Howard's Astronomical Adventures

Temperature Regulator for Film Hypersensitizing

Circuit schematic courtesy Rockett Crawford.
Usable temperature range is 0 to 70 C, regulation +/- 1/3 C


The standard Lumicon film hypersensitizing kit uses an unregulated heater. This can cause higher than desired hypering temperatures as well as variations between uses. This is not acceptable if you want to ensure repeatable results. The circuit shown below will provide accurate temperature regulation for very little cost.

Parts List

All of the parts needed to construct the temperature regulator are available from Radio Shack.

Part NoComment
271-110AThermistor, 10k @ 25 C
276-007741 op amp
275-0059v, 18ma, mini relay, NO
276-19958 pin dip socket
 R1 10k, 1/4w resistor
 R2 10k, 1w resistor
 R3 1k, 1/4w resistor
 R4 1k potentiometer
 R5 50k potentiometer
 Project Box, Power supply

Construction Notes

I used a compact switching 12v/5v power supply I had laying around. Just about anything supplying 12v at 200ma or more will do. However a regulated power suppy would ensure repeatable accuracy.

The temperature sensor (thermistor) should be attached with long wires.


Slide the thermistor into the regulator hole on the lumicon tank until it touches bottom. For optimum regulation accuracy find a way to keep it in contact with the tank.

Initial Calibration: pot R5 is the coarse temperature adjustment and R4 is the fine temperature adjustment. On the first use you will need to set R4 at about half setting and get the initial temperature close with R5. Take your time doing this and try to not overshoot your desired temperature as it may take the tank awhile to cool down. Once close, say within a few degrees, adjust R4 to get the final temperature. This process could take a few hours with the small heater (15w?) in the Lumicon 600 tank. After this you will not need to touch the adjustment, just turn it on.
The relay buzzes for a short time when the temperature is nearly exact; I suspect adding a capacitor (say 47 mf @ 25v) between pin 6 of 741 and ground would help eliminate this but I have not tried it.