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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Capacitive key for security device. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Security devices and object signaling Every radio amateur who has ever designed security devices for a summer house, garage, apartment or car wondered - which key to choose for this device? The same question arose before the author of the article. The simplest security devices are built with a time delay. This delay is given to the owner so that after opening the door he has time to turn off the device. Unfortunately, this solution is not applicable in all cases. If, for example, the alarm has a shock sensor, then after hitting the door of the security object, such a device will work only after a few seconds, which is unacceptable. Other simple disconnect devices common in amateur designs include reed switches, touch sensors, and infrared transmitters of uncoded IR radiation with a frequency of several kilohertz. But these methods also have obvious shortcomings. All these keys are universal and fit together. If, for example, I return home with a friend and he saw me putting my hand with a key fob to a certain place, then the secret is open, since there are few options. I have either a reed switch or a sensor there. And leaking information in this case can be costly. Based on the foregoing, when designing a security system, it is necessary to proceed from the fact that the key is difficult to repeat, like the key to the door lock, but at the same time compact and not laborious to manufacture. Specialized microcircuits (encoders and decoders) are not affordable for many, and not all cities can buy them. On the chips of the common K561 series, the key fob is large, which is not convenient. In addition, microcircuits require power, and the battery can fail at the most inopportune moment. In my opinion, the option of a key in the form of a resistor of a certain resistance is interesting. The dimensions are compact, the price is low, no power is required, the "decoder", made in the form of a bridge, is relatively simple. But the resistor is pretty easy to pick up using a variable. The key capacitor is also compact, cheap, does not require batteries, but it is more difficult to pick up, since large-capacity variable capacitors are rare, and for those from old radios that radio amateurs have, the upper limit of capacitance change is small, usually 360 ... 495 pF. The capacitance of even two KPI sections connected in parallel does not exceed 1000 pF. In addition, devices with a key in the form of a capacitor of a certain capacity are not described in the mass literature (at least the author does not know this), respectively, information about this method of disarming the device is still not widespread. A diagram of a security device with a key in the form of a capacitor of a certain capacity is shown in Fig. 1.
On the elements DD1.1 and DD1.2, a rectangular pulse generator is assembled. Elements DD1.3 and DD1.4 are a single vibrator that generates pulses of a reference duration. On element DD2.1, a comparison node is made, and on DD2.2 and DD2.3, a Schmitt trigger. Let's consider the operation of the device in more detail. Under normal standby conditions, capacitor C1 is absent. At the same time, at the output of the DD1.2 element, in the author's version of the device, the voltage is slightly less than half the supply voltage, i.e. log.0. It's related to that. that the DD1.1 element operates in a linear mode due to the presence of resistors R1 and R2 Depending on the microcircuit instance, the voltage at the output of the DD1.2 element can be anything. High-frequency generation may even occur due to the parasitic capacitance of the cable and the connector for connecting the capacitor. Let's analyze the different states in which the elements DD1 1 and DD1.2 can be. If the jacks for connecting the capacitor are closed, then the generator turns into a Schmitt trigger. At the output of the DD1.2 element, it can be like a log level. 0 and log. 1. In the steady state of log.1 and the absence of the C3R3 circuit, the comparison node can recognize this state as the "correct frequency", since the output of the one-shot, in the absence of pulses from the generator, will also be in the state of log.1. Chain C3R3 eliminates this possibility. When connected to the resistor sockets, the elements DD1.1 and DD1.2 also turn into a Schmitt trigger with a stable output state. When a capacitor of random capacity is connected to the sockets, the generator will start working and pulses will appear at the output of element DD1.2. They will start the one-shot, and the node on DD2 1 will compare them with the pulses generated by the one-shot. If the pulse durations from the generator and the single vibrator are not equal, at the output of the comparison node (the EXCLUSIVE OR element DD2.1) there will also be pulses that will charge the capacitor C1 through the VD7 diode to the log level. 1. Thus, in any state of the elements DD1.1 and DD1.2, in addition to generating the "desired" frequency, either a log will be present at the output of the comparison node. 1, or impulses. At pin 9 of the DD2.1 element, there are pulses with a duty cycle close to two, and at pin 8, the duty cycle varies depending on how close the frequencies are to each other. If the generator frequency becomes less or more than the nominal value, then positive pulses will appear at the output of the DD2.1 element, as shown in the waveform. These pulses will charge the capacitor C7 to the level of log.1, respectively, and log.1 will be formed at the output of the device. As the frequency of the generator increases, the frequency of the pulses at the output of DD2.1 will also increase, and when it decreases, it will decrease. The minimum frequency is limited by the C3R3 chain. Its time constant is chosen several times greater than the time constant of the C1R2 master oscillator circuit. However, it is not worth overestimating the ratings of the C3R3 elements, since there may be false positives of the key if log.1 is set at the output of DD1.2. Capacitors C1 and C4 are chosen the same for ease of calculation, then the resistances of resistors R5 and R2 should also be equal. Resistor R6 is needed to adjust the duration of the single vibrator pulses. The ratio of the resistances of the resistors R7 and R8 determines the maximum possible deviation of the capacitance of the capacitor C1 from the nominal value, since due to various destabilizing factors (changes in the supply voltage, temperature, humidity; displacement of the resistor R6 engine, differences in keys from each other, etc.) generator may deviate relative to the duration of the single vibrator pulse. Instead of a silicon diode, a low-power germanium diode can stand in place of VD1, then the need for resistor R7 will disappear, since capacitor C7 will be discharged by the reverse current of the diode. However, this will degrade the temperature stability of the device. In the absence of the K561LP2 chip, the comparison node and the Schmitt trigger can be performed on two K561LA7 chips. A diagram of such a device construction option is shown in Fig.2.
