Practical telephone anti-theft alarm circuit

The phone anti-theft alarm introduced here can automatically identify whether someone else is using the phone to make a phone call or whether the owner is talking to the phone. There is no need to set a switch, which is convenient to use. When used, it can interfere with the phone thief and prevent the phone thief from making normal calls. Circuit Principle The practical telephone anti-theft alarm circuit is shown in the figure below (click on the image to enlarge). [url=../news_uploadimg/20041228231714219.GIF] When the telephone outside line is normal, there is a 48" 60V DC voltage on the telephone line. This voltage passes through VD1 and is conducted by the bidirectional trigger diode DV3, so the transistor VT2 is saturated and conducts, VT3 and VT4 Because the base has no bias and is cut off, the light-emitting diode LED does not emit light and an alarm occurs. When a parallel phone call occurs on the outside line, the phone line voltage drops to about 10V, and the bidirectional trigger diode VD3 turns off. After a delay of about 1s such as C2, VT2 is cut off, VT3 and VT4 are saturated and conductive, and the line voltage is forced to be clamped below 3V. The voltage regulator tube VD2 is cut off, and the discharge of capacitor C1 will keep VT3 and VT4 on. When the voltage of C1 drops to a certain value, the two tubes exit After saturation, the line voltage rebounds, causing VD2 to conduct again, and VT3 and VT4 to saturate again, thus forming an oscillation, which interferes with the phone thief and prevents him from making normal calls. At the same time, the LED also emits a red light alarm to tell the owner that the phone thief has hung up. , the bidirectional trigger diode VD3 conducts to cancel the alarm, and the circuit returns to the original monitoring state. When a call comes in, the negative half cycle of the ringing signal causes the circuit voltage to drop, because C2 maintains VT2 saturated conduction through VD4 and other discharges. , will not cause malfunction. When the owner picks up the phone to talk, the resistor R7 will receive a DC voltage drop, causing the transistor VT1 to saturate and conduct, and also block VT3 and VT4, so there will be no malfunction in the selection of components in this circuit. When making VT1 and VT2, 9011, 9013 and other silicon NPN transistors can be used, and the amplification factor is greater than 60; for VT3 and VT4, it is best to use 2N555 and other silicon NPN transistors, and the amplification factor is greater than 60. VD4 uses IN4004 rectifier tube, VD2 uses 3V, 1/ 2W type zener diode. VD3 uses a bidirectional trigger diode with a breakdown voltage of 26V. The other components are selected as shown in the figure (click on the picture to enlarge). This circuit only needs to be installed. Correct, it can work normally without debugging. The unit should be connected between the telephone line and the telephone. L1 (corresponding to pad 1) should be connected to the positive line of the telephone line, and L2 (corresponding to pad 2) should be connected to the negative electrode of the telephone line. If the wiring is reversed, the alarm will not work. Download the printed circuit board diagram: