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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Security device with key-resistor. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Safety and security Recently, the demand for various kinds of security systems has increased. This article describes a device that uses a resistor of a certain value as a key. The device can be used to protect premises. In the described device, an analog "key" is used - a resistor. When a resistor with a given resistance is connected to the "lock" contacts, the security mode is disabled. If the door is opened without such a "key" by an intruder, the device immediately gives an alarm. It should be noted that the analog "key" has some disadvantages. For example, at high humidity, when moisture may appear on the elements of the block, an alarm may be triggered when using your "key". This disadvantage, however, will not allow an attacker to enter the protected premises unnoticed. A dynamic head is used as a sound emitter of the siren. The device is powered by a GB1 battery. When the supply voltage drops below the permissible level, a buzzer sounds. The diagram of the device is shown in the figure. Immediately before leaving the premises, the owner must set the SA1 toggle switch to the "Security" position. The device switches to armed mode after 25 s. Before entering the room, it is necessary to insert the mating part of the connector - the key into sockets X1, X2 and remove it no earlier than after 2 s. After that, there is another 20 s to enter and switch SA1 to the "Off" position. If you open the door without inserting the "key", the siren will immediately turn on. To turn it off early, you must move SA1 to the "Off" position and press the SB1 button.
A feature of the system is the need to hold the "key" in the socket for at least 2 s. Due to this, its resistance cannot be selected by simply rotating the variable resistor. This is explained by the fact that the identification interval of the "key" by the system is within 6...7 kOhm. When using a variable resistor, for example 100 kOhm, it must be rotated at a speed of 0,5 kOhm / s in order for the system to recognize the "key". In this case, the entire resistor will scroll in 200 s, while only 20 s are allocated to enter the room with the "key" and turn off the device. Block A1 - electronic lock. Operational amplifiers (op amps) DA1.1 and DA1.2 are included in the voltage comparator circuit. Chip DD1 is used to send an alarm to block A2. The voltage divider on resistors R4-R6 sets the voltage of 3 and 6 V at pins 1 and 4,4 of the DA3,5 chip, respectively. If the "key" is not inserted (resistor R1 is off), the divider R2R3 provides a voltage of 2 V at pins 5 and 5,3. With this inclusion of the op-amp, if the voltage at the non-inverting input is greater than at the inverting one, then the output voltage will be close to the voltage supply, if, on the contrary, the output voltage is close to zero. In the armed mode (no resistor R1) at the output of the op-amp DA1.1 - 9 V, and at the output DA1.2 - 0. As a result, there is a high level on the resistor R7. Diodes VD3 and VD4 decouple the outputs of the op amp DA1.1 and DA1.2. Capacitor C1 is needed to protect against interference on pins 2 and 5, as they are connected to the input jack. The resistance of the resistor R1 is chosen so that when it is connected to the lock, the voltage across the resistor R3 is in the range of 3,5 ... 4,4 V. In this case, the voltage at the terminals of both op-amps will be near zero. The DD1 chip is four identical keys capable of switching both direct and alternating voltage. The key is open at a high level at the control input V and closed DD1 are connected in parallel. Reed switch SF1 must be connected so that when the doors of the protected premises are closed, its contacts are open. If the owner is present at the protected facility, the SA1 toggle switch is in the "Off" position. - at the key control inputs V low level - and even when the door is open and the reed switch is closed, the high level from connector X4 does not pass to X6 "Alarm". Resistor R8 limits the charging current of capacitors C5 and C6, which can damage the DD1 chip. Before leaving the premises, the owner puts SA1 in the "Protection" position. At the same time, the capacitor C9 begins to charge through the resistor R3, after 25 seconds the voltage on it reaches a level sufficient to open the keys DD1. The device goes into armed mode. If you now open the door, then through the resistor R8 and the DD1 chip, a high level will go to the contact of the X6 "Alarm" connector and the siren will turn on. The owner, before entering the room, must insert the "key" R1 into sockets X2, X1, while the outputs of the op-amp DA1.1 and DA1.2 will be low. Capacitor C3 through diode VD5 and resistor R7 will be discharged in 2 s while the "key" is inserted. At the same time, at the inputs of V elements DD1.1-DD1.4, a low level will close the keys of the DD1 microcircuit and it will be possible to open the door. Having entered the room, it is necessary for 25 seconds (until C3 is charged again) to set SA1 to the "Off" position. A voltage regulator of 2 V is assembled on the DA9 chip. The DD2-DD4 microcircuits form the necessary time intervals for the siren to work. Siren multivibrators are made on a DD5 chip. An RS flip-flop is assembled on logical elements DD3.1, DD3.2. The R11C7 circuit sets it to the zero state (low level at the output of the DD3.1 element) when the power is turned on. If an "Alarm" signal is received, a high level will appear at the input of the DD2.1 element, and a low level at the output. In this case, the high level that appeared at pin 9 DD3.3 will allow the operation of the multivibrator assembled on the elements DD3.3, DD3.4. A low level at the input R DD4 will allow this counter to work. The inputs of the elements DD5.1 and DD5.4 will receive a high level, which will allow the siren to work. After 4 pulses arrive at the counter DD210, a high level will appear at its output 15, and a low level will appear at the output DD2.2. This will reset the RS trigger to its initial state and the siren will turn off. You can turn off the siren ahead of time with the SB1 button. It should be noted that both of these options turn off the siren if the output of the X6 connector