Soil Probe Interface to Wireless Mote

Overview

This report provides a detailed description of the hardware interface we developed between the Decagon Devices ECH2O-TE soil probe and a TelosB-class mote. The goal of this report is to provide enough information for an interested reader to reproduce the interface. This interface has been working for several months at the Desert Botanical Garden in Phoenix, as you can see in the graphs of recent readings.

If you decide to build this interface yourself, remember safety comes first. The interface is straightforward and low-powered, but building it requires an understanding of the safety issues involved in working with electronic tools and components.

This description is made freely available as described in the license at the bottom of this page. We also have made available a software driver for use with TinyOS. You can download it from the TinyOS contributions repository.

Requirements

In order for the probe and mote to communicate, the interface must adapt to the requirements and limitations of each component. The probe provides a three-pin stereo plug for UART output, excitation/power input, and ground. So first at the physical level we need an external stereo jack to mate with the plug. For power the probe requires 10 mA of current for 9 ms at 3.5 V or higher. Unfortunately the mote output pins are capable of a maximum of 6 mA at 3.0 V. This means we need an external power source and can use the mote pins only to trigger the external source rather than to power the probe itself. Finally, at the data communication level we can match the probe's UART output with the mote's built-in UART input.

Design and Implementation

Given the requirements above, we adopted a straightforward solution as shown in the schematic diagram below. The design was inspired by an example in The Art of Electronics by Horowitz and Hill. To switch the probe on and off, we use a couple of MOSFETs. Q1 (n-channel) allows us to use the 3 V source from the mote to in turn switch Q2 (p-channel), which then provides 6 V of battery power.

We implemented this design on a stripboard, and the layout below shows the particular components we used. In addition to the components on the schematic, on the bottom right is the voltage regulator VR and capacitors C1 and C2 that produce the 3.0 V power for the mote. Finally, you can see all the components together in the photo of the weatherproof enclosure.

Code Description
P1Battery input connector (6 V)
P2Mote power connector
P3Mote UART RX, I/O pin (ADC0) connector
P43.5mm, 3-pin right-angle stereo jack (CUI, Inc. P/N SJ1-3513N)
Q1Supertex TN0606 3.0 V MOSFET
Q2Supertex TP0606 3.0 V MOSFET
R1100 kΩ, 1/4 W, 5%
R227 kΩ, 1/4 W, 5%
VRMicrochip MCP 1702, 3.0 V voltage regulator
C11 μF ceramic capacitor
C2

License

Copyright (c) 2008, Arizona Board of Regents
All rights reserved.

Redistribution and use with or without modification, are permitted provided that the following conditions are met:

  • Redistributions must retain the above copyright notice, this list of conditions and the following disclaimer.
  • Neither the name of Arizona State University nor the names of its contributors may be used to endorse or promote products derived from this description without specific prior written permission.

THIS DESCRIPTION IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS DESCRIPTION, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

Version History
8/2008Initial version.