Take a crystal for which you need a constant temperature. Solder the
cathode of a cheap diode (1N4148 etc.) to the crystal case. Solder one end
of a moderately low value half-watt resistor to the case. Bring out the
anode of the diode, the free end of the resistor, and the crystal case on
wire leads and encapsulate the crystal-diode-resistor in heat shrink. Use
the diode as your temperature sensor, the resistor as your heating element,
an opamp/driver transistor as the comparator/amplifier and bingo, the
world's cheapest crystal oven. Bang-bang or linear, your choice.

Jim



eh Henry Kiefer wrote:
Hi all -


Do you know of other interesting devices or circuits good for misuse?

Best regards -
Henry