Basic Solar Thermal Controller
Believe it or not the relay on this differential controller can handle a 500 Watt load although I recommend keeping it under 200 Watts. A simple adjustment of the differential pot is all that's required to maximize a solar heat harvest. Bigger is better when it comes to the surface area of collectors, but it's not necessarily better when it comes to the size of the differential controller. For latest controller click on above illustration.
This Basic Differential Controller is easy to build from a schematic and even easier to build with a KIT and set of instructions. The quarter is not part of the KIT and you'll have to supply your own solder, but I'll throw in a set of thermistors, 4 mounting screws and 4 standoffs and mounting block.
When mounted the Basic Differential Controller would appear as illustrated above. If you need a differential controller that displays temperature check out the AC/DC Differential Controller