Quantity   Component name
1 × Geyserwise Thermostat Just the thermostat, available from some online sellers without buying the rest of a Geyserwise controller.
1 × Sonoff THR320 Smart relay, designed to interface with external 1Wire sensors.
1 × ATTiny85 Optional, used to interface between the Sonoff THR320 and the analog thermistor in the Geyserwise thermostat.
1 × DS18b20 1Wire temperature sensor Alternative to the ATTiny85. You want a bare IC, not one encased in a stainless steel casing, because it won't fit in the tube.