W1209 Thermostat module

Insanly cheap module with a relay, 3 digit display and a sensor connector. All of that for $1.60 including shipping!

STM8S000F3P6 microcontroller, 8 KB Flash, 1 KB RAM, 128 bytes EEPROM

But please be aware that the PCB layout violates all common safety rules on the high voltage side. No sufficient creeping distance, the PCB traces are way too small to actually carry 15A and the screw terminals are not up to that task either.

connecting the board

SWIM connector

Pinout of SWIM connector

Pin Signal
1 Vcc (5V)
2 SWIM
3 NRST
4 GND

Schematic

Schematic of W1209

Further reading

Functionally identical firmware, GPL2'ed