Car pager with microphone. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Car pager with microphone. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Automobile. Security devices and alarms

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A simple and convenient radio autoguard can be used to monitor the car within the line of sight - about 150 m. Its advantage is that it does not require the installation of contact sensors, but is able to respond to any mechanical impact on the car.

The car pager operates in the VHF FM broadcasting range at frequencies of 88-108 MHz and provides reliable communication between the receiver located on the ninth floor of the house and the car standing in the yard. The radio autoguard responds to any mechanical impact on the protected object, including blows, rattles, tool touches, etc.

The main technical characteristics of the pager:

Time to switch to security mode, s.......25
Alarm duration, s.......15
Radio transmitter power, mW.......20
Range of the pager, m.......150
Broadcasting VHF FM range, MHz....... 88-108
Car pager dimensions, mm.......45x80

The circuit diagram of the device is shown in fig. 1. A condenser microphone M1 with a built-in amplifier (MKE-3, M1-B "Sosna", etc.) is used here as a sensitive sensor. From its output, the electrical signal through the sensitivity regulator R2 is fed to a two-stage ULF, assembled on transistors VT1 and VT2. From the output of the amplifier, the signal is fed to a diode rectifier on diodes VD1 and VD2 and to a transistor switch VT3. In standby mode, on the VT3 collector - log level. "1", and in the presence of an acoustic signal - log. "0" (negative pulse).

Fig.1 (click to enlarge)

The logical device of the pager is made on DD1 and DD2 microcircuits of the K176LE5 type. A negative pulse from the collector VT3 is fed to the shaper, assembled on the elements DD1.1 and DD1.2. From the output of the shaper (pin 3) a negative pulse is fed to the element DD1.3. The function of the element is to create a delay (25 s) for the device to switch to armed mode after turning on the power with the SA1 toggle switch. This delay is needed to close the car doors. When the power is turned on, the capacitor C7 is charged through the resistor R9, the high-level voltage at which blocks the passage of a negative pulse through the element DD1.3. After charging the capacitor C7, when the microphone sensor M1 is triggered, a positive pulse appears at pin 11 of the DD1.3 element, which is fed to the input of the one-shot (pin 12) on the elements DD2.1 and DD2.2.

At the output of the single vibrator (pin 10), a positive pulse with a duration of 15 s is formed. This pulse through the resistor R11 is fed to the transistor key VT4, VT5, which connects the relay winding K1 to the power source, and an alarm sounds. The same pulse through the VD3 diode allows the operation of the multivibrator on the elements DD2.3 and DD2.4, the signal of which is fed to the VD4 varicap, modulating the frequency of the radio transmitter. At the same time, a positive pulse through the resistor R1S opens the VT7 transistor switch, which supplies power to a single-stage VHF FM transmitter on the VT6 transistor. The transmitter is a capacitive three-point master oscillator and provides an output power of about 20 mW. If the receiver described above is used as a receiver on a specialized micro-assembly KXA058, then stable communication can be provided within the line of sight at a distance of at least 150 m.

To prevent the autoguard from looping, the level of the log. "1" from the output of the odivibrator (pin 10) on the elements DD2.1 and DD2.2 through the resistor R8 enters the capacitor C6, which charges and closes the element DD1.1, preventing the passage of a negative pulse through it from the collector of the transistor VT3.

After a time interval of 15 s, determined by the parameters of the circuit C8, R10, and as the capacitor Sb is discharged through the resistor R8, the circuit switches to armed mode.

When installing the device on a car, it is necessary to fix the M1 microphone housing to the metal surface of the car body with a clamp. In the atom case, the microphone will respond to all touches and blows on the car body. To turn off the radio guard, you need to bring a permanent magnet to the reed switch SA2, which will close the base of the transistor VT4 to the ground and turn off the sound alarm. For this, a reed switch with normally open contacts of the KEM1 type is used, which is installed on the inside of the windshield.

Transistors KT315 can be replaced with KT312, KT342 and KT3102, and KG829 - with KT819 or KT805. The device uses fixed resistors of the type MLT-0,125, RI2 - type MLT-0,5. Capacitors C6, C7 and C8 must be low leakage current, type K53-4 or K50-35. Coil L1 frameless with a diameter of 8 mm contains 6 turns of PEV wire 0,8 mm.

When setting up the watchman, the transmitter is tuned to a free section of the VHF FM band by compressing or stretching the turns of the coil L1 and adjusting the capacitor C10. Time delays in the passage of signals, if desired, can be adjusted by selecting the resistances of resistors R8, R9 and R10.

The printed circuit board of the pager is made of double-sided fiberglass and has a size of 40x75 mm. The printed circuit board of the automobile pager is shown in Figure 2. Installation is carried out in the usual way. Under the transistor VT5 type KT829, it is recommended to place a small aluminum plate (according to the size of the transistor housing) and fasten the board, plate and transistor with an M3 bolt and nut. The location of the parts on the device board is shown in Fig.3.

Ris.2,3

The M1 microphone is connected to the board with three wires twisted together. As an antenna, you can use a piece of wire or a knitting needle 30-45 cm long. It is desirable that the length of the antenna be equal to 1/4 or 5/8 of the wavelength of the transmitter.

See other articles Section Automobile. Security devices and alarms.

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