I built a heater for my telescope mirror using 20 1/2 watt resistors in parallel. I am using a 12 volt battery and I want to build a simple controller with a LED light to show on/off and a way to vary temperature. Any help would be appreciated.
Do you want to turn the heater on or off - or would you like to vary the power to the resistors??
Seems to me that there is a fan controller IC that might be easily adapted. How much power will the circuit have to handle? Is it 10 watts or something different?
You need to measure the temperature, then compare it to a setpoint. A simple bang-bang controller could be made with a 3 terminal temperature sensor, a pot to set temperature, a comparator to compare the two values, and a power mosfet to drive the resistors. If you want to go advanced, a PWM setup can vary duty cycle etc., but I am not sure how much you want to put into this, or if tight temperature control is required.
That should be no problem, any of the idea listed above will work. With my circuit I recommend replacing the PNP transistor driving the relay with a P-channel MOSFET and using that to drive your heater.