Integrated timers in the car anti-theft system. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Automobile. Security devices and alarms
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Based on just two cheap 555 type integral timers, you can assemble an inexpensive car anti-theft system. Its schematic diagram is shown in Figure 1.
Ris.1
Timer A performs a dual function - it provides an exposure time (it is approximately equal to 1,1 RACA), sufficient not only for the driver, having turned on the anti-theft system, to get out of the car, but also for him, after getting into the car, managed to turn off the system. Thanks to this exposure time, the need to install a special switch outside the car is eliminated, which is always inconvenient, not to mention the fact that an attacker can always find this switch. In this case, the toggle switch to turn the system on and off can be hidden somewhere behind the dashboard of the car.
When the system is turned off, then timer B is started by the fall of the output pulse of timer L. When the system is turned on, the thyristor allows timer B to be started only by the operation of one of the sensors - grounding-type switches located at vulnerable points of the machine.
As a result, the periods of charge and discharge are the same and depend only on Rt and RR. Therefore, for the operating frequency of the circuit, we can write the equation
or
Thus, the change in frequency with temperature is consistent with the change in voltage in a resistor divider consisting of a thermistor and a resistor (a divider of this type is often used in a bridge circuit in order to obtain a linearized dependence of voltage on temperature). The voltage at the output of the divider can be expressed as
Since the denominators in equations (3) and (2) are the same, the frequency vs. temperature behavior in the converter circuit will have the same form and the same degree of non-linearity as the voltage vs. temperature behavior for a conventional voltage divider consisting of a thermistor and resistor.
If a thermistor is used, which at a temperature of 25 ° C has a resistance Ro = 5 kOhm and provides in the temperature range
from 0 to 50 ° C, the change in resistance is 9,06 times, then the converter circuit gives an error due to non-linearity no more than ± 25 ° C in the temperature range up to 0,5 ° C. This error is reflected in the graph of the dependence of frequency on temperature, shown on figure.
The fact that the frequency value in hertz in the entire operating temperature range agrees well with the temperature value in degrees Fahrenheit (the interval 3-46°C corresponds to the interval 37-115°F) is purely coincidental. In the general case, the frequency will depend linearly on the temperature in any of its intervals, but the frequency value will differ from the temperature value.
Highly stable polycarbonate capacitors may need to be used to minimize transducer errors. In the circuit shown in the figure, general purpose capacitors with a nominal tolerance of ± 5% were used, and specially selected small capacitors were connected in parallel to obtain an accurate frequency value for a given temperature.
The timer IC itself introduces only a negligible error into the frequency reading as a function of temperature. If you do not provide for the correct decoupling of the power circuits, then the circuit may be somewhat sensitive to changes in the supply voltage.
Author: MLHarvey; Publication: N. Bolshakov, rf.atnn.ru
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