HR202L Humidity Sensor without shell - Analog Polymer Sensor

The HR202L is a resistive-type humidity sensor. It uses an organic polymer film whose electrical resistance decreases as ambient relative humidity increases. This change in resistance provides an analog output proportional to the humidity level, which must be measured using an AC excitation circuit (typically 1kHz) to ensure stable operation and prevent sensor degradation.

You cannot connect the HR202L directly to a digital GPIO pin. It requires an interface circuit to convert its variable resistance into a voltage readable by an ADC. A common method is to place the sensor in a voltage divider network with a fixed resistor, powered by a PWM pin filtered to create an AC signal, and then read the divided voltage with an analog input pin on an Arduino or through an external ADC for Raspberry Pi.

The sensor has a maximum power dissipation rating of 0.2mW. It is typically driven with a low-voltage AC signal (1.5V max) at a frequency between 500Hz and 2kHz. Exceeding the voltage or using DC excitation can lead to inaccurate readings, reduced sensitivity, and permanent damage to the polymer film. Always use a current-limiting resistor in series.

While the sensor provides a baseline accuracy of ±5% RH, calibration at one or two known humidity points (using a calibrated reference) can improve measurement precision, especially over its full 20-95% range. Environmental factors like temperature can affect the reading, so for high-accuracy applications, implementing temperature compensation based on the sensor's datasheet characteristics is recommended.

The shell-free design minimizes response time by allowing the active polymer film direct exposure to air. It also provides flexibility for engineers to integrate the sensor into custom enclosures with specific filters or membranes for dust protection, or to pot it directly onto a PCB. Users must design appropriate mechanical protection and environmental sealing for their specific application.