode at a frequency determined by R1 and C1. The receiver receives and amplifies signal, it converts the signal's frequency into a current which is displayed on a 0-1 mA meter, Use lense to increase range.

For full details, see "The forest mims circuit scrapbook" (M'Graw-hill, 1983)