Pager for protection. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Security devices and object signaling

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Vehicle security is a very urgent problem, despite the large number of anti-theft devices on the market. The operation of the sound alarm on the car does not give the owner practically any advantages compared to cars without an alarm: the surrounding people usually do not react to the howling of the siren, and the owner is far enough away. The way out is to use a radio channel and transmit an alarm signal to the owner without unnecessary noise. The advantage of this signaling method is that the hijacker is unaware of the transmitter in the car, and it is possible to find the stolen car using a directional antenna. To receive a signal from the security system, you can use a converted pager, which, with the ubiquity of "mobile phones", is increasingly turning into a toy lying idle.

A frequency of 26945 kHz has been allocated for the protection of cars. But in order to be able to recognize a specific transmitter, it is necessary to encode the radio signal. Chips used in this design: MC145026 - encoder and MC145028 - decoder. They allow you to form 19683 different combinations using only one operating frequency of the internal oscillator of the microcircuit. When the generator frequency is changed, the number of code combinations increases.

The pager is a receiver with a pulse sequence decoder, on which the code inherent in your car is set by jumpers, and a sound signaling device that turns on when this code matches the one received from the transmitter. The transmitter in the car is activated by the rocking sensor. It transmits a frequency modulated pulse train. When the sensor is triggered, the transmitter turns on for a few seconds. If the "impact" on the car stops, the transmitter turns off.

The transmitter circuit is shown in Fig.1. A swing sensor is assembled on the DD1 chip and the PA1 microammeter. When changing the position of the body, and hence the microammeter, negative pulses appear at the output of the comparator, setting the RS-trigger on the elements DD2.3, DD2.4 to a state in which pin 10 DD2.3 is high. It opens transistors VT5 and VT6. Through VT5, power is supplied to the transmitter, and it turns on. The voltage of the logical "0" from pin 11 DD2.4 is supplied to the enable input of the encoder DD4, as well as to the input R of the counter DD3. Prior to this, the counter was constantly reset to zero logical "1" at the input R. Now he counts the pulses from the generator to DD2.1, DD2.2. When "6" appears at pin 3 of DD1, the transistor VT1 opens and returns the RS flip-flop and the counter to its original (standby) state.

Fig.1. Schematic diagram of the transmitter (click to enlarge)

If the impact on the sensor has stopped by this time, the system remains in this state for an arbitrarily long time, and if not, then the RS-trigger is again switched by pulses from the output of the comparator DD1, and the transmitter will work again.

Capacitor C4 is necessary for the initial reset of the counter and the transfer of the RS flip-flop to standby mode. Code packets from the encoder DD4 are sent to the frequency modulator of the transmitter on the elements VD1, L1, L2, VT2, R12 ... R16, C7, C8, and then to the RF amplifier on VT3, VT4, R17 ... R19, C9 ... C20, L3...L8.

The receiver circuit is shown in Fig.2. Its high-frequency part is similar to that described in [3]. The AGC circuit is not needed in this circuit, therefore the amplifier of the DD1 microcircuit operates in the comparator mode, the operating point of which is set by the tuning resistor R1 to minimize high-frequency noise. From the output of DD1, the signal is fed to the logic level driver on transistors VT2 and VT3. The code sequence is decoded by the DD2 chip, and if the code packets match, a logical "11" appears at pin 2 of DD1. This level starts the generator on the DD3 chip, and an alarm sounds.

Code combinations are set by changing the levels at the address inputs DD2. The encoder and decoder microcircuits perceive three states: logical "0" and "1" and an unconnected address input. The addresses must be set identically in both the encoder and decoder, and the internal oscillators must be set to the same frequency.

Setting up an alarm system begins with the transmitter. The engine of the resistor R4 (Fig. 1) is set to a position in which the output 9 of the DD1 comparator is high, but with a light tap on the microammeter, negative pulses appear at the DD1 output. Further, by disconnecting terminal 12 DD15 from the resistor R4, the AF generator is connected to it. By changing the inductance of the coils, they achieve maximum UHF amplification.

Then, the operating point of the receiver chip DD1 is set with the resistor R1 (Fig. 2) and the receiver circuit is tuned with a sweep frequency generator [3]. To check the correct decoding of the code, the output 15 DD4 of the transmitter is connected to the input 9 DD2 of the receiver, having previously disconnected it from the logic level driver (VT3). During normal operation of the alarm, the activation of the rocking sensor causes the appearance of a logical "11" at the output 2 DD1 and sound in the piezo emitter B1. Next, all connections are restored and the receiver is debugged together with the transmitter, receiving the signal via the radio channel.

Fig.2. Schematic diagram of the receiver (click to enlarge)

The device uses electrolytic capacitors of the K50-35 type, non-polar - KM. TKE capacitors C5 (transmitter), C15, C16, C17 (receiver) should be minimal, you can use K73-17. Resistors - type MLT. The microammeter type M476 of the swing sensor is being slightly modified. A weight is fixed on the arrow so that when the scale of the device is lowered down, the arrow is in its center.

The winding data of the transmitter coils are given in Table 1, the receiver - in Table 2.

Table 2. Winding data of the receiver coils

The printed circuit board of the transmitter is made of double-sided foil fiberglass with dimensions of 64x94 mm. Its drawing is shown in Fig.3. The receiver board with dimensions of 59x60 mm is shown in Fig. 4. From the side of the parts, the holes are countersunk, except for the places where the parts are connected to a common wire, in these places the parts are soldered on both sides.

Fig.3. Transmitter circuit board

Fig.4. Receiver circuit board

Literature

1. V. Bruskin. Foreign microcircuits of communication radio receivers. - Radio amateur, 1999, N1 S. 14.
2. V. Zhigachev, A. Paremsky. Encoding and decoding devices based on LSI from MOTOROLA. - Radio amateur 1994, N6, p.62.
3. G. Minakov, M. Fedotov, D. Travinov. Radio station "Hummingbird". - Radio, 1999, N1, p.59.

Author: S. Abramov, Orenburg, asmoren@mail.ru; Publication: cxem.net

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