Remote anti-theft alarm via telephone

When no one was at home and the thief pried open the door and window, the owner's cell phone suddenly rang thousands of miles away. After confirming the incoming call number, he knew that something was wrong at home and quickly asked for help from neighbors or property management.

Such alarm devices are expensive. The author introduces two simple and practical alarm circuits for your reference.

1. Reed switch direct trigger circuit

In general, a desktop phone can use the hands-free key and the redial key to complete a call without picking up the headset. Taking advantage of this condition, we lead these two sets of contacts from the printed board of the telephone and connect them to the moving plates of the two reed switches. Then install these two reed switches above the door frame. Arrange them in parallel at a certain distance, and install magnets at the corresponding positions of the door, so that when the door is pushed open, the reed switch K1 is attracted, which is equivalent to pressing the hands-free key; then continue to push the door. K1 is released and K2 is attracted, which is equivalent to pressing the reed dial key, and the telephone completes the call, so the set mobile phone rings immediately, in order to prevent the thief from being alarmed when dialing. A small switch should be connected in series in the speaker circuit of the telephone to cut off the sound after each redial is stored.

This circuit is low-cost, simple to make, and does not require an external power supply, but it is limited to the protection of doors and windows. Since four signal transmission lines are required to connect to the telephone, the distance cannot be too long. Otherwise, it is easy to cause interference and malfunction.

2. Break pulse trigger circuit

Still take out the hands-free contact and redial contact from the phone, and connect them in parallel with the ④ and ⑤ contacts of the photocoupler (see attached figure). When the ① and ② pins of the photocoupler are turned on, the resistance of the ④ and ⑤ pins decreases sharply, replacing the closing of the contacts.

The circuit analysis is as follows: At the moment of power supply, the A and B ends of the reed switch are closed; transistor T1 is turned on, and its negative collector potential turns off T2 through Cl (at this time, capacitor C2 is charged instantly, causing T4 to turn on for a short time, but it has no effect on the circuit). After entering standby mode, except for T1 conducting a small current (about 5mA).

The other components are in the cut-off state. When A and B are disconnected, T1 is suddenly cut off, the collector potential increases, and Cl is quickly charged. T2 is turned on for a short time, and the optocoupler G1 is turned on, which is equivalent to pressing the hands-free button of the telephone. At the same time, the second foot of the monostable circuit 555 is triggered by a negative pulse, and the third foot outputs a high voltage to turn on T3 for 2 seconds, T4 is cut off, and C2 completes the discharge. After 2 seconds, T3 is cut off again, the collector potential increases, and C2 charges to turn on T4 and the optocoupler G2. It is equivalent to pressing the redial button of the telephone and sending a call signal.

The characteristics of this circuit are: as long as there is a disconnection pulse trigger (any positive pulse is OK) can generate two short interval start signals, so at points A and B, you can either connect a reed switch and use the magnet's clutch as the signal source, or use a thin wire to surround the protective object. Using the disconnection pulse as the signal source, the resistance of the wire can be above 10kΩ.