Practical overheating alarm circuit

The overheat alarm is a very useful alarm instrument that will sound an alarm in case of overheating. The overheat alarm sets its alarm point at approximately 150°C, if necessary. The temperature of the alarm point can be changed according to the working range of the alarm. You can imagine adding a small relay to replace the alarm to control other electrical appliances.

The core part of the overheat alarm is the thermistor T1 shown in the figure below. The thermistor is a temperature-sensitive resistor. Its resistance increases as the temperature decreases. The thermistor exhibits a resistance of approximately 17.89kΩ at 150°C and 77°C at 77°C. 100kΩ resistor.

In the figure below, the model number of U1.A and U1.B of the circuit is CD4013B. CDCD4013B is a CMOS double D flip-flop. U1:A and peripheral devices form a monostable multi-frequency oscillator, which can generate a set of periodic 5s pulse output at pin 1 of U1:A. The frequency of the oscillator depends on the values ​​of R1 and C2. The oscillator pulse is transmitted to U1:R through the field effect transistor Q1. Note that when the temperature on the thermistor is low, the pulse generated by the oscillator lags behind the clock edge, and when the temperature outside the thermistor T1 is high. The pulse generated by the oscillator will lead the edge of the clock pulse, which causes the output of U1:B to change. When the temperature changes and reaches the set initial voltage value, a signal is output at the Q terminal of U1:B to drive ZVNI. 110A type field effect transistor Q20. This circuit is powered by a 3V, 120rnA/h lithium battery. It can be used continuously for a long time when the circuit is working normally.

The resistors used in the circuit are all 0.25W, and the accuracy type is 5%.

Except for C1, which is an electrolytic capacitor, the others are polyethylene capacitors. It is recommended to use an integrated circuit socket when installing CDCD4013B, so that components can be easily replaced if they are damaged in the future. When installing the chip, pay attention to the order of the pins. Except for the thermistor T1, all components are soldered to the circuit board. See the table below for a list of other components.

After building the circuit board, be sure to check whether there are any loose solders on the circuit board and whether the solder pads are short-circuited. The overheating alarm is mounted in a box with an aluminum chassis. The switch potentiometer R4 and the buzzer are installed in the front of the upper part of the box. The circuit board is mounted on a bracket and placed on the bottom of the aluminum box together with a 3V lithium battery holder.

If necessary, the overheat alarm can be used for higher or lower temperature alarms by simply changing the thermistor alarm point. When choosing a different thermistor, the alarm must be calibrated. To ensure that the designed alarm point can be reached.

Next, prepare a shielded wire. One end of the shielded wire is connected to the wire that controls the timing, and the other end is connected to the thermistor. Depending on the purpose, the thermistor can be packaged in an old metal pen. Pass a shielded wire through one end of the pen. The thermistor wires are generally very small diameter wires, so you must be careful when soldering the shielded wire to the thermistor. Put a heat shrink tube on the connection between the thermistor and the shielded wire.

Once the thermistor is packaged. It is now time to test and calibrate the overheat alarm. Connect the thermistor to the terminal strips on the chassis box package. Make sure switch S1 is closed, then insert the 3V lithium battery. Now you are ready to calibrate the overheat alarm.

In order to test the circuit, the thermistor needs to be heated to 150~C to make the buzzer sound. One way is to put a calibrated ordinary thermometer and the packaged thermistor together. Then place this assembly next to the gas stove or electric stove, about 1cm away from the stove. Turn on the stove.

Once the combination reaches 150~C, the buzzer will sound, indicating that the overheat alarm has been calibrated. If you wish you can experiment with different uses and at different temperature points.