is not high. The ratings of the frequency-setting ratings of the multivibrator R12, C8 ensure its operation at a frequency of approximately 1,2 Hz, while the siren operates for about 20 minutes. This time can be changed over a wide range by selecting R12 and C8 or connecting element DD2.2 to another output DD4. The VD6, R15, R18, C10 chain gives the siren a characteristic howl. It is possible to change the tone of the siren by selecting capacitors C11 and C12. A power amplifier is assembled on transistors VT1-VT4. The power output 14 of the DD5 chip is connected directly to the positive terminal of the GB1 battery. This is necessary so that the power amplifier transistors are securely closed. Fuse FU2 protects the battery from short circuits in the device circuits. An audible signaling device is assembled on the DD6 chip, which is triggered when the supply voltage drops to 10,2 V (at -25 & No. 176; C to 10 V). It was described in the article by I. Alexandrov "Two devices for a battery" ("Radio", 1989, No. 5). The reverse-biased emitter junction of the VT5 transistor plays the role of an economical zener diode. Its stabilization voltage of 7,3 V is almost constant when the supply voltage changes from 16 to 7,8 V. The divider R20R21 generates a voltage of 2 V at pin 6.1 of the DD4,3 element. If a voltage of 1 V is supplied to pin 6.1 of DD6 and the power pin of the DD12 microcircuit, then the voltage of 4,3 V at pin 2 is perceived as a low level. When the supply voltage of the microcircuit drops to a certain threshold value, the potential at pin 2 (4,3 V) begins to be perceived as a high level. A low level occurs at the output of the DD6.1 element, a high level occurs at the output of DD6.2, and the buzzer starts working on the elements DD6.3, DD6.4. By selecting the resistor R22 within 1 MΩ ... 5 kΩ, the loudest sound of the piezo emitter is achieved. The device is not critical to the choice of elements. Some of the digital microcircuits have analogues in the K176 series, and they can be used. The DA2 chip can be replaced with KR142EN8G. Transistors VT1-VT4 - from the KT972, KT973, KT825, KT827, KT829, KT853 series, with any letter index, of course, of the corresponding structure. Diodes VD1, VD2 - any universal or pulsed with a permissible direct average current within 10 ... 20 mA and a permissible reverse voltage of 10 ... 20 V. Diodes VD3-VD6 can be from the KD521, KD522, KD503, KD510 series with any letter index. The BQ1 piezo emitter is applicable to any of the ZP series. Ceramic capacitors - K10-43a, K10-47a, K10-50a, KM, oxide - any of the K50, K52, K53 series. Resistors can be S2-ZZN, MLT, OMLT, VS. Button SB1 and toggle switch SA1 - any, as they switch weak currents. When the siren is operating for 20 minutes or more, a BA1 dynamic head with a power of at least 10 W at a resistance of 8 ohms and at least 20 W at a resistance of 4 ohms should be used, since the coil is very hot and less powerful heads usually fail after 3 .. .5 min work. Since the device consumes a significant current in alarm mode (from 1 to 2,5 A, depending on the dynamic head used), it is better to use a GB1 battery from the car. In this case, no power switch is needed. The device in armed mode with the low battery buzzer on consumes 14 mA. Theoretically, this current will discharge a car battery in 5 months, but it should be recharged every two months. Block A1 is conveniently mounted on the door, and block A2 should be placed in a secluded place with the battery and preferably closer to the dynamic head. For ease of installation of the device at the facility, it is desirable to make all connections of the blocks through connectors. Pairs of transistors VT1, VT3 and VT2, VT4 should be installed on heat sink plates with an area of at least 15 cm 2. If the case of block A2 is metal, then the DA2 chip and transistors VT2, VT4 can be attached to the case. Setting up the device comes down to choosing the key resistance R1 and setting the buzzer threshold to 10,2 V. When setting up the electronic lock unit, the resistor R1 is replaced by a variable of 10 kOhm. By rotating the engine of this resistor, a voltage across the resistor R3 is achieved, equal to the middle of the interval between the voltage values at pins 3 and 6 of the DA1 chip. Then it is desirable to install a constant one with the same resistance instead of a variable resistor. To establish a buzzer, it is necessary to use a variable resistor with a resistance of 1 MΩ. It is switched on according to the variable resistor circuit instead of resistors R20 and R21. The battery is replaced with an adjustable voltage source and the voltage is set to 10,2 V. By rotating the variable resistor slider, the buzzer is turned on. After that, the correctness of the threshold setting is checked by changing the voltage of the power source. If necessary, adjust the resistor a little again. Then it is desirable to replace the variable resistor with two constant ones, as shown in the diagram. This increases the thermal stability of the operation of this node. The electronic "lock" proposed by the author can be simplified. It is better to perform it in one package, while you can replace the DD1 chip and the DD2.1 element with one two-input AND-NOT, leave one of the two synchronously operating multivibrators DD3.3, DD3.4 and DD5.1, DD5.2, exclude the elements DD6.1 and DD6.2, remove the voltage regulator DA2, since CMOS chips and operational amplifiers operate in a wide range of supply voltages. If you leave DA2, you do not need a voltage regulator on the transistor VT5, using the output voltage of DA2. If the SA1 switch is placed in the power supply circuit of the device, the interval between battery recharges will be much longer and there is no need for the SB1 button. To protect the device from damage by applying external voltage through contacts X1 and X2, it is advisable to replace the VD1 diode with a 3,3 kΩ resistor, respectively reducing R1, and connect a 3 ... 9 V zener diode in parallel with R12. It is desirable to protect the inputs of the DD1 chip with diodes. To do this, you need to connect two diodes to the X4 connector: one - with the anode to X4, the cathode to the power source, the other - the cathode to X4, the anode to the common wire. Author: A.Rudenko, Kharkov, Ukraine
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