This circuit was obtained in an old English magazine (TSI) 1998 and consists of a Schmitt trigger adapted to operate with a 1k5 thermistor. The Rfix resistor determines the trigger point may be replaced by a trimpot for adjustment. The output can excite a power stage with relay or SCR input from a suitable microcontroller. The original transistor BC109 can be replaced by more modern equivalent of the BC548, no change with in the other components.
