The following is an excerpt:
The alarm adopts the wireless feedback alarm principle and consists of two parts: the first part consists of an anti-theft intrusion detector and a miniature wireless alarm transmitter; The second part is the wireless alarm receiving controller. When in use, the first part is installed in the storage room, garage and other places that need to be guarded; The second part is placed in the living room of residents.
Circuit principle
Intrusion detector and miniature alarm transmitter.
Figure 1 is the circuit schematic diagram of the intrusion detector and the miniature wireless alarm transmitter. A small magnet and a normally closed reed switch E form an intrusion detector. The small magnet is installed on the door leaf of the storage room, and the reed switch E is installed on the opposite door frame near the small magnet. Usually, the door is closed. Because the small magnet is close to E, and the two normally closed contacts in E are disconnected by external magnetic force, the micro transmitter does not work because there is no power supply. When cheating, the small magnet will lose the function of external magnetic field as the door leaves away from E, and the two internal contacts will be closed by their own elasticity, and the micro transmitter will send out coded alarm signals immediately after being electrified. IC 1(VD5026) is a digitally coded integrated circuit with eight address codes, namely A1~ A8; 4 data codes, namely D 1 ~ D4. (Editor's Note: For the introduction of VD5026/5027, please refer to the article "Codec Circuit and System Security" in the 35th issue of this year's Electronic News). The address coded data is output by IC 1 pin. The oscillation frequency of IC 1 is determined by the external resistance R 1. The smaller the resistance value of R 1, the higher the oscillation frequency. The resistance value of R 1 can be selected from120kΩ to 470k, but it should be noted that the resistance value of R 1 must be strictly consistent with the resistance value of the oscillating resistance R 15 (see Figure 2) of the alarm receiver decoder VD5027, otherwise it will not be able to decode reliably. Transistors V 1, C 1, C2, L 1, L2 and other components form a modulation and RF transmission circuit, and its transmission frequency is about 300MHz. In order to increase the stability of the transmission circuit, the antenna L2 can be directly printed on the circuit board.
Second, the wireless receiving alarm controller
Fig. 2 is a schematic circuit diagram of a wireless receiving alarm controller, which consists of a super regenerative receiving, amplifying, shaping and decoding circuit and an alarm signal generating circuit.
The 300MHz RF signal input from the antenna is sent to the transmitter of T 1 via C 1, amplified by T 1 at high frequency, and sent to the transmitter of super regenerative RF demodulator T2 via C4. The demodulated coded data pulse signal is sent to A 1 and A2 of operational amplifier IC 1 for amplification and shaping. Decoding integrated circuit VD5027 is the corresponding circuit of coding integrated circuit VD5026. In use, the address codes A 1 ~ A8 of VD5026 and VD5027 should be absolutely the same, and their status codes should also be the same. When the VD5026 stops sending signals (the transmitter is off), the pin VT of the VD5027 is reset to zero. In the circuit, one-way SCR is used for alarm holding. When the pin of VD5027 is at a high level, the thyristor is triggered to conduct, and the music piece 956 1 outputs an alarm signal due to power supply, which pushes the horn to sound, and the alarm indicator D2 lights up. After that, even if the foot of VD5027 is reset to zero, the alarm horn will sound until the alarm release switch K2 is pressed, because the SCR is turned on.
In order to prevent power failure and alarm leakage, the wireless alarm receiving controller adopts AC and DC power supply and can automatically switch. When there is an AC power supply, the rectified 9V power supply is added to the cathode of diode D5, and the diode is turned off, so the circuit works by the AC power supply. When the AC power supply is cut off, the negative terminal of D5 will be turned on due to the loss of 9V voltage, and the 9V battery will supply power to the circuit through D5, thus realizing the automatic switching of AC and DC power supplies.
Component selection
The small magnet and reed switch E in 1 can buy the finished normally closed door and window safety sensor in the market. It is required that the gap between the door leaf and the door frame should be small, and it should be firm after closing, otherwise it may cause false alarm when it is blown by the wind. L2 can be sealed with silver-plated wire or enameled wire ∮ 1.5mm, or printed directly on the circuit board. The encoder does not need an encoding switch, as long as one point is connected to the positive or negative pole of the power supply. 15V laminated battery power supply. The whole transmitting circuit can be made on a circuit board smaller than a matchbox.
In the alarm signal receiving and alarm control part in Figure 2, it is required that the beta values of T 1 and T2 should not be greater than 100. Inductor L 1 adopts 4.7μH color-coded inductor, L2 adopts ∮0.5mm enameled wire, which is wound around ∮4mm drill for 3 times. Antenna can be replaced by 30cm long cord, K2 is alarm release switch, and normally open contact button should be adopted. The battery is a 9V laminated battery. Except the power switch, alarm release switch, power indicator and alarm indicator are installed on the panel, other components of the whole circuit are installed on a circuit board.
Production debugging
According to Figure 1 and Figure 2, select components and solder them on the circuit board. Set the address code and status code of VD5026 and VD5027 to be consistent, and then start debugging.
The difficulty of debugging is that the transceiver frequency should be strictly consistent, and it is necessary to coordinate the debugging of alarm transmitting part and alarm receiving part. The initial installation and debugging, in order to ensure the success of debugging, it is best to cooperate with the oscilloscope. First, short-circuit the two ends of the reed switch in the transmitter to keep the transmitter in working condition and connect the power supply of the receiver. First, solder R 16, connect the probe of the oscilloscope to pin ⑦ of IC 1(LM358) in Figure 2, and put the transmitter close to the receiver (about 20cm apart). Adjust the fine-tuning capacitor C3 of the transmitter and C9 of the receiver with a non-inductive screwdriver until the oscilloscope. Then widen the distance between the transmitter and the receiver to make the distance between the transmitter and the receiver about 10m, and connect the probe of the oscilloscope to pin ② of IC 1(LM358) in Figure 2. Carefully adjust the fine-tuning capacitor C9 and inductor L2 of the receiving part until the waveform amplitude of the demodulation coded pulse signal displayed by the oscilloscope is the maximum. Then adjust the fine-tuning capacitor C 1 of the transmitter, and observe the waveform change of the oscilloscope while adjusting to maximize the waveform amplitude. Repeat several times to make it reach the best state. If everything is all right, connect the resistor R 16. At this time, the speaker should give an alarm sound, the alarm indicator light is on, and the transmitter power supply is disconnected. The alarm can still be maintained. When the alarm release switch is pressed, the alarm should be released. Finally, experiments can be carried out from a distance. Under normal debugging conditions, the alarm distance can reach more than 50 meters.
The alarm circuit is simple, and the cost of the whole machine is only tens of yuan. By changing the sensor at the transmitter and setting the address code and status code of VD5026/5027, the use of this alarm can be greatly expanded (such as multi-channel alarm and data transmission). ).