Working principle: This automatic temperature control circuit for water temperature includes a step-down rectifier circuit, a humidity control timing control circuit, and a thyristor control circuit. When the temperature of the cylinder water is below 25 degrees, it automatically heats up for 10 seconds to maintain the water temperature at around+25-30 degrees. Its circuit is shown in the figure.

The step-down rectifier circuit consists of a step-down transformer T, rectifier diodes D1, D2, etc., and a voltage stabilizing circuit composed of R4, DW (2CW57), which stabilizes a voltage of+9V.

IC1 (555) and RP1, RT, R1, R2, C1 form a temperature controlled single temperature state timing control circuit. RT is a temperature sensitive element, which adopts Negative temperature coefficient (NTC) glass sealed thermistor; 555 operates in monostable mode. Normally (temperature t<25 degrees Celsius) 555 is in a reset state, which means that the 3-pin output is low. When the water temperature in the cylinder drops, the resistance of the temperature sensitive resistor RT in the water increases. Adjust RP1 to make RP1+RT=2R1 when t=25 degrees Celsius. When the water temperature is slightly below 25 degrees Celsius, there is RP1+RT>2R1, making the trigger level of 555 lower than the trigger flip level by 1/3Vdd (3V). Set 555 trigger, and the 3-pin output turns to high potential. LED1 emits light, and at the same time, trigger the thyristor to conduct, The resistance wire RL is electrified and heated, and the heating time of RL depends on the transient duration of the 555 monostable circuit, which is td=1.1R2C1

The parameters shown in the diagram are designed in 10 seconds. When the temporary stabilization time is up, the output of 555 turns to low level, LED1 goes out, SCR turns off, RL powers off, and heating stops. When the water temperature drops again to 25 degrees Celsius after a period of time, there is another temporary stabilization and heating process, and the water temperature in the cylinder is kept above 25 degrees Celsius by being powered back and forth.

In the selection of components, the Negative temperature coefficient (NTC) resistor should be selected for the temperature sensitive resistor RT, and it is better to use the glass sealed structure to prevent electric leakage. The WSW organic solid core Potentiometer can be selected for the Potentiometer RP1; It is best to use ready-made specialized heating devices for fish tanks, and the power should not exceed 30W; Thyristors can choose 1A directional thyristors with a voltage resistance of no less than 400V, such as 3CTS1A-D type; Transformer T selects a 5VA step-down transformer with a central tap (10V) of the secondary coil.