water boiling alarm circuit

Published:2011/5/9 9:56:00 Author:Christina | Keyword: water boiling, alarm circuit | From:SeekIC

This water boiling alarm circuit is composed of four transistors, the circuit is as shown. The audio oscillator is composed of the transistor T2, T3, the resistor R2 and the capacitor C, the audio signal is output by the speaker. The switch circuit is composed of the transistor T1, the resistor R1, W and the transistor be node Rt, and it can be used as the audio oscillator control switch.Rt is the temperature sensing element (T1's bias resistor). Usually Rt and T1 are in the reverse cutoff state; when the temperature rises, Rt's reverse resistance gets smaller, the leakage current increases, T1 has some bias to conduct, the audio oscillator starts working and the speaker sounds. In this figure, transistor T1's magnification needs to be more than 50, T2,T3's magnification needs to be more than 100. The temperature sensor Rt uses the metal shell germanium transistor's be node, such as the 3AX81.etc, also you can use the 2AP-type germanium diode.



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