Analog and Humidity Sensor

The LEDs are rated for 1 watt and they’re attached to a heat sink with epoxy. One is near-UV, around 403 nm (it came from Dealextreme). They’re switched with small bipolar transistors and powered with a 6 volt 800 ma transformer. I stuck everything in a lantern hanging in my front yard and had it running the alternate-pulse program as a beacon for visitors.

I made a sensor that detects humid air by soaking a paper towel in salt water and letting it dry. I pulled the layers of 2-ply paper apart and stuck one layer between two squares of old brass-plated window screen with terminal wires soldered on. A plastic laundry clip applies constant pressure to the sandwich.

The sensor’s baseline output varies widely depending on the ambient humidity, so I connected the output to the center pin of a 10 kilohm potentiometer. One of the side pins was connected to ground, and the other was connected to a transistor base to amplify the signal before passing it to an Arduino input pin. The digital output goes from high to low at a certain point when turning the potentiometer from the position nearest the input pin toward the ground pin, which represents the point of maximum sensitivity in the current atmospheric conditions. It probably wouldn’t be hard to do this automatically in the code.

It’s surprisingly sensitive- the draft of my weakly humidified forced-air furnace turned it on from across the room. The video shows it in use as a digital input. The air in the room was fairly dry so the reading went from high to low quickly each time I stopped breathing on the sensor.

Sketches: Arduino Sketch: <a title=”Arduino Sketch: Luke_Analog_I:O_1″ href=”http://code.arc.cmu.edu/~cheng/uploads/Luke_Analog_I:O_1.pde&#8221; target=”_blank”>Luke_Analog_I:O_1</a>

Arduino Sketch: <a title=”Arduino Sketch: Luke_Analog_I:O_2″ href=”http://code.arc.cmu.edu/~cheng/uploads/Luke_Analog_I:O_2.pde&#8221; target=”_blank”>Luke_Analog_I:O_2</a>

Arduino Sketch: <a title=”Arduino Sketch: Luke_Analog_I:O_3″ href=”http://code.arc.cmu.edu/~cheng/uploads/Luke_Analog_I:O_3.pde&#8221; target=”_blank”>Luke_Analog_I:O_3</a>

Arduino Sketch: <a title=”Arduino Sketch: Luke_Analog_I:O_4″ href=”http://code.arc.cmu.edu/~cheng/uploads/Luke_Analog_I:O_4.pde&#8221; target=”_blank”>Luke_Analog_I:O_4</a>

All of my schematics are in one PDF, which I’ll post shortly.

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