Design of burglar alarm for valuable household appliances

This valuable home appliance anti-theft alarm has the characteristics of small size and loud sound. It is suitable for use in homes, hotels, offices, etc.
Working Principle

The circuit principle is shown in Figure 1 (click to download the schematic diagram). SCR, R1 and AN form a thyristor trigger switch circuit; IC1, R2, VT1, VT2 and BL form a simulated siren sound circuit. Usually, AN is oppressed by home appliances, causing its two normally closed contacts to be disconnected, SCR is blocked without a trigger signal, and the alarm does not work. When the home appliance is lifted, the two contacts of AN automatically close, the trigger end of SCR obtains a trigger signal from the positive pole of the power supply through R1, SCR is turned on, IC1 is powered on, and the siren sound electrical signal output from its output end is amplified by VT1 and VT2 power, driving the speaker to emit a loud alarm sound. At this time, even if the home appliance is put back to its original place or AN is destroyed, the alarm sound cannot be stopped. Only when the owner turns on the switch S installed with the alarm and difficult for others to find for a while, can the alarm be released.
Component selection and production
See the table below for the component list.

serial number	name	model	quantity
R1	resistance	4.7K	1
R2	resistance	240K	1
R3	resistance	10K	1
VT1	Transistor	3DG6, 9014, etc.	1
VT2	Transistor	3AX81, 9012, etc.	1
SCR	One-way thyristor	3CT2K, BT169 0.1A/50V-1A/400V	1
IC	Audio IC	KD-9561 or other similar products	1
AN	Micro switch	Normally closed	1
BL	speaker	8Ω	1
S	Power switch	Toggle switch	1
E	Battery Pack	4.5V	1

The alarm part can be installed in a small plastic box and placed in a place where the alarm sound can be easily heard (such as a duty room, etc.). Use a double-strand thin wire to lead the AN to the place where the home appliance is placed, and cleverly place the AN on a desktop, countertop or TV cabinet, etc., to ensure that the AN can operate correctly.