This is a "sensor mode only" version of my plant watering alarm Chirp. If you are interested in long cable runs, lower power and improved stability, you should check my RS485 soil moisture sensor -- also it's the easiest way to get your soil moisture readings into Raspberry Pi.
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.
Note Upon reading the moisture or temperature value, a value form the previous read command is returned and the new measurement is started. If you do rare measurements and want to act instantly, do two consecutive readings to get the most up to date data. Also you can read GET_BUSY register via i2c - it will indicate when the measurement is done. Basically the process goes like this: read from GET_CAPACITANCE, discard results, then read from GET_BUSY until you get '0' as an answer, then read form GET_CAPACITANCE again - the returned value is the soil moisture NOW.
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. Light sensor is covered by heatshrink, so the light reading will always report total darkness.
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.