Water boiling alarm circuit design

 Boil water on the kitchen gas stove. Once the water boils, if you don't turn off the flame in time, the boiling water will overflow and put out the flames. Gas leakage is very unsafe. This problem can be solved by using the water boiling alarm.

Working principle of the circuit : This circuit uses a thermistor as the temperature sensing element. When the water temperature increases, the resistance of the thermistor decreases and the potential of point A increases. When the potential of point A is higher than the IC-1 inverter conversion When the voltage is high, IC-1 will output a low level and IC-2 will output a high level. Make the audio oscillator composed of IC-3 and IC-4 work, and the piezoelectric ceramic sheet produces sound. When IC-2 outputs low level, the audio oscillator composed of IC-3 and IC-4 does not work, and the piezoelectric ceramic piece is silent. The IC uses a C066 two-input four-NAND gate with an operating voltage of 3V to 18V. In this circuit, the power supply is 3V to 6V; the RT thermistor has a resistance of about 1kΩ; the piezoelectric ceramic piece has a diameter of 27mm; the resistor has an ordinary 1/8 or 1/4W metal film resistors.

Production and debugging method : Find two scrap fluorescent lamp starter shells, use iron sheets as clamps, stick the tops of the two starters together, and tighten them with screws. One of the starters can be placed on the mouth of the kettle to obtain the temperature of the water. The two pins of the thermistor are welded to the cover of the other starter and installed into the shell. Note that the thermistor must be close to the inner shell wall to facilitate heat transfer. Solder the outer leads of the thermistor and the temperature sensor is ready. After all the components are welded and checked, you can turn on the power for debugging, put the temperature sensor on the kettle spout, and when the water boils, adjust the RP so that the piezoelectric ceramic piece makes the right sound. Repeat debugging a few times before it can be officially used. If you want to change the sound frequency, you can change the capacity of C2. If you feel that the sound is not enough, you can connect an external triode to the output of IC-4 to amplify the sound effect.