Abstract:
This circuit is very important to measure the relative humidity or temperature in air found
in applications like cold storage of food items and green houses. Because in place of
electrical signals cables that are most important optical fibers for such harsh environments,the design its signal on low cost, plastic optical fibers. In this circuit can be taken a value directly by amplifying the signal received from optical fiber and photodiode. This design can be used a relative humidity sensor or thermistor. The sensor acts as a capacitor whose value varies from 180 to 220 pf for relative humidity in the range of 0% to 100%. The sensor is linear within ± 3% of full scale. Thermistor acts a resistor. A Motorola NL 27WZ 14 single gate is implemented as a free running oscillator. The output from the oscillator goes to the 2N 2222 transistor, which drives the LED to flash the light in to the optical fiber. If a Nicd battery is used, a wall adaptor can charge it.
The relative humidity sensor is slightly non linear with respect to temperature. However,
this isn’t a series problem in the above applications where temperature is generally regulated and humidity is controlled. There’s base line reading due to the minimum capacitor value at 0% relative humidity. This value must be subtracted from the frequency reading to get the correct value for a 0% to 100% rage. Results were within a ± 5% limit for 10 tested sensors over 10 readings, the average was within ± 3%. In view of relative humidity sensor can be used a thermistor for measuring the temperature within small range. Because of relative humidity sensor corresponds the thermistor which change linearly resistivity with increasing temperature. There fore this circuit can be used to measure the temperature or relative humidity in any where utilizing a thermistor or relative humidity sensor.
