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Window Switch Sensor Based on Temperature

2014-11-16 03:14  
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Figure 1 

The circuit has been designed to produce a window switch with the use of a thermistor for the main purpose of sensing temperatures.

Thermistor – one type of resistor used as temperature-sensing element which demonstrates a large change in resistance proportional to a small change in temperature because of the composition of its semiconductor materialBC214 – a complementary silicon planar epitaxial transistor used in AF small signal drivers and amplifiers as well as for low noise preamplifier applications due to its feature of good linearity of DC current gain

The circuit mainly serves as a basic resistance-sensing window switch. The function of the resistor is being taken in the form the Negative Temperature Coefficient (NTC) thermistor which is an electronic component and temperature sensitive passive semiconductor that is exhibiting a large change in electrical resistance in accordance with a change in its body temperature. The term thermistor originates from the words “thermal resistor”. When subjected to an increase in body temperature, theNTCthermistor decreases in resistance. It can perform several unique functions that is impossible to be achieved with standard electronic components because of its extreme sensitivity to minute temperature changes.

Accurate temperature variation measurements can be done using a simple measuring device because of the very high temperature coefficient of the thermistor. Its temperature is dependent upon the surrounding ambient when a very small amount of power is dissipated in a thermistor. As a result, its electrical resistance becomes a function of the ambient temperature.

The thermistor TH1 in the circuit should have a resistance in the range of 500 Ohms to 9K Ohms although it works over a relatively small temperature range compared to other temperature sensors and it can provide very precise and accurate sensing. The use of 10K Ohms potentiometer RV1 is to set half of the supply voltage on the junction of TH1 & RV1 during the presence of average temperature. The voltage output of the circuit ranges from 0 V to 12 V.

The use of the diode D3 across the relay RL1 is to protect the ICs and the transistors from the sudden high voltage produced when the relay coil is switched off. This is due to the magnetic field created by the current flowing through the relay coil, which suddenly breakdown when the current is switched off. This sudden breakdown of magnetic field induces a short high voltage across the relay which could most probably damage the ICs and transistors. The diode serves as protection which permits the induced voltage to drive a brief current via the coil & diode so the magnetic field will pass away rapidly rather than instantly to prevent the induced voltage from becoming high.


Figure 2 

The temperature window switch circuit is widely used in German cars and automobiles as auto electrical parts such as the thermo sensor, coolant temperature switch for radiator fan, water temperature sensor, cooling system, thermostats, automotive relays, and thermal switch. Those German brands include Mercedes Benz, Volkswagen, Opel, Saab, andBMW. The use of thermistors may also be seen in scientific, industrial, and commercial specialty temperature sensing probes.


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