Design of home anti-theft alarm system based on single chip microcomputer

Based on the actual situation, a remote intelligent anti-theft alarm device for home use, connected to the telephone line, simple to operate, and stable and reliable in operation is designed. When people go out, they often hope to implement automatic monitoring and alarm to protect their family property from loss. In response to this demand, a series of automatic alarm systems have been developed, such as door magnetic, touch and infrared monitoring automatic alarm systems. The remote intelligent anti-theft alarm device to be introduced in this article can monitor multiple important points (such as doors, windows, etc.) at the same time, dial a number in time when theft is found, and can use ordinary telephone lines to make alarm signal calls. Its performance is stable and reliable, practical and applicable, and has high flexibility.

Basic working principle

As shown in Figure 1, the remote intelligent anti-theft alarm device consists of a signal detection circuit, a reset circuit, a telephone number input circuit, an 89C51 single-chip microcomputer, a voice circuit, an analog off-hook and on-hook circuit, a DTMF encoding sending circuit, a ring current signal detection circuit, and a busy signal detection circuit.

The DTMF coding sending circuit sends the telephone number that is pre-set by the user through the telephone number input circuit and stored in the CPU. The number can be set at will and can be a local program-controlled telephone (such as a work telephone or 110, etc.), a mobile phone or a pager, etc.

When the signal detection circuit detects that someone has broken in, it will send a trigger signal as an alarm signal, which will be amplified and sent to the CPU. The CPU will delay for 10 seconds. If the owner has entered, the reset circuit can be used to initialize the CPU within these 10 seconds, otherwise it will be ready to alarm. The CPU will first check the output of the ring current signal detection circuit. If the output is a high level, it means that the user's phone is ringing and cannot be picked up for dialing. Delay processing is required. Only when the output is a low level, the CPU will instruct the analog off-hook circuit to pick up the phone, send the DTMF phone number, and call the preset recipient. At this time, there will be two situations: the recipient is busy and the recipient is ringing. If the first situation occurs, the system will simulate hanging up, delay, and then pick up the phone to dial; if the recipient rings and picks up the phone, it will start sending the signal tone or voice signal recorded in the APR9600, and finally hang up.

Circuit composition and working principle

1 Signal detection circuit

The sensor in Figure 2 is a P2288 passive pyroelectric human infrared sensor. It works in a balanced differential detection mode and only senses the infrared radiation of the active human body with a wavelength of 7 to 14 μm. It will not be affected by the ambient temperature and visible light. The sensor can sense the infrared radiation of the human body within 10 meters with the addition of a Fresnel lens. After the infrared radiation of the active human body is detected by the sensor, the sensor will generate a weak electrical signal, which will be filtered, amplified, and sent to the bidirectional comparator. To prevent false operation, the signal will be compared with the threshold level, a pulse signal will be generated, and the high level will be sent to the CPU as an alarm signal.

2 Phone number input circuit

The DTMF encoder in Figure 3 is CSC5087, which can generate a set of dual-frequency signals according to different keys. CCITT (International Telegraph and Telephone Consultative Committee) stipulates the combination of keys and high and low frequency groups. For example, when the "8" key is pressed, the DTMF signal frequency is fL=852Hz and fH=1336Hz.

The DTMF decoder is SC8870, which translates each DTMF signal into a 4-bit binary code output. The 16 DTMF signals correspond to 16 binary codes from 0000 to 1111. For example, for the DTMF signal with fL=852Hz and fH=1336Hz sent by CSC5087, SC8870 decodes and outputs the code "1000", which corresponds to the key "8".

3 Voice Circuit

The voice circuit is used to record and play the voice signal or other alarm information that the user wants to alarm. The corresponding information can be preset by the user and can be modified at any time.

The voice circuit in this system uses the voice recording and playback chip APR9600. This chip is a new voice circuit that uses analog storage technology and has good sound quality, low noise, is not afraid of power failure, and can record and play repeatedly. The single-chip circuit can record and play for 32 to 60 seconds. APR9600 has a variety of manual control methods. The circuit is simple when multi-stage control is used. The sampling speed and recording and playback time are adjustable. Each single key has multiple functions such as start, stop and loop.

4 Ringing signal detection circuit

The ring current signal detection circuit is used to detect whether there is a 25Hz/90V ring current signal on the telephone line. As shown in Figure 4, the polarity protection circuit is composed of rectification, filtering and voltage stabilization circuits. The high-level signal output by the ring current signal after passing through this circuit is not a regular rectangular wave, but also needs to pass through a shaping circuit composed of two-stage Schmitt inverters G1 and G2. At this time, the high level output is sent to the CPU for delay processing, waiting for the ring current signal to disappear.

5 Busy signal detection circuit

As mentioned earlier, after dialing the DTMF telephone number, if the recipient is answering the call, a busy tone signal (a 450Hz audio signal with a period of 0.7s and a duty cycle of 50%) is sent back on the line. The CPU issues a simulated hang-up and delay instruction based on this signal.

As shown in Figure 5, the busy tone detection is composed of a polarity protection circuit and a NE567 phase-locked loop. The detection frequency of NE567 (I) is 450Hz, which is used to determine whether there is a 450Hz audio signal in the line. Because the busy tone signal is not a continuous audio signal, the output of NE567 (I) is not a continuous low level, but an oscillation signal with alternating high and low frequencies and a frequency of 1.429Hz. NE567 (II) is used to complete busy tone recognition, and the detection frequency is 1.429Hz. If the output signal frequency of the previous phase-locked loop is exactly 1.429Hz, the output is a low level, which is sent to the CPU as an indication signal of the existence of a busy tone to complete the relevant actions.