This is a "sensor mode only" version of my plant watering alarm Chirp.
The sensor can be read via I2C protocol and provides these features:
This is the second version of my sensor with some improvements:
More documentation and example code is available on github page
Both light and moisture sensors give relative values. Meaning, more moisture will give you higher reading, more light, lower reading.
Moisture is somewhat linear. I test all sensors before shipping and they give about 290 - 310 in free air at 5V supply.
I didn't measure linearity of the light sensor, it gives 65535 in a dark room away form desk lamp. When it's dark, it takes longer to measure light, reading the light register while measurement is in progress will return the previous reading. Be aware, light sensor is pretty noisy.
Temperature is measured by the thermistor on the body of the sensor. Calculated absolute measurement accuracy is better than 2%. The returned value is in tenths of degrees Celsius. I.e. value 252 would mean 25.2°C.
The sensor works fine with Arduino and RaspberryPi. Examples are available on github page. Also check out this Arduino library: https://github.com/Apollon77/I2CSoilMoistureSensor
The sensor comes coated with PRF202 - a moisture resistant varnish for electronics. It's ok for play around in a flower pot but not enough for outdoor use. You must add an additional protection to the whole sensor after soldering cable to it! Some suggestions on making the sensor more robust after attaching the cable:
Be sure to coat the whole thing - the sensor part, the electronics and the cable connection itself so no bare copper or solder is accessible to the water.
Pre-coated version is available since December 2016. The sensor is coated in epoxy resin, cured and additionally protected by adhesive-lined heat shrink. The 1m (3 feet) long cable is pre-soldered.
The pinout of the rugged version (with white cable):
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I've beed a web-based software developer for 10+ years, but now I'm an engineering ronin doing mostly digital electronics projects.