Simple Temperature Indicator

SIMPLE TEMPERATURE INDICATOR For the absolute measurement of tempera- tures a thermometer is indispensable. There are, however, many situations where an abso- lute value is not needed and a relative indica- tion suffices.The getting hot of an electric drill or vacuum cleaner may be indicated to the user by the lighting, or changing of colour, of a simple indicator light. It would be a further advantage if on such equipment a green light would indicate that all is well as far as tem- perature is concerned. As the temperature rises, the light should change colour slowly to indicate that the equipment is getting too hot. The proposed circuit does this and has the additional advantage that it does not need a separate low-voltage supply: it works direct from the mains. The indicator proper is a two-colour LED, Dl, while the sensor is a com- bination of a negative temperature coefficient (NTC) and a positive temperature coefficient (vrc) resistor, R4 and R3 respectively. At a relatively...