This is a simple humidity sensor that is very easy to build, and inexpensive (around $25 for parts from DigiKey). This is the same design I used on Arcadia's humidity sensor. The output of the HTM1505 is almost ideal - reading between 1 and 3.7 volts, dependent on the humidity in the air. The only external circuitry required is a simple voltage follower via an op amp to buffer the 70 ohm output impedance. Any discrete op amp will do - I have found LM358's to be very reliable and accurate for these type of meters, since they can read almost to ground from a single power supply. Mounting the HTM1505 is the hardest part - you must be sure it is in free moving air, and not sealed in an enclosure. I used an upside down mesh paper clip holder from Wal-Mart to mount my circuit, it provides the circuit protection, but the mesh allows the air and humidity to pass through the enclosure freely. There are two conversions to give you relative humidity (%), one is a very simple linear equation, and the other is a messy polynomial. The polynomial is more accurate, and can also compensate for temperature drift, but the linear equation is easier to calculate, so use your own discretion when picking which equation to use. The power supply can be from a power transformer, 9 volt battery, or any other 7-25 volt source.