Circuit diagram of telephone line anti-theft alarm
The telephone alarm can monitor and prevent others from illegally embezzling your telephone line in real time. When someone makes a phone call on the outside line of your phone in parallel, the alarm will send out an interference signal, so that the pirate can't dial, and use the sound and light to alert the owner that someone is making a phone call. The following figure is the principle diagram of the anti-theft device. Normally, the post office program control machine sends a standby voltage of about 60V on the external line. The polarity is converted to the positive side after the polarity conversion of D1-D4. On, T2 cut off, T3 cut off, the oscillation circuit composed of T4, T5 does not work. When the telephone pick-up is stolen, the external line voltage drops to about 9V, which cannot cause D5 to break down, T1 is cut off because of no bias, and T2 base potential rises and turns on. The conduction of T2 causes T3 to conduct and drive the post-stage audio oscillation circuit to work. The LED and DY emit sound and light alarms, and the interference signal is added to the external line through the LED and DY. At the same time, the external line voltage is lowered, so that the phone thief cannot use it. Dial and force to hang up. Although the master lifter also lowers the voltage and cuts off T1, because T6 or T7 (T6 and T7 are used for easy installation, the incoming line can be connected regardless of polarity, D6 and D7 play an isolation role) through the phone The machine gets the base current and turns it on, making T3 unable to turn on. The oscillation circuit does not work. Therefore, the host can dial the call normally without being affected. Because only R8 and R9 with very low resistance are connected between the host's phone and the outside line, the vibration and zero conversation will not be affected when receiving the call.
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