Water boiling alarm circuit

The following introduces a dual-purpose alarm. In addition to the water boiling alarm, it also has the function of anti-theft alarm. The entire alarm only costs a few yuan and only requires a
dozen components
.     The circuit principle of the water boiling alarm is shown in the figure. In the figure, a complementary tube multivibrator is composed
of a PNP transistor VT1 and an NPN
transistor VT2 and resistors, capacitors, speakers
and other components
. This oscillator is similar to an electronic doorbell with a pleasant sound.
When the oscillator is required to output a larger alarm sound power, the transistor
connected to the speaker must be a tube that dissipates larger power and allows a larger collector current to pass through. These tubes are cheap and can be purchased in general electronics stores.