Kettle automatic alarm circuit - PTC Thermistor
July 30, 2021
When a household gas stove boils water, the boiling water often overflows to extinguish the flame, causing gas leakage, which is a waste of energy and can cause potential serious accidents. The automatic kettle alarm made by PTC thermistor can effectively prevent accidents.
1 Principle circuit
The automatic alarm circuit of the kettle is composed of a NAND gate integrated circuit block and a PTC thermistor as the core. The schematic diagram is shown in Figure 2.8.1. Before the water temperature reaches the preset value, the resistance of RT is small, and the input of IC If the voltage is lower than the threshold voltage, B will not alarm. When the water temperature rises to the boiling point, the RT resistance value increases rapidly and exceeds the threshold voltage. About 30 seconds later, the IC outputs an audio signal, and B emits a water boiling alarm sound.
2 Selection of main components
The RT of the automatic alarm circuit of the kettle uses PTC thermistor as the temperature probe. The resistance at room temperature is ≤500Ω, and the standard resistance-temperature characteristic curve is shown in Figure 2. The buzzer B uses Ф27mm.
3 Installation and commissioning
When in use, insert the temperature sensing end of the water temperature probe into the mouth of the kettle, and install the alarm in a place where it is easy to hear the alarm sound, and pay attention to moisture.