Output frequency or tone of the circuit temperature controlled oscillator varies with the temperature at which the input germanium diode is held. Reverse resistance of D1 varies from 500-Ω to 10 KΩ, when temperature varies between 200C and 800C. (At higher temperature germanium devices show very low resistance.)
This variation of resistance changes the tone of the output sound of the oscillator which is built around the AF amplifier IC (IC1). Instead of germanium diode, base-emitter junction of an ordinary medium-power transistor such as AC128, AC188, 2N360 or 2N610 can be used. The circuit temperature controlled oscillator will not work with silicon diodes.
The circuit temperature controlled oscillator can be made light-controlled by just replacing the diode with an LDR or a reverse biased photodiode.
A 9V to 12V ordinary unregulated power supply can be used for this circuit.
PARTS LIST OF TEMPERATURE CONTROLLED OSCILLATOR
Resistors (all ¼-watt, ± 5% Carbon)
R1 = 56 KΩ
VR1 = 10 KΩ
C1 = 0.005 µF
C2, C3 = 220 µF/16V
IC1 = LM380
D1 = DR25 (germanium only)
LS1 = 8Ω/ 3Watts