I've created a circuit (schematic below) to measure humidity with a HIH-5030 according to its datasheet and an ATtiny85 and tested it on a breadboard, the readings are very accurate.
Soldered on a small PCB (below), the readings drop within about 5 minutes after connecting it from correct 50% relative humidity to about 30% and I am trying to figure out why.
- I've measured the output voltage of the sensor to verify that AD conversion is correct, which it is
- The board warms up slightly but if I warm it up even more while disconnected and then connect it, the behaviour is the same
- The sensor is installed close to the controller but even in power down sleep mode the behaviour is the same so I suppose interference by the controller can't be the issue
- Below the sensor there is a ground plane and some tracks, can this cause issues because this is a capacitive sensor?
- When the sensor is connected to the board with wires about 3 cm long, the readings are correct
I suppose I've done some real beginners mistake in my PCB design? For the "minimum load" resistor R7 I have chosen 80k instead of 65k because the sensor is operated with 5V instead of 3.3V.