Here, four elements of the DD2 chip are included so that they form one EXCLUSIVE OR element. The inputs of two unused elements of the DD3 chip are connected to a common wire or "plus" of the power source. Capacitors and resistors installed in the timing circuits must be with a minimum TKE and TCR. Capacitors of the K31-11 series are best suited for this purpose. PM, K73-17, K73-11, K73-9 and resistors S2-14, MLT. If there were no such elements at hand, then the easiest way to determine which of the capacitors meet this requirement to a greater extent, and which to a lesser extent, is to heat the output of the element with a soldering iron and look at the screen of the oscilloscope connected to the output of the comparison unit, the duration of the difference pulse. Special requirements are placed on the capacitor C1, since its capacitance should change little with changes in temperature, humidity and other weather changes. In addition, if more than one key is planned to be used with an electronic lock, then the key capacitors should have a minimum variation in capacitance relative to each other. During testing, the author's version of the device showed high resistance to supply voltage instability. Its change from 7 to 15 V did not cause the appearance of pulses at the output of the comparison unit when the capacitor C1 was connected, however, it is still better to take power from a stabilized source. Structurally, the device is made in a small plastic box of suitable dimensions and is placed near the sockets for connecting capacitor C1. In the author's version, the sockets were a plug from headphones with a diameter of 3,5 mm. The wires from the board to the connector must be of a minimum length. Capacitor C1 (grade PM) is located in the metal case of the pin part of the connector. With a different design of the key, it must be taken into account that touching the terminals of the capacitor with your hands while it is connected is undesirable, since it can cause pickups and change the frequency of the generator. If the device is supposed to be operated in conditions where high humidity is possible, then it is better to cover the printed circuit board with a protective varnish after assembly and adjustment. Setting up the device comes down to setting the resistor R6 to a one-shot pulse of such duration that when the capacitor C1 is connected, there are no pulses at the output of the comparison unit. If it is impossible to use an oscilloscope at the installation site, then this operation can be performed using a multimeter, adjusting the resistor to the minimum reading at the output of the DD2.1 element (see Fig. 1) or DD2.4 (see Fig. 2). You can also choose the resistor R7 to set the maximum tolerance for the deviation of the capacitance of the capacitor C1 from the nominal. The output of the device in the author's version is connected to an integrating circuit with a time constant of 100 ms. This is desirable, because in an unfavorable situation, negative pulses of short duration may be present at the output. For example, an attacker installed a capacitor with a similar value of 1 pF in place of capacitor C3300. In this case, the capacitor C7 will be charged to a voltage slightly more than half the supply voltage. Log.1 will be stored at the output of the device. If we now close the capacitor C1, then the trigger on the elements DD1.1 and DD1.2 can switch to a log.1 state and during the discharge time of the C3R3 circuit, the output of the comparison node will be log.0, which can have time to discharge the capacitor C7 to a voltage less than half the supply voltage, and the Schmitt trigger on the elements DD2.4, DD2.1 will go into the state of log.0. After discharging the capacitor C3, the capacitor C7 will be charged again with pulses or a constant voltage to the level of log.1 and log.1 will also be set at the output of the device. If it is required to turn off the alarm with a positive pulse, then the signal can be removed from the output of the DD2.2 element (see Fig. 1) or the output of the DD3.1 element (see Fig. 2). The device makes it possible to put into practice the generation frequency from hundreds of kilohertz to tens of hertz with a corresponding change in the ratings of passive elements. The author assembled three devices according to the scheme of Fig. 1 and one - according to the scheme of Fig. 2. All of them worked at once and required only adjustment by resistor R6. The key to one device did not trigger the others. Author: V.Sidorov, Kirovo-Chepetsk, Kirov region
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