Ice alarm. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Automobile. Electronic devices
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For more than 10 years, Rover vehicles have been equipped with a signaling device that warns the driver of possible icy conditions. With the advent of versatile and cost-effective quad op-amp blocks, such a device can be installed on any car model.
The ice warning device shown in the diagram, by monitoring the ambient temperature, warns the driver about the onset of conditions for the formation of ice on the road: in wet weather at temperatures from 0 to 2,2 ° C. The device has a robust design and provides good noise immunity.
The circuit controls the duration of the flashing period of the light emitting diode. At a temperature of 2,2 ° C, the circuit generates short periods of flashes. When the temperature drops to 0°C, the flashes of the light-emitting diode follow continuously. The flashes repeat about once per second.
(click to enlarge)
A thermistor is used as a temperature sensor, the nominal resistance of which at 25 ° C is 15 kOhm. The thermistor is installed in the enclosure and is in contact with the surrounding air. The guard prevents errors caused by air movement.
Three of the four operational amplifiers of the block are used in the circuit. Since the operation of the circuit depends on the ratio of currents, the circuit is insensitive to changes in battery voltage, which eliminates the need for voltage stabilization.
Amplifier A1 compares the resistance of the thermistor with resistors R1 and R2 connected in series. Resistor R3 in the feedback circuit serves to set the appropriate slope of the output voltage of the amplifier, depending on the temperature. At a temperature of 0 ° C, the voltage at the output of the amplifier is 0,6 Vbat, and at a temperature of 2,2 °C it is 0,3 Vbat.
Amplifier A2 is connected according to the multivibrator circuit, the repetition rate is approximately one pulse per second. Resistors R4, R5 and R6 are used to set the upper and lower voltage limits on the capacitor C1 at 0,6 Vbat and 0,3 Vbat, respectively.
Output voltages of amplifiers A1 and A2. compared on amplifier A3. When the output voltage of the multivibrator (A2) is less than the output voltage of the amplifier L, the output voltage of the amplifier A3 is positive and the light emitting diode lights up. Resistor R7 limits the current through the light emitting diode to approximately 25 mA.
Resistor R2 is used to calibrate the circuit. With the thermistor immersed in ice, the value of R2 is set such that the light emitting diode is permanently on. Other operating points can be obtained by adjusting the resistors R1, R2, R3.
Author: E. Summer; Publication: N. Bolshakov, rf.atnn.ru
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