The circuit shown in the figure measures relative humidity in air at the low temperatures found in applications like cold storage of food items and greenhouses. Instead of using electrical signal cables that are rated for such harsh environments, the design transmits its signal on low-cost, plastic optical fibers.

This design uses an HS1101 relative-humidity sensor from Humirel (www.humirel.com). The sensor acts as a capacitor (C1) 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.

A Motorola NL27WZ14 single gate is implemented as a free-running oscillator. The time constant, (R1 + R2) × C1, sets the frequency of oscillation. The output from the oscillator goes to the 2N2222 transistor, which drives the LED to flash the light into the optical fiber. The duty cycle is nearly 50%, which cuts the power drawn by the LED in half. A 3.0- to 3.6-V battery can power the circuit. If a NiCd battery is used, it can be charged by a wall adapter. But in place of the battery or cells, a 5-V regulated power supply can be used.

Note that the HS1101 sensor is slightly nonlinear with respect to temperature. However, this isn't a serious problem in the above applications where temperature is generally regulated and humidity is controlled.

Relative humidity is computed by measuring the received signal's frequency through the optical fiber via a photo diode. There's a baseline reading due to the minimum capacitor value at 0% relative humidity. This must be subtracted from the frequency reading to get the correct percentage for a 0% to 100% range. Results were within a ±5% limit for 10 tested sensors. Over 10 readings, the average was within ±3%.