Over at Karosium there is a DIY DS1821 based Thermostat project. The code is available and I am assuming if there are some requests that the schematic will also be posted.Â
“After configuration for thermostat mode the center (1Wire DQ) pin becomes a thermostat output pin. Since it’s still an open collector pin it needs a pull up resistor, and since it’s still basically a 4mA GPIO a transistor at least is needed to switch anything meaningful with it.”