Humidity sensor - H1
Humidity sensor - Serial - Philips H1
Fig. 1 Humidity Sensor
Description:
This board is a humidity sensor, based around a Philips H1 sensor. This sensor has its capacitance changing with relative humidity. Connected to a timer in astable mode, we can get an oscillator with the output frequency depending on the relative humidity.
The board interface is an 8 bits type, but the output of the oscillator is to connect to the external input of a timer, like timer 1 for example. A jumper can select either bit 0 or 6 as the output bit on the bus.
The board can be powered by 3.3 or 5V processor board. The board itself can actually be powered between 3 and 15 V.
Specifications:
Input voltage 3 to 5V
Interface8 bits (bit 0 or 6)
Range10 to 90 %
Precision3 % when calibrated
Schematic:
The power supply is filtered by C2. R1 offers some protection for the output signal. JP2 allows to select between bit 0 or bit 6 for that signal. It allows to connect easily the board to PIC16F628 or PIC18F2320 processor boards. IC1, a ICM7555 or NE555, is a classic timer in an astable configuration. The ICM7555 is compatible with the old NE555, with improved performance and low supply current.
The output frequency is determined by the following formula:
F = 1.38/((R3+2xR2) x Cx) where Cx is the capacitance of the sensor. R2 and R3 have the same value, and have a tolerance of 1%.
The formula becomes F = 1.38/(300 000 x Cx).
The capacitance of the sensor goes from 112 to 143 pF for a relative humidity between 10 and 90 %.
This gives an output frequency of 41 KHz to 32.1 KHz.
Fig. 2 Humidity sensor schematic
Construction:
The PCB fits on a small single sided board. All components are on the top side.
Fig. 3 Humidity sensor layout
Start with the lowest components, and continue with the higher ones. Keep the sensor for last.
Pads 1 to 4 can be drilled to 3mm to fix the board, or put spacers.
| Humidity Sensor Component list |
|---|
| C1, C2: 100 nF R1: 22 K R2, R3: 100K 1% IC1: ICM7555 or NE555 + DIL socket JP1: Philips H1 JP2: jumper 1x3 K1: Connector AMP MT 10pins male |
How to use / Testing:
To check the board, leave IC1 out and connect the board to an 8 bits interface and power up the board. Check you have the Vcc supply between pin 1 and 8 of the DIL socket.
Switch the power off and fit IC1 on its socket.
To check teh sensor, you can use a frequency meter and read the frequency of the output of the sensor when powered on. You can also run the Humidity tutorial on a processor board. Just make sure you set the switches on JP2 to suit the processor used.
Files and links:
Eagle PCB and schematics files for Humidity sensor.PDFs of PCB, layout and schematics of Humidity sensor .
PIC Tutorials for Humidity sensor.
Philips H1 humidity sensor.