Using NTCs or LDRs as sensors, the frequency of this circuit depends on the temperature or the intensity of the light. The signal is rectangular and can be transmitted remotely for remote sensing. The circuit works with modern transistors like the BC558 and can be powered with voltages from 6 to 12 V. The capacitors can be changed according to the desired frequency. The sensors must have normal resistances between 20k and 100k.
