Fig.1: this diagram shows the pin out of the humidity sensor and its response characteristics.

There are various humidity sensors on the market but the recommended device for use with the PICAXE-18X Datalogger is the Honeywell HIH-3610-001. This sensor is a direct humidity-to-voltage device with built-in conditioning circuitry.

It is supplied in a small 3-pin single-in-line package. Two of the pins connect to a regulated +5V power source, while the third gives a linear output voltage that’s proportional to humidity. This means that it can be connected directly to the Datalogger (via connector CT4) without any additional circuitry (see photo above).

As with all humidity sensors, take care not to physically touch the sensing area of the device, as moisture/oils from the hand could damage the sensitive sensor element.

A sample graph of the response of the humidity sensor is shown in Fig.1. When used with the PICAXE, the voltage output of the sensor is measured by the internal analog-to-digital converter (ADC) and stored in a variable (eg, b1) as a number between 0 and 255. Each ADC step is 5V/256 = 0.0195V (using a regulated 5V supply).

The graph in Fig.1 shows an offset of approximately 0.8V, which equates to an ADC value of 41 (0.8 / 0.0195). The RH (relative humidity) slope is set at about 0.0306V per %RH, or 1.57 ADC steps per %RH.