Liquid cooling vest help

[BrianG]

Switching on and off in static cycles would only be good for a very small ambient temperature range. A thermistor is a much better idea, but you'll have to do some electronic tweaking.

Thermistors are simply resistors that vary resistance with temperatures. There is usually a datasheet that shows a graph of resistance vs temperature. They are almost never used by themselves, but in a voltage divider to create a voltage node than varies with temperature.

So, you'd assemble a Schmitt trigger (ie, a comparator with hysteresis) with a reference voltage node for one of the trigger's inputs, and the thermistor and another fixed (maybe a pot for fine tuning) resistor to create the varying voltage node for the other trigger's input. The trigger's output can then drive a transistor (or FET) to switch the motor on and off. This controls the motor based on temperature rather than a fixed time cycle.

BTW: When you are dealing with temperature controls, you can get a lot of "cycling" because the circuit is trying to maintain your exact set temperature. The Schmitt trigger gives you a range of temperatures, similar to how your house thermostat works; your temperature set point may be 70*F, but the A/C may kick on at 72*F and shut off at 68*F. If the A/C kicked on at 70.01*F and off at 69.99*F, it would forever be turning on and off in fast succession, which isn't good. Heck, you might even be able to use a modified thermostat!

Granted, making a circuit like this is going to require calculations and some kind of experimenter's breadboard to get a prototype. Then, once it works like you want, you recreate the circuit on an etched PCB or just a generic non-etched PCB (the kind with holes all over).