Over-temperature and under-temperature alarm

This is an over-temperature and cooling alarm composed of CD4011 four-NAND gate integrated circuit, which is used to monitor the temperature of the constant temperature box. When the constant temperature circuit fails or the power is off, and the temperature in the box is higher or lower than the allowed range, it can sound an alarm to remind people to take corresponding measures. The circuit has a resolution of 0.1℃.
Working principle The circuit is shown in Figure 1 (click to download the schematic diagram). Diodes VD1 and VD2 are both placed in the constant temperature box. When the temperature in the box is lower than the lower limit, the potential at point a in the figure is greater than the threshold voltage U at the input end of gate A, and the output end of gate A is low, so that gate B outputs a high level. At this time, the multivibrator composed of gates C and D starts to oscillate, and the piezoelectric piece emits an alarm sound; when the temperature in the box is higher than the upper limit, the potential at point b is less than the threshold voltage U of gate B, which can also make gate B output a high level, and the piezoelectric piece YD emits an alarm sound. Only when the temperature in the box is between the upper and lower limits, YD will not make a sound. The lower and upper limits can be changed respectively by adjusting the values ​​of RP1 and RP2. The power supply circuit is an AC/DC dual-purpose power supply. E is the backup power supply. When the AC power supply fails, VD3 is turned on and the backup power supply supplies power to the circuit. Since this circuit consumes very little power, the backup power supply can use a stacked battery to reduce the volume. Component selection and production The component list is shown in the table below.

serial number	name	model	quantity
R1, 2	resistance	1K	2
R3, 4	resistance	120K	2
R5	resistance	3K	1
R6	resistance	680Ω	1
RP1, RP2	Potentiometer	470K Linear	2
C1, 2	Polyester capacitor	0.22u	2
C3	Polyester capacitor	1700P	1
C4	Electrolytic Capacitors	200u	1
C5	Electrolytic Capacitors	470u	1
VD1,2	Detector diode	2AP9	2
VD3,5	Rectifier diode	IN4001	2
VD4	Zener diode	2CW55 6V	1
IC	Four NAND gate integrated circuit	CD4011	1
YD	Piezo Buzzer	HTD-27A or HTD-35A	1
T	Power Transformer	12V 2-5W	1
E	Battery Pack	6V	1

When the wires connected to sensors VD1 and VD2 are too long, it is best to use shielded wires to enhance anti-interference capabilities.
Finally, take the chick hatching temperature monitoring as an example to explain the adjustment method: first, stabilize the temperature of the incubator at 36.5°C, move the sliding arm of RP2 to the bottom, and then adjust the lower limit adjustment potentiometer RP1 to the critical point where YD sounds or does not sound; then stabilize the temperature of the incubator at 39.5°C, and adjust the upper limit adjustment potentiometer RP2 to the critical point where YD sounds or does not sound. In this way, when the temperature of the incubator is lower than 36.5°C or higher than 39.5°C, an alarm will sound, and the oscillator can be officially used.