Constant Temperature Circuit
(c) G. Forrest Cook 1997

Description:
This circuit is a generic low power temperature controller that 
can be used for stabilizing temperature sensitive electronic
circuits.  Specifically, it was built to stabilize a VFO
(Variable Frequency Oscillator) for ham radio applications.
The circuit has also been used to stabilize the otherwise
drifty Ramsey FM10a micropower FM transmitter.

Theory of operation:
The 7805 voltage regulator provides a reference voltage that is
fed into a resistive bridge formed on one side by the 20K trimmer
and the other side by the 3.3K resistor and the 1K/thermistor
combination.  The termistor is an NTC (Negative Temperature Coefficient)
type.  The op-amp is run in a differential mode and tries
to keep its inputs at the same potential by the thermal feedback
loop formed by the heater and the thermistor.  The three 1N4001
diodes are used to bias the emitter of the transistor up enough
that it can shut off fully with the limited voltage swing from the
741 op-amp.  The heating indicator LED also taps off of the same diode
ladder to enable it to shut off entirely.  The value of the (1uF) capacitor
in the op-amp feedback loop may need to be adjusted if the circuit
"rings", or swings back and forth before stabilizing on a temperature.

Construction:
I recommend building this circuit on perforated circuit board
or make a PC board if you have the tools.  I epoxied all of the
heat generating components (the 40 ohm resistor, the TIP122, and
the 3 1N4001 diodes) on an L shaped aluminum piece and put the
circuit board with the op-amp on standoffs on the aluminum.  Be
sure to use a heat conducting insulator for the transistor tab.
Also be sure that the thermistor is located near the 40 ohm resistor
and has good thermal contact.  Bolt the whole assembly near the
item that you wish to control the temperature of.  I recommend
using styrofoam insulation around the temperature controlled
device, it is best to encapsulate the temperature regulator and
the target circuit inside of the insulation.

Use:
Adjust the 20K pot to center and power up the circuit.  The LED will
probably come on bright and gradually dim down as the circuit reaches
equillibrium.  Put a thermometer on the 40 ohm resistor and measure
the temperature.  Adjust the 20K pot to the temperature that you
desire.