Humidity sensor HS15p


Thread Starter


Hi everyone,

I'm doing the project that used humidity sensor to control speed of the fan that depend on humidity. Right now i'm facing the problem to use the humidity sensor HS15p and to display the humidity on lcd Display. I'm using pic16f877 for this project.

I didn't know how to make a connection between the sensor and pic. i've gone through one circuit and it said I must connect the sensor in series with resistor and it use three pin on the pic. I've tried but the sensor isn't working at all. Anyway at which part should I supply the the ac voltage since the connection i've got did not show the connection for ac supply?

Please help me and your answer are much appreciated. My email add: [email protected]

Robert Scott

The HS15p is a resistive humidity sensor that can be thought of as a variable resistor whose resistance depends on the humidity. However continuous DC current through this sensor will destroy the sensor. Therefore you must measure the resistance of the sensor using any circuit that ensures only balanced AC current will flow through the sensor. One good was to ensure that is to place a low-leakage capacitor in series with the sensor and then operate at a frequency that is high enough so that the capacitive reactance of the blocking capacitor is small compared to the resistance of the sensor.

The AC excitation can be generated by the output pin of the PIC. You could measure the RC time constant of the circuit by driving the pin and then turning it around in software to make it an input and watching how long it takes for the voltage to decay to the input threshold of the PIC pin.

Robert Scott
Real-Time Specialties
Embedded Systems Consulting