Water alarm

Circuit Overview:

When we boil water, we often forget to turn off the fire after the water boils or even dries up, which is a waste of resources and unsafe. Here we introduce a water-opening alarm, which is very suitable for home use.

Working principle:

The circuit is shown in the figure below. Transistors BG2, BG3, resistor R2 and capacitor C2 form an audio oscillator. Changing the capacity of capacitor C2 can change the pitch of the oscillator. Transistor BG1, resistor R1, diode D and others form an electronic switch circuit to control the operation of the oscillation circuit. Diode D is led out from the plug as a temperature sensing element, and is also equivalent to the bias resistor of BG1. At room temperature, diode D is reverse biased and cut off, making transistor BG1 cut off. When the temperature rises, the reverse resistance of D becomes smaller and the leakage current increases. When the temperature rises to a certain level, transistor BG1 reaches a certain bias voltage and turns on. The power is added to the audio oscillation circuit through transistor BG1, the oscillator starts to vibrate, and the speaker sounds an alarm. Therefore, as long as you try to put D in the spout of the kettle and adjust the value of resistor R1, when the kettle water boils, BG1 turns on, the circuit starts to vibrate, and the speaker sounds an alarm.

Component selection and production:

Transistors BG1-BG3 can be general-purpose tubes. Diode D can be IN4148. When installing, place D in a copper tube of suitable length and thickness with one end closed, and pay attention to the insulation of both. The pins are connected to the plug through the lead wire.
After the assembly is correct, you can start debugging. First, short-circuit the E and C poles of transistor BG1, adjust the value of resistor R2 to make the audio oscillator vibrate, then insert the assembled sensor head (D) into boiling water, adjust the value of resistor R1, make transistor BG1 conduct, the audio oscillation circuit work, and the speaker sound.