Abnormal temperature reminder

The device described below can emit light and sound reminders when the temperature is abnormal, adjust the temperature or check the constant temperature equipment.

1. Working Principle

The circuit is shown in the figure. Two NAND gates IC1a and IC1b are connected to form an oscillator with a period of 1s. Due to the addition of diode D1, an asymmetric rectangular wave with a duty cycle of 1:9 (100ms:900ms) is generated.

The output of the oscillator is in each 1s cycle. The first 100ms is the temperature sampling time. In the last 900ms of a cycle, the circuit is in rest time to save battery.

The circuit is powered by a 1.5V battery. The light-emitting diode LED3 is used for voltage monitoring and indication. When the battery voltage is lower than 1.2V, the LED starts to flash to remind you to replace the battery. Even when the battery voltage drops and the supply voltage changes, the temperature information voltage from the voltage divider circuit and the threshold level of the gate circuit will also change proportionally. The operating temperature point will hardly change.

The output pulse of the oscillator is added to the voltage divider circuit. The temperature sensor and resistors R1 and R2 form a two-stage voltage divider circuit. The voltage divider output contains a voltage of 100ms temperature information. The first stage voltage divider output is directly added to the input of the IC1c gate circuit. When the temperature information voltage exceeds the threshold value of the gate circuit, the output of the gate circuit flashes the red LED2 through the boost circuit, indicating that the temperature has changed. At the same time, another gate circuit IC2a ​​inverts the input voltage and turns off the white LED1 through the boost circuit, indicating that the temperature has changed.

The temperature detection circuit can also realize the fire alarm function. IC2c and IC2b form an oscillator. The temperature information voltage is divided by the second stage and added to the control input of the oscillator. When the threshold value is reached, an alarm sound with a period of 1s is output. The piezoelectric speaker is driven by the crystal transistor T1.

In this design, the temperature set for the first stage partial pressure is 25°C, and the second stage partial pressure is set 21°C higher than 25°C, which is 46°C.

The temperature sensor uses a negative temperature coefficient thermistor 103AT, and the corresponding values ​​of its resistance and temperature are shown in the attached table. The analog information of the temperature sensor is converted into a digital signal using the threshold characteristics of the gate circuit, and the LED uses the instantaneous change state of the gate circuit from low level to high level to indicate the change in temperature.

2. Circuit Adjustment

Repeatedly adjust the resistance of the 20kΩ and 10kΩ potentiometers to ensure the temperature detection accuracy.

1. Slide switch S2

●Position “1”: The position that is turned on during operation.

●Position "2": Alarm working detection position. In this position, an alarm sound is emitted, indicating that the alarm function is normal.

●Position 3: Potentiometer adjustment position. First adjust the 20kΩ potentiometer, and use a digital multimeter to measure the resistance between points A and B to be 10kΩ. This is equal to the resistance of 103AT at 25℃. Then set switch S1 to position "6". Adjust the 10k potentiometer so that LED2 just changes from off to on. Pay attention to adjust to this turning point.

2. Slide switch S1

●Position “5”: Temperature drop monitoring. LED1 lights up.

●Position “5”: Neutral. Used in seasons when temperature control is not required.

●Position “6”: Temperature rise monitoring. LED2 lights up.