Introduction: How to Make a Simple Distributed Sensor Buoy System

As part of a project at RPI a Distributed Buoy Sensor Network was designed and built for measurement of water quality over time and to assist in the development of new sensor technologies. The system is intended to communicate wirelessly via Bluetooth and to generate it's own power via solar energy. This is only a first stab, so expect more work on the way.

Materials list:

  • Simple Power & Communication System
  • Sensor System (varies a lot)
  • Solar & Wireless Enclosure
    • Display case cut in half (size may vary but mine is 6"x6"x6") (walmart)
      • Or triangle extrusion
    • Rubber Gasket (size depends on display case, but for me >6"x8.5") (hardware store)
    • Plastic sheet (size depends on display case, but for me >6"x8.5") (hardware store)
      • Or any stiff sheet
    • Angle Bracket (hardware store)
    • 8 M4 screws, M4 washers, and M4 Nuts (hardware store)
  • Sensor Enclosure
    • Rounded end PVC endcap (hardware store)
    • PVC tube (hardware store)
    • PVC female-female connector (hardware store)
    • PVC female screw section (hardware store)
    • PVC male screw endcap (hardware store)
    • Eye Hook (hardware store)
    • 4 M4 screws (hardware store)
    • Anchor
      • Or sufficiently heavy object with a hole in it
    • Rope, cord or cable (hardware store)
    • Plumbers Putty (hardware store)

Step 1: Power & Communication Circuit

This is the simplest possible power and communication system possible. It could easily be expanded to communicate via the cellular network and to have a PV optimizer circuit.

The batteries and solar panel are in parallel and provide power to the processor, with diodes for protection. The panel charges the batteries when it can supply excess current at voltage. As this is a simple trickle charge scheme, meaning that you should only ever use this design with NMH batteries because they can take the strain of over and under charging. The diodes are critical to ensure that the solar panel doesn't over charge and to control voltage output.

The bluetooth connection is very simple and follows the basic hookup guide.

Step 2: Sensor Circuit

Because the buoy sensor could technically be nearly any seven sensors or even more with multiplexing thing, this is a difficult section. For my first test, I used a simple temperature sensor to ensure that data collection was working as expected. This is the easiest part. The temperature sensor I chose for testing is this one. For greater accuracy, you want the sensor to be on the outside of the enclosure, with low thermal mass, hence the use of this slightly more expensive sensor.

Connect this sensor in series with another resistor (350ohm), and use this as a voltage divider whose output is linearly related to the temperature. To seal it in, drill a hole in the end cap for a press fit, and coat very lightly with epoxy to keep it in position.

Step 3: Software

You will need to install drivers for the promicro, if you've never done this before, follow instructions here. Then download the code, and upload it.

The code I have been using to get data is for an android phone. It could easily be modified to get data to a computer or Iphone. If you've never installed a homebrew program on your android phone, follow these instructions. Then install the program included above, or build it yourself from the source code.

Be sure to try this out and test the whole thing thoroughly, especially if you are trying different sensors, before going any father. Keep an eye on your power, and an eye on your data collection.

Step 4: Build Solar & Wireless Enclosure

  1. Cut your display box into a triangle extrusion.
  2. Drill holes for the right angle side brackets such that they are positioned in each corner
  3. Apply sealant to the side brackets and then screw them in
  4. Cut the bottom to size
  5. Then drill holes such that they line up with your angle brackets
  6. Cut rubber gasket to size and puncture holes in such that they line up with the angle brackets
  7. Epