Infrared receiver "electronic password" with a decoder. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / infrared technology

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A schematic diagram of a device that receives a coded IR signal from a keychain generator is shown in fig. 49.

The DA1 microcircuit, which is a photoamplifier, converts the current pulses in the BL1 photodiode, excited by the IR flashes of the emitter, into voltage pulses suitable for direct injection into a digital analyzer. On fig. 50, a shows the pulse sequence at the output of the photo amplifier, corresponding to the code 111011100111001, which we will consider here and below as an example.

The receiver has two generators. One of them, made on the elements DD1.1 and DD3.1, expands each of the incoming pulses (recall: their duration is about 10 μs) to tf1@R3 C5@0,6...0,8 ms (the condition must be met:

0,5 tsign

And the second, assembled on the elements DD1.2 and DD3.3, forms a pulse with a duration tf2@R4 C6=30...50 ms On the front of this pulse, a short pulse (tr @R5 C7@10 μs), which sets the shift register DD4-DD5 and the counter DD6 to zero (Fig. 50, e).

Elements DD1.3, DD1.4, R7, ZQ1 form a master oscillator operating at the frequency of the ZQ1 quartz resonator - 32768 Hz (recall, the master oscillator of the IR emitter operates at the same frequency).

The received signal (or interference) is fixed in the shift register DD4-DD5 as follows. By the front of the first IR flash, all the storage elements of the device are switched

Rice. 49. IR receiver "electronic password" (click to enlarge)

Rice. 50. Diagrams of signals for the code "111011100111001"

to the zero state (zeros are set at the outputs of the microcircuits DD4-DD6) and the counter DD6 starts counting. After about 0,5 ms (tzn / 2), zero at the output 2 ^ 4 (pin 5) of the counter DD6 will be replaced by one. In the shift register K561IR2, a change in the voltage at input C of the type J leads to the displacement of the number stored in it by one digit towards the older ones (down in Fig. 49), and the value that at that moment will be on its input D (pin 4). It can be 7 - extended to tf1 "single" pulse, and 1 - if there was no IR flash in this familiarity of the code message. The next shift of the number will occur in tzn=0 ms - a "step", which will be preserved in the future.

The system will make only 16 bit shifts (shift pulses generated by the counter DD6 are shown in Fig. 50, c) - with the appearance of one at the output 29 of the counter DD6 and, accordingly, zero at the input DD2.2 (pin 9), the counter will self-lock and will remain in this state until the next start of the system.

Thus, the received sequence of IR flashes is converted into a number stored in the register DD4-DD5. It remains to be seen whether it is code.

This is carried out by a diode-resistor decoder D1, the circuit of which (for the same code 111011100111001) is illustrated in Fig. 51. The idea of ​​decoding is simple. All register outputs, which, in accordance with the code combination, should be one, are connected to the inputs of the diode-resistor connector (VD1, VD4-VD6, VD9-VD11, VD13-VD15, R1), and the outputs, which should

Rice. 51. Decoder for the code "111011100111001"

be zero, - to the inputs of the diode-resistor disjunctor (VD2, VD3, VD7, VD8, VD12, R2). If a number-code is fixed in the register, then a high level voltage will be set at the output of the conjunctor - 1, and a low level voltage will be set at the output of the disjunctor - 0. And only in this case, a signal 1 will appear at the output of the IR receiver. This "single" state will last until then , until the SB1 "Reset" button is pressed (several buttons of the same purpose can be turned on in parallel with it) or any extraneous signal * passes through the channel.

The receiver is mounted on a board measuring 83x54 mm, made of double-sided foil fiberglass 1,5 mm thick (Fig. 52). The manufacturing technology of the board and the methods of mounting parts on it are the same as in the manufacture of the key fob-generator board.

When mounting the receiver, special attention should be paid to the electrical shielding of the photohead (BL1, DA1, etc.): having high sensitivity and significant broadband, it is subject to the action of electrical signals of various origins. The screen can be made of tin, its cutting is shown in fig. 53: bend the box along the dashed lines, solder it in the corners, align the bottom and set it as shown by the dash-dotted line in fig. 52 are soldered at two or three points to the null foil. If necessary, the amplification of the photohead can be reduced by shunting the input of the DA1 microcircuit with a resistor R1 = 0,3 ... XNUMX MΩ.

Rice. 52. Printed circuit board IR receiver "electronic password"

All resistors in the receiver are of the MLT-0,125 type. Dimensions of capacitors C4 and C10 - Ж8x12 mm. Capacitor C2 - type K53-30, the rest - KM-6, K10-176 and KD. Quartz resonator dimensions - Ж2x6 mm.

A place is allocated on the board to accommodate the elements of the sound generator described above (see Fig. 43, a). On fig. 52 it is outlined in dash-dotted lines.

It is necessary to take measures to attenuate the illumination of the photodiode by extraneous light sources that can significantly

Rice. 53. Photo-amplifier screen pattern

reduce the sensitivity of the photodetector. The photodiode can be placed in a "well" glued together from black polystyrene, which will protect it from the effects of sources located away from the optical axis. In addition, the invisible "corridor" formed, within which only optical contact of the receiver with the emitter key fob will be possible, will increase the already considerable difficulties of information "hacking" of the system.

It is useful to seal the photodiode window with a film that attenuates mainly visible light. Moreover, the distance at which the receiver is able to detect and decode the IR flashes of the remote control, in not too bad conditions, exceeds 10 m. In which, most often, there is no need.

A variety of signaling devices can be connected to the receiver output (pin 12 of the DD3.6 element). For example, the LED shown in Fig. 49 with a dashed outline, or a sound generator, announcing the appearance of "one's own". But if, at the signal of the receiver, the security system must make a decision on its own (turn on, for example, the electric lock drive), then a delay must be entered into the electronic key that controls the actuator (IM). So, for example, as shown in Fig. 54. The delay in turning on the IM depends here on the time constant R2C1 and can be several tenths of a second. The duration of the delay will increase even more if a diode VD1 is included in the emitter circuit of the transistor VT3, designed for the operating current of the IM, for example, KD213A. The supply voltage of the IM, taking into account the extra voltages that occur when it is turned off (the VD2 diode is mandatory for inductive loads), should not exceed the maximum allowable for the VT1 transistor (60 V - for KT972A, 45 V - for KT972B). The operating current of the actuator must not exceed 2 A here.

Rice. 54. Electronic key with delay

Actuator delay is an effective tool in combating attempts to guess the code involved in the system. In the coding system adopted here, even a second delay will force an attacker to stand at someone else's door for more than one hour. And this - if he has the appropriate equipment, knowledge of the principles of coding and time-pulse characteristics of IR radiation. It is incomparably more difficult to "spy" on the operation of an IR key fob - generator, without entering into visual contact with its owner, than code generators operating in the radio range allow.

The receiver remains operational when the supply voltage drops to 4 V, the current consumed by it does not exceed 1,4 mA.

*) Note that the decoder does not care about the state of the high-order digit of register DD5, since after the end of writing to its pin. 2 will necessarily be 1 - the start bit of the code combination or the first bit of interference.

Publication: cxem.net

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