Temperature-sensitive audio amplifier
This project demonstrates how transistor collector current depends on temperature, by using the leakage current of a transistor to control the frequency of an audio oscillator.
The NPN transistor operates as a pulse-type oscillator. The bias voltage is controlled by the series circuit made up of the 22K resistor and the resistance of the PNP transistor between collector and emitter. The base current and collector current of a transistor vary with the temperature.
Adjust the 50K control volume so that the speaker output is a low sound or pulses. Now warm up the PNP transistor by holding it between your fingers. You should hear the tone become higher as the transistor temperature is increased.