The diode (1N4148) is the temperature probe. Insulate it, and clamp to wherever you want to measure the temperature at. Try just next to the water outlet from the engine block. It works by the diode's forward voltage changing with temperature, the warmer it gets the larger the change. I used it for a boiling milk alarm, using two settings via a switch, so you can use boiling water to calibrate it at 100'C, with the switch giving an offset for the boiling milk temperature. Trial and error to get it just alarming before the milk boils, but no more burnt milk.
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where and how do you place the temp sensor? does it use thermocouple? any calibration needed? Thanks