Home thermostat

The household thermostat controller introduced in this article has a temperature adjustment range of 30-60℃ and can be used for home-made yogurt, flour fermentation and other insulation.

How it works

The thermostat controller circuit consists of a temperature sensor, a differential amplifier, a rectangular wave generator, a pulse width modulator, an execution and power supply circuit, as shown in Figure 1.

The temperature sensor uses two silicon switching diodes VD1 and VD2 connected in series. VD1 and VD2 form a temperature measurement bridge with resistors R1, R3, R4, etc. The comparison amplifier D2 inside the quad operational amplifier integrated circuit IC1 is used as a differential amplifier, D1 is used as a buffer isolation amplifier, D3 is used as a rectangular wave generator (also called a triangle wave forming circuit), and D4 is used as a pulse width modulator.

After turning on the power switch SA, the AC 220V voltage is stepped down by R10, rectified by VD3, filtered by C3 and stabilized by VD4 to generate a +12V voltage as the working voltage of IC1. The +12V voltage is also stabilized by IC2 into a +5V voltage to provide the working voltage for the temperature measuring bridge. The thyristor VS serves as the executive component to control the operation of the heater EH.

The rectangular wave oscillator composed of IC1's internal D3 and its peripheral components generates a rectangular wave signal, which is output from IC1's 8th pin, processed into a triangular wave signal by resistor R9 and capacitor C2, and then added from IC1's 12th pin (D4's positive input terminal). The output signal of the temperature measuring bridge is amplified by IC1's internal D2, output from IC1's 7th pin, and then added to IC1's 13th pin (D4's negative input terminal).

Pin 14 of IC1 (output end of pulse modulator) outputs rectangular wave pulse voltage, and its pulse width is modulated by the output voltage of pin 7. When the voltage of pin 13 of IC1 is lower than the voltage of pin 12, pin 14 of IC1 outputs high level, turning on thyristor VS, energizing heater EH, and lighting up light-emitting diode VD5, indicating that the thermostat is in heating state. When the voltage of pin 13 of IC1 is higher than the voltage of pin 12, pin 14 of IC1 outputs low level, turning off VS, stopping heater EH, and turning off VD5.

The thermostat box adopts a periodic switch control method, with a fixed switch cycle, and controls the temperature by controlling the switch time ratio. When the temperature in the box is higher than the control value, the pulse width of the rectangular wave output by the 8th pin of IC1 decreases, and the "on" time of the heater EH (that is, the power-on working time) decreases accordingly, and the "off" time (power-off time) becomes longer, causing the temperature to drop; conversely, the working process is the opposite, the EH "on" time becomes longer, and the "off" time becomes shorter, causing the temperature to rise; because the on-off cycle is only about 30s, and the change in the on-off time ratio is a gradual process, the temperature control accuracy is high and the temperature fluctuation is small.

       Component Selection

IC1 (including D1-D4) uses LM324 integrated quad operational amplifier circuit; IC2 uses 78L05 or LM7805 small three-terminal integrated voltage regulator. VD1 and VD2 use 1N4148 silicon switching diode; VD3 uses 1N4007 silicon rectifier diode; VD4 uses 12V, 0.5W silicon voltage regulator diode, such as 2CW60-12V or 1N5242, 1N5242B, 1N6002, 2CW5242, etc.; VD5 uses red light-emitting diode. VS uses 2A, 400V thyristor, such as CR2AM or CSM2B, etc.

RP1 and RP2 are both linear solid core potentiometers. R1-R9, R11 and R12 are all RTX-1/4W carbon film resistors; R10 is a 5W wirewound resistor. C1-C3 are all CD11-16 electrolytic capacitors. EH uses a 300W ordinary electric iron heating core (since VS is a thyristor, half-wave AC passes through the heater EH, so the heating power is only half of the nominal power of the heater).

Production and debugging

All electronic components are installed on a self-made printed circuit board, and then placed in a thermostatic control box of suitable size. The power switch SA and light-emitting diode are fixed on the front panel of the box. As long as the components are in good condition and the wiring is correct, the system can work normally when powered on.