Vibration anti-theft alarm circuit

How it works

The piezoelectric ceramic piece B1 is used as a vibration sensor, which is fixed close to the back of the door near the door lock. The integrated circuit IC, the power amplifier transistor VT2, and the speaker B2 form an audio alarm circuit, and the switch tube VT1 provides a positive pulse trigger signal to the IC.

Normally, the transistors VT1 and VT2 are both in the cut-off state, the IC does not work, the speaker B2 is silent, and the static current of the entire circuit is only about 3μA. Once the thief pries the lock and opens the door, it will cause the piezoelectric ceramic piece B1 fixed on the back of the door to vibrate. Bl outputs a weak electrical signal to turn on the originally cut-off VT1, and the battery GB1 provides a positive pulse signal to the trigger end of the IC through VT1, and the IC is triggered to work. After the IC works, its output end outputs the internally stored "ding-dong-" sound electrical signal, which is amplified by VT2 power and drives B2 to emit a loud alarm sound. Every time the IC is triggered, B2 will emit three "ding-dong-" sounds in succession, for about 4 seconds.

In the circuit, VT1 is not set with a bias circuit for two purposes: first, by using the characteristic that VT1 conduction requires B1 to provide a forward voltage greater than 0.65V, the circuit only responds to strong vibrations caused by picking the lock, but not to slight vibrations caused by other external interference, thereby improving the accuracy of the alarm; second, it greatly reduces the static current of the circuit and greatly extends the battery life. Generally, each time a new dry battery is replaced, it can work for more than one year. Battery GB1 provides a suitable 3V operating voltage for IC, and GB2 is mainly connected in series with GB1 to increase the operating voltage of VT2 to 6V, which significantly increases the sound of B2.

Component Selection

IC uses KD-153H "Ding Dong" doorbell special integrated circuit. The integrated circuit is encapsulated in a small 24mm x 12mm printed circuit board with black paste, and has holes for inserting and soldering peripheral components, which is very convenient to install and use. The main parameters of KD-153H are: working voltage range 1.3V~5V, typical value 3V, trigger current not more than 40μA; when the working voltage is 1.5V, the measured output current is not less than 2mA, and the static total current is less than 0.5μA; the working temperature range is -10℃~60℃. KD-153H can also be directly replaced by the chip with built-in "Ding-Dong-" sound in the HFC1500 series integrated circuit. Transistor VT1 uses 9015 or 9012, 3CG21 type silicon PNP triode, and the current amplification factor β>50 is required; VT2 uses 9013 or 3DG12, 3DK4. Type silicon NPN medium power triode, and the current amplification factor β>100 is required.

BJ uses ordinary HTD27A-1 or FT-27 piezoelectric ceramics. Other models can also be used, but the diameter should be as large as possible to improve the trigger sensitivity of the alarm. B2 can use 8Ω/0.25W small-caliber dynamic speakers. GB1 and GB2 are composed of two No. 5 dry batteries connected in series (with a plastic battery rack).

Production and use

Since the alarm uses very few components, there is no need to make a separate circuit board. When welding, solder the transistors VT1 and VT2 directly to the chip substrate of the integrated circuit IC as shown in the figure on the previous page, and then put it together with the batteries G1 and G2 into an insulating small box with a volume of about 70mm×60mm×20mm. When welding, special attention should be paid to the fact that the outer shell of the soldering iron must be well grounded to prevent the AC induced voltage from breaking through the CMOS circuit inside the IC! The piezoelectric ceramic piece Bl is led out of the box with a double-strand wire about 20cm long; the speaker B2 is also led out of the box with a double-strand soft plastic wire (the length depends on the distance between the building and the storage room). B2 can be put into a plastic soap box, and a sound hole is opened in the panel in advance to make a beautiful small speaker.

In actual application, as shown in the right figure, fix the piezoelectric ceramic plate B1 with three 1cm long wood screws (or glue it with strong glue) to the door lock part close to the back of the storage room door. Note that the metal substrate surface should be flat against the door, and the alarm circuit box should also be fixed on the back of the door; the speaker box is led to the upstairs room through a double-strand plastic-skinned wire. In this way, once a thief pries the lock and opens the door, the speaker B2 will emit a loud "ding-dong-" alarm sound. If the test requires a lot of force to trigger the circuit by knocking on the door panel with your hand, you can swap the two wiring heads of B1, and the circuit trigger sensitivity will definitely increase a lot.