Using D flip-flop to make electronic temperature controller

The article "Replacing Mechanical Thermostat with Electronic Thermostat" in the 47th issue of "Electronics News" in 2002 introduced an external thermostat device, which is not actually a direct temperature measurement and then regulation, but an adjustable timer switch, which indirectly adjusts the temperature by adjusting the power supply and power outage. The circuit uses three integrated circuits, five transistors and dozens of other components, and the production workload is large and the cost is high.

Now we introduce a device of this type made using a D flip-flop, which is simple and economical to make and has good effects.

Regarding the principle of using the D flip-flop to generate oscillations, readers can refer to the article "Square Wave Generator Composed of D Flip-Flops" in the 17th issue of "Electronics News" in 2004. The composition of this circuit is very similar to that of this circuit, and it has the original characteristics, and it is also an expansion of the application of the D flip-flop.

1. The principle circuit of the main controller
is shown in Figure 1 (the power supply part is not drawn, and the method used may vary from person to person). Through the adjustment of two potentiometers, according to the state change conditions of the reset terminal R1 and the set terminal S1 of the D flip-flop CD4013, the working time of the output terminal Q1 at the low level and high level is controlled. When Ql outputs a high level, the red indicator light is on, and the driving transistor VT is turned on, the relay is closed, and the socket obtains AC power. The change of the Ql switch signal enables the electrical appliance to be automatically adjusted.

　　According to the circuit composition, it can be seen that the charging circuit of RPl to C1 determines the time for the reset terminal R1 to rise (that is, the time for Q1 to remain at a high level), thereby adjusting the power supply working time.

　　The RP2 charging circuit for C2 affects the state of S1, and determines when Q1 jumps to a high level, that is, the time to maintain the power supply. The two time adjustments in the circuit are independent and effective, and do not affect each other. The capacitor discharges quickly through the diode. When the DC working power supply of the D trigger is +12V, and the two capacitors Cl and C2 are both 220μF, after testing, the adjustment effect of the RP potentiometer is shown in the attached table.

　　If all capacitors are 100μF, the timing time will be reduced to half of the time in the table.

　　2. Delayed power supply protection circuit
    In order to avoid damage to the compressor caused by power failure and then power supply, delayed power supply should be added. At the same time, the other half of the dual D flip-flop CD4013 can be fully utilized. The circuit is shown in Figure 2.

　　When the DC is powered on, the S2 set terminal is high level, making Q2=1, forcing the R1 terminal of the main control circuit to be high level, Q1 is reset and the control output is 0, the VT transistor is cut off, and C4 is gradually charged through R4. When the reset R2 terminal is 1, Q2=0, and the main control circuit oscillates according to its own rules. The size of R4 changes the time when the main control circuit starts to enter the oscillation operation, that is, the purpose of delay is achieved. According to the parameters of R4 and CA in the circuit diagram, the delay time is about 5 minutes, which is the same as the data in the table.

　　3. Implementation of strong cooling
    If the refrigerator needs to be powered on for a longer time, such as quick-frozen dumplings, there is a gear with a "strong cooling" function on some mechanical thermostats. For this device, a switch K can be added. When it is closed, it forces the setting S1 to be high. At this time, the charging and discharging process of the main controller is stopped, and the output Q1 is kept at a high level for a long time, and the socket is continuously powered. Disconnect K, and the control circuit works as usual. Manual operation enters strong cooling, which will not be affected by the delayed power supply protection part.

　　4. Installation matters
   The two potentiometers, RP1 for working adjustment and RP2 for power failure adjustment, are preferably logarithmic, which is convenient for distinguishing the adjustment of high resistance areas and making the "time scale" of a longer time more accurate and detailed. In order to facilitate different usage situations, a resistor of appropriate size can be connected in series in the RP branch as the starting resistance value, which will also expand the range of adjustment time. The potentiometer, indicator light, and switch K are all installed outside the equipment for easy observation and adjustment operation. The capacity of the capacitor does not need to be very accurate, but the leakage current is required to be small. The working power supply of this circuit is mainly determined by the parameters of the relay. The CMOS integrated circuit can work with a power supply of 3 to 15V, and the control time effect is somewhat different.

　　5. Advantages of this device
   (1) Mechanical thermostats often require temperature compensation (e.g., to operate the electric heater) when the ambient temperature is low in winter, otherwise it is difficult to start the compressor. The time control method introduced here does not have this phenomenon and will not cause additional useless heat consumption. (2) Temperature adjustment is controlled externally by the working time and power outage time. There is no need to open the refrigerator door to change. (3) After selecting the appropriate timing parameters, it can be directly used for various other timing controls, such as lighting, heat preservation, colored lights, etc.