Tropical fish tank water temperature automatic control circuit

Working principle of the circuit: After rectification by diodes VD2~VD5 and filtering by capacitor C2, a voltage of about 12V is provided to the control part of the circuit. The 555 time base circuit is connected as a monostable trigger, and the transient state is 11s.

Assume the control temperature is 25ºC, adjust the potentiometer RP so that RP + Rt = 2R1, and Rt is the thermistor with a negative temperature coefficient. When the temperature is lower than 25ºC, the Rt resistance increases, pin 2 of the 555 time base circuit is low level, and pin 3 changes from low level to

The output becomes a high level output, the relay K is turned on, the contacts are closed, and the heating tube begins to heat. When the temperature returns to 25ºC, the Rt resistance becomes smaller, and pin 2 of the 555 time base circuit is at a high level, and pin 3 is at a high level. Output low level, relay K loses power, contact

Disconnect and heating stops.