The basic concept of our design is to have a hygrometer driven self-regulating humidifier controller, which can be attached to a humidifier unit. The system takes humidity readings and compares them to a set humidity value. The controller regulates the humidity by turning the humidifer unit on if the air is too dry and off when the air is too humid.
A Philips 2322 691 90001 capacitive humidity sensor (see datasheet) is used to measure the humidity. The sensor has a varying capacitance that is used to vary the frequency of an oscillator. By determining the frequency of the oscillator output, we calculate the relative humidity and use the value for comparing to a set value. We originally planned to use the Actel on-chip oscillator (see application note) . However, because of the low capacitance of our sensor, we needed to switch to a 555 timer.
The humidity setting is entered into our system by using a 3x4 keypad. The user can enter 2-digit humidity values (00% to 99%) as a sequence of in order to set two digits to set the desired humidity level. The user can enter a value at any time and can re-enter values as desired. On reset, the system initally regulates the humidity at 25%.
A 7-segment LED interface provides output to the user. The humidity sensor readings are displayed as they are calculated. The current humidity setting is also displayed.