Contactless key. Encyclopedia of radio electronics and electrical engineering

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

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Unlike a conventional mechanical key or an electronic one (magnetic card), this one does not need to be taken out and inserted into the lock. When approaching the door at 0,5 m of a person who has this device with him, the door lock with an electric drive (electromagnet) will automatically open.

The key itself has dimensions slightly larger than a matchbox (70x54x17 mm) and can be easily placed in any pocket.

Rice. 2.11 Transmitter layout

In the simplest version, the system consists of a miniature transmitter (key) and a receiver tuned to its frequency, as well as an actuator with a power source.

The device can also find other uses, such as disarming. Without knowing the location of the receiver, such a key cannot be used.

The transmitter circuit, Figure 2.11, is assembled on a single transistor operating in microcurrent mode. Inductance L1 and capacitors C2, C3 ensure the operation of the oscillator at a frequency of about 200 kHz. Four battery tablets of type D-0,115 were taken for power. The current consumed by the transmitter does not exceed 1,6 mA, and one battery charge is enough for continuous operation of the circuit for three days

Fig. 2.12. Charger

Rice. 2.13. Receiver (click to enlarge)

As a charger, the simplest circuit with transformerless power supply from a 220 V mains can be used, Fig. 212. In this case, it is not necessary to open the key case - the connection to the charger is made through a miniature connector X1. The HL1 LED is mounted on the charger case and serves as an indicator of the presence of voltage, and the VD2 zener diode limits the output voltage. It may take 4-10 hours to fully charge the batteries.

The receiver circuit is shown in fig. 2.13. The signal induced in coil L1 is amplified by transistors VT1 VT3. Signal detection is performed by the transistor VT4 (active detector). On VT5 and VT6 (in diode connection), the operating point of the amplification stages is stabilized. Two resonant circuits (L1-C1-C2 and L2-C8) are tuned to the transmitter frequency using ferrite cores. This ensures narrow-band amplification of the receiver and operation (appearance of zero voltage on the collector of transistor VT7) only from the transmitter with a certain frequency.

A variant of the power supply and actuator circuit with the inclusion of the YA1 electromagnet is shown in fig. 2.14. Capacitor C1 provides a delay in the operation of the transistor VT2 in the presence of random noise at the output of the receiver.

Rice. 2.14. Executive device

Rice. 2. 15. Topology of the transmitter circuit board

Design features. Fixed resistors can be of any type, electrolytic polar capacitors are of the K50-16 type, the rest of the capacitors are of the K10-17 (KM-4) type.

Rice. 2.16. PCB Topology and Receiver Element Location

The device circuits are mounted on fiberglass 1,5 mm thick. The topology of the printed circuit board for the transmitter and receiver is shown in fig. 2.15 and 2.16.

In the transmitter, the mounting of radio elements is located on the side of the printed conductors. The batteries are pressed against the board with a plate of foil fiberglass (not shown). Microswitch S1 type PD9-2 can be replaced with a button that will have to be pressed when approaching the receiver

For the manufacture of coils L1 (receiver and transmitter), a ferrite rod of brand 400NN (or 600NN) with a diameter of 8 mm was taken from any magnetic antenna of a household receiver (usually a piece 20 ... 30 mm long is enough for tuning). The coils are wound with PELSHO wire with a diameter of 0,06 (0,08) mm and contain 300 turns - at the receiver, and 200 turns - at the transmitter (on a paper frame 45 mm long). After winding, the turns are fixed with glue of the "Moment" type. For the manufacture of the L2 coil in the receiver, a frame and ferrite cups were taken from the intermediate frequency circuits of miniature radio receivers, fig. 2.17. The winding contains 1 - 160 turns, 2 - 200 turns with PEV-2 wire with a diameter of 0,08 mm. Windings are desirable to be carried out separately in the compartments, while winding 1 is wound in the upper section.

Rice. 2.17. Receiver L2 Coil Design

When installing the receiver, it is better to place it at the level of the key position, in addition, to obtain the maximum detection range, the location of the transmitter and receiver coils matters - they must have the same directivity patterns, which happens in the case of a similar location: horizontally or perpendicular to the ground.

In conclusion, it can be noted that in order to increase the secrecy of the key, it is easy to introduce modulation of the frequency of the oscillator with a code packet or a lower frequency (pulse modulation), which slightly complicates the circuit, but makes it difficult to fake the key for a person familiar with the principle of operation of this system. Transmitter modulation will also increase transmitter uptime without recharging the batteries. As a modulator, it is convenient to use the oscillator circuit shown in Fig. 1.40. In the receiver in this case, after the detector, a simple filter is installed, tuned to the modulation frequency.

Publication: cxem.net

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