temperature controller circuit schematic

555 Temperature Controller Circuit

You can build a temperature controller circuit with the 555 IC together with a thermistor resistor divider. The advantage is that a well regulated power supply is not needed. The dividing network consists of adjustable resistor R3, thermistor R4 and R5.

Temperature Controller Circuit Schematic

temperature controller circuit schematic

DANGER: Electric Shock Risk! This circuit is connected to the main 220V line, therefore it must be handle only by experts!

how does the temperature controller circuit works?

When the thermistor temperature is below a set value the voltage at pin 2 of the 555 drops below 1/3 of Vcc. This turns on the triac controlled heater and also starts the timing cycle.

If the thermistor temperature rises above the set point before the end of the timing cycle the heater shuts off at the end of the timing period. Otherwise the heater continues to stay on.

Thermistors at different values can be used as long as R3 + R4 = 2 * R5 holds true at the desired temperature.


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  • Tushar Ghosh

    Can’t we use a separate ckt to convert the AC to a low volatge regulated DC supply and then couple it to the given ckt?

  • tejas

    Can this circuit needed programing..or not
    And can time are adjustable

  • Emmanuel

    where can I connect the load (I mean, the heater whose temperature is to b controlled)?

  • brettegerton

    For this circuit, if I wanted to use a 230V (UK) supply voltage would I need to change any of the values for the resistors/capacitors/diodes etc..? If so would you be able to provide the values, many thanks, Brett

    • Allan

      No, you won’t need to alter any component values but just remember that 230 volts can be fatal! Never forget that 230 volts AC actually peaks at ~325 volts! The 230 volts figure is just an RMS [Root Mean Square] value. Even though diode D1 will only be conducting on every other half cycle of the 230 volt AC the 5W resistor [R1] will still get quite hot.

      Be sure to keep all of the high voltage components well separated and insulated from the low voltage components on the circuit board.

  • sargoth

    Does D1, R1, and the zener diode need to change for 120V?

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