Touch sensor in the security alarm. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Safety and security

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The proposed article describes a device that reacts to touching an object - a door lock, a safe, etc. It can be used in a security system.

On fig. 1 shows a diagram illustrating the principle of operation of the device. Here G1 is an alternating voltage generator (power grid); Ср - high-voltage separating capacitor of small capacity; D - sensitive threshold element; KP - a controlled object with electrical conductivity; C0 - its electrical capacitance; Sk is the electric capacitance of a person touching the CP.

Zin is much less than the capacitance of the capacitor C0. Therefore, the current I that occurs in the standby mode (without Sk) is approximately equal to I=314.Ur.Sr. С0/(Ср+С0), and the voltage at the input of the sensing element: Uin=I.Zin, where Zin is its input resistance, and Ur is the generator voltage. The voltage Uin must be below the threshold of the sensitive element. Connecting a foreign object to the gearbox will increase the capacitance up to C0'=C0+ Sk and, accordingly, the current in the circuit up to I'=314. Ur.Cp.C0'/ (Cp+C0'). In this case, the input voltage Uin'=I'.Zin. This voltage must be above the threshold, and then touching the CP will trigger an alarm.

On fig. 2 shows a schematic diagram of the device. An AC voltage generator with a coupling capacitor at the output is a C9C10 capacitive divider connected to the electrical network. Input resistance of the threshold element Zin=40 kOhm (it has a complex character). The low-pass filter R1C1R2C2 attenuates possible high-frequency and impulse noise.

Operational amplifier DA1 and transistor VT1 make up an economical comparator, the signal from which is fed to the counting input of the 14-bit counter DD2. On the DD1 chip, an oscillator is assembled that generates short pulses. They enter the input R of the counter and periodically return it to its original state. The repetition period of these pulses is Tr=2.R12. C6 in the manufactured sample was 7 s. The R14C8 circuit is introduced to reset the DD2 counter when the power is turned on.

Transistor VT2, operating in the electronic key mode, turns on the piezo siren HA1 when a high level appears at the output of the counter. But this will happen only if, in the interval between the reset pulses, at least 256 pulses with the frequency of the AC mains arrive at the input C. That is, if the contact with the gearbox lasts at least Tc=Nc/F=256/50=5,12 s, where F=50 Hz is the frequency of the alternating current in the network. Hence the maximum duration of the alarm Tmax=Tr-Tc=1,9 s. In fact, its duration will depend on the moment of touching the CP, it can be any within 0...Tmax. If the touch on the control panel is maintained, the alarm will be repeated periodically.

Resistor R4 - SP3-38a or SP3-19b, the rest - MLT 0,125. Capacitors C9 and C10 (they are mounted in a mains plug or socket) - K5-15-N70 (operating voltage 1,6 kV), K73-9 (630 V) or KSO-2 (500 V). Capacitors C3 and C5 - K50-35, K50-40, etc., the rest - KM-6, K10-17b, etc. Transistor VT2 - silicon composite structure npn with a current transfer coefficient of at least 400 and an allowable collector current of at least 200 mA. As DD2, the KR1561IE20 chip can be used. Piezo siren HA1 - AST-10.

In the assembled device, you need to check the voltage at the output DA1 (pin 6) in standby mode. If it exceeds 1 V (the minimum voltage at the output of the operational amplifier is usually not standardized), a resistor R8 of lower resistance should be installed - in this mode, the transistor VT1 must be securely closed.

The sensitivity threshold DA1 U=Upit.R4/(R3+R5) is set with a trimming resistor R4. The device will begin to respond to the input signal only if its amplitude exceeds this threshold.

In the security system, one or another delay in turning on the alarm signal is almost always done. This is one of the tricks that are used when trying to transfer to automata an algorithmically difficult function - the selection of only potentially dangerous, threatening events from a variety of possible events. The delay adopted here will not work for households who habitually quickly (less than Tc) unlock the door of their apartment, but it will react to a thief who takes too long (more than 2Tc) picking up a master key.

The temporal characteristics of the device (the delay in turning on the alarm Tc and the maximum sound duration Tmax) can be changed by installing frequency-setting elements with other ratings and taking the output signal from another output of the counter DD2. So, with Tr=7 s and Nc=128 (for this, the resistor R10 is connected to terminal 13 DD2) we have: Tc=2,56 s, Tmax=4,4 s. At Tr = 4,8 s (for this, the capacitor C6 must be 0,68 μF) and Nc = 128 we have: Tc = 2,56 s, Tmax = 2,2 s. If such a significant delay is not needed and you should immediately respond to a touch on the gearbox, it is enough to connect the resistor R10 to output 23 (pin 7) of the counter DD2. In this case, the alarm will sound after 0,16 s.

Since the mains is taken as the alternating voltage generator necessary for the normal operation of the sensor, it is convenient to use it to power the sensor. It is only necessary to choose a network power supply unit (adapter) suitable for voltage and current load from any radio receiver or electronic toy. Capacitors C9 and C10 in this case may not be required: experience shows that capacitive coupling with the mains, which occurs in the power supply itself, is quite sufficient.

The table will help you choose a power source, where Upit is the voltage of the power source, Idezh is the current consumed by the device in standby mode, Itr is in alarm mode. We only note that with an increase in Upit, power consumption in standby mode increases, and the AST-10 piezo siren sounds loud enough even at Upit = 4,5 V.

The described device is, in essence, an independent security system. It can also be used as an integral part of a more complex device, removing the signal from the outputs of the counter DD2.

Elements R7, R8, R9 and VT1 can not be installed if the supply voltage is more than 8 V, while the output DA1 should be connected directly to the input C DD2. The elements DD1.3, DD1.4, R14, C8 are also optional - the differentiating circuit C7R13 can be connected between the output 3 of the element DD1.2 and the input R DD2. In this case, the upper terminal R13 according to the diagram must be connected to a common wire.

Author: Yu.Vinogradov, Moscow

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