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Temperature Sensitive Circuit

Discussion in 'General Electronics Discussion' started by Bely Stockman, Jan 11, 2015.

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  1. Bely Stockman

    Bely Stockman

    Jan 11, 2015
    I want to know how to make a circuit that turns on a light based on temperature. Basically it's a circuit that reads temperature and turns a light on if a certain temperature is reached.
  2. davenn

    davenn Moderator

    Sep 5, 2009

    Jan 11, 2015

    U1 is a PTC type thermistor. At lower temperature its resistance is low. R1 is to be selected carefully such that at lower temperatures the voltage across U1 is not enough to satisfy the requirement of Q1 to turn on. D1 is also added to provide an additional diode drop to further control the turn on/off of the circuit. It may or may not be needed depending on the design. At lower temperature the Q1 is off and the relay coil won’t be energized and the lamp will not turn on.

    Once the temperature increases the resistance of U1 also increases and its voltage drop until it reach the VBE requirement of Q1 to turn on. Once q1 turns on, the relay coil will be energized and the lamp will turn on.

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