Sensor and sensor interface
The Watermark sensor is the basis for the Davis Instruments soil moisture station.
We measured about 10M ohms resistance across a Watermark sensor when bone dry, settling to 5.6M ohms after more than an hour. You could use the Javelin Stamp analog to digital converter (ADC) class with a voltage divider and regulated voltage source to measure that resistance and operate the sensor.
Alternatively, you can use the complete subsystem offered as the Davis Instruments Vantage Pro Envoy wireless sensor data logger coupled with their soil moisture remote (Figure 3). Within Figure 3, (a) shows the Watermark sensor, and (b) is the remote wireless station to which you connect up to four Watermark sensors and four temperature probes. The indoor Vantage Pro Envoy receiver is in (c), while (d) shows a close-up of the switches you must configure in the remote wireless station to set the operating frequency.
The Envoy gathers indoor temperature, barometric pressure, and humidity directly, and reads up to four each soil moisture and temperature sensors from a wireless remote you station in the yard. The Envoy provides a serial port interface to your PC (consider a USB-to-serial port adapter such as the Parallax FTDI unit available from Parallax, with a documented interface for both current data and archived samples.We chose to use the subsystem rather than build up our own interface, because the subsystem already had the features we wanted, including sensors besides just soil moisture, and because the wireless link to the remote station simplified connecting the outdoors sensors to the indoors PC.
FIGURE 3: Vantage Pro hardware suite
We wanted to gather data from the soil moisture sensor to understand how it behaved in our yard while we worked on the rest of the system design and implementation. Davis Instruments documents the interface to the Vantage Pro so to both gather data and verify we understood the interface we wrote some test software. Figure 4 shows a Microsoft Excel plot of the data we gathered, revealing some interesting properties of the sensor:
1. Although it’s specified as having a data range of 0 (very wet) to 200 (bone dry), the useful range of the sensor appeared to be 0 to 10 or so, because at readings higher than 10 the soil itself was too dry to grow anything.
2. The target readings for the sensors in our yard appear to be between 1 and 3.
3. The actual range as reported through the Vantage Pro interface is 0 to 200, but if you leave the sensor to dry further after it reaches 200, the interface will eventually report the same value as for no sensor connected.
4. It takes only a few minutes for the sensor to respond to being dunked in a container of water, but it’s far slower to dry out. The speed of response both ways is fast enough for garden watering applications.
5. The movement of water through soil to a sensor placed 6 to 12 inches down is much slower than the sensor response time — we found it takes hours before the sensor reports a change after a good soaking from the sprinklers. You might want to stack two sensors, one to track surface water and another to track water at a depth for the roots of larger shrubs.
FIGURE 4: Soil moisture sensor test data plot
The abrupt drop in the soil moisture values near the middle of the plot are points where the interface reported no sensor data available. That wasn’t because the ground suddenly dried out, though, it was because we powered up a second wireless transmitter station and forgot to change its operating frequency. Its transmissions jammed the signal from the operating one, a reminder that — as with any wireless link — you should both verify the link’s integrity before trusting it, and build your control systems to respond in a known way if the wireless link fails. In the software we wrote, for example, missing data values result in guaranteed successful comparisons against thresholds, letting you program operating rules that leave the sprinklers on or off as you choose.