We finally got around to making a batch of tempeh. In order to maintain the proper temperature for incubating it in a cooler, we came up with this control device made from a spare thermostat, a light bulb, and a 120V to 24V doorbell transformer (thanks Jack!). All of which are scrap parts that can be attained inexpensively.
Most home heating and cooling system thermostats require 24V, so this makes the transformer necessary. At 24V, a 60W incandescent lamp will use 6W, which is close to the 7W light bulb recommended by the tempeh culture instructions.
The thermostat we used only goes up to 82°F, whereas incubation is supposed to be in the 85-90°F range, but hopefully it works out! We also added some thermal mass (water) to the container to moderate the temperature swings.
Off the shelf options include:
deluxe Fridge/Fermenter Controller from Holy Scrap Hot Springs when it is ready!
The reptile controller would not be good for controlling refrigeration as it isn't designed with the proper hysteresis to prevent wear and tear on the compressor. However, you could get away with using a peltier/thermoelectric refrigerator for cooling.
Update: The 6W wasn't enough to keep the temperature up, so we switched to a 75W lamp (9W) plus 11-16W of uncontrolled Christmas lights, which let us get up to 90°F.