Infrared "business card" receiver with decoder. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / infrared technology

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Schematic diagram of the IR receiver "visiting card" is shown in fig. 40. Here DA1 is a microcircuit that converts the current pulse that occurs in the VDI photodiode under the influence of an IR flash into a voltage pulse whose amplitude is sufficient to directly control CMOS microcircuits (Fig. 41, a). On the elements DD1.1 and DD1.2, a single vibrator is assembled that converts a short pulse corresponding to the duration of the IR flash * into a pulse with a duration of tf = 50 μs (tf @ 1/2 tp, where tp is the repetition period of IR flashes in the code package (Fig. 41b)). On the elements DD1.3, DD2.3-DD2.5, a device is assembled that generates a pulse at the input R of the counter DD3 (Fig. 41, d), with which it is transferred to the zero state along the front of the first IR flash, and the time interval Tpr (Fig. 41, c), within which the DD3 counter can freely count the pulses (according to their decline) arriving at its input C.

Rice. 40. IR "business card" receiver (click to enlarge)

Rice. 41. Plots of signals in the IR receiver

The decoding of the code message, finding out whether it contains N code - the code number of pulses, is assigned to the decoder D1. As an example demonstrating its structure, Fig. 42, and the configuration D1 for Ncode = 284 is shown. Since the "weight" of the Qi output in DD3 is 2^(i-1), then in binary notation Ncode=000100011100 (2^(3-1)+ +2^(4-1)+2^(5-1)+ 2(9-1)=4+8+16+256=284). The decoder consists of a 4-input connector** (Rl, VD3-VD5, VD9), the inputs of which are connected to all Qi=1, and an 8-input disjunctor (R2, VD1, VD2, VD6-VD8, VD10, VD12), inputs which are connected to all Qj=0.

It is easy to see that a high-level voltage (log.1) will arise and remain at the output of DD1.4 (see Fig. 41, e) only if N code is fixed in the counter DD3, in any other it will be reduced one way or another to zero. On fig. 42, b shows the configuration of the conjunctor in the encoder of the IR emitter, which generates Ncode = 284; its other position at the outputs of the counter is due to the fact that the "weight" of the output Qi here is equal to 2^(i-5).

Rice. 42. Decoder for Ncode=284 (click to enlarge)

The decoder D1 will have a similar structure for another N code, with other, of course, positions of the diodes in the conjuncter and disjunctor.

In order for the system to respond to the N code only with a sufficiently long exposure, the chain R9 C11@texp. Usually take texp = 0,3...3 s. Such a system simply will not respond to a short-term appearance of an N code (in an attempt, for example, to quickly select a code).

The output of the device - an open collector transistor VT1 - can be supplemented by one or another actuator. For example, a tone generator (Fig. 43, a), warning about the arrival of "one's own", or an electronic key that controls the electromagnetic lock Y1 (Fig. 43, b).

Rice. 43. Tone generator (a) and electronic key to the electromechanical lock (b) (click to enlarge)

In a properly assembled IR receiver, it may be necessary to reduce its sensitivity. This can be done both electrically - by shunting, for example, the input of the DA1 amplifier with resistor R12 (shown in dashed line in Fig. 40), and optically - by covering the photodiode with a "gray" filter, which can be used, for example, as plastic wallpaper, which at the same time performs the function filter, almost completely "cutting off" the visible part of the spectrum of spurious illumination.

Experience has shown that the radiation of the IR generator is capable of "breaking through" even 1,5 ... 2-mm plastic. In addition, a large protective overlay, which does not allow visually determining the position of the photodiode, will be another obstacle to unauthorized entry into the system.

Protection against code selection is the main concern of the designers of such devices. The coding system adopted here is relatively simple: Ncode is just one number out of a thousand possible. But code selection is complicated here and by a number of other circumstances. Note that the duration of the code message Tcode cannot be either too small (otherwise the pulses at the input C of the counter DD3 will "stick together"), or too large, more than Tpr (the next IR flash is transformed into an R-pulse, returning DD3 to its original state). Complicates the selection of code and t^, in any case, it slows it down a lot. Protective functions are incorporated even in the brightness of the IR flash - it should only be sufficient. Increased illumination of the photodiode can bring the receiver's photo head out of operation and. lead to errors in the account;

And all this, we note, in the absence of any special countermeasures, which, of course, are not difficult to foresee here. It is possible to introduce one more (or more than one) photosensor located to the side, the illumination of which will immediately block the system. Or a lock that reacts to too many attempts made. More than four in a minute, for example. Locks can, of course, complement each other.

On fig. 44 shows the IR receiver circuit board. It is made of double-sided foil fiberglass with a thickness of 1.5 ... 2 mm. The foil on the side of the parts is used only as a zero bus-"ground" (a "-" power source is connected to it), in places where the conductors pass, it has etched circles with a diameter of 1.5 ... 2 mm (not shown in the figure). Null-foil connections to "ground" leads are shown as solid squares.

Rice. 44. Printed circuit board IR receiver "business card"

The photo amplifier (VD1, DA1, etc.) with its high sensitivity, broadband, high input impedance must be shielded. Otherwise, electrical interference, including from the operation of its own decoder, can make the IR receiver completely inoperable. The screen, which has a "window" for the photodiode, is made of tin in the form of a box and soldered to the null foil at two or three points. On fig. 44 the dashed line shows its approximate location.

Table 9

Upit, V	Ipot, mA
4,2	0,9
5,0	1,1
6,0	1,3
7,0	1,6
8.0	1,9
9,0	2,3

It is also recommended to take measures to minimize the illumination of the photodiode by extraneous light sources, since this can significantly reduce the sensitivity of the receiver to the signals from its IR generator. As a hood limiting the side illumination of the photodiode, you can take a segment of a plastic or metal tube blackened inside with a diameter of 10 ... 15 mm.

The photosensitive part of the receiver can be made in the form of a separate head, connected to its other elements with a thin three-wire cable ("+", "-", pin 10 DA1). The small dimensions of such a photo head will allow it to be installed in the cutout of a door "peephole", behind a masking plate in the thickness of the door, in a door frame, etc.

The IR receiver remains operational in a wide range of supply voltages. The dependence of the current consumed by it Icon on the supply voltage Upit is shown in Table 9.

*) Recall that the duration of the pulse at the output of the photo amplifier DA1 depends not only on the duration of the IR flash, but also on its brightness - the illumination of the photodiode. The reason is the relatively slow recovery of its dark conductivity.

**) Physical elements that implement conjunction and disjunction - this is how logical functions AND and OR are usually called in works on mathematical logic. If we intend to continue to use the results of mathematical research and do not intend to repeat them (which, by the way, would be quite difficult), then we must at least understand their language.

Publication: cxem.net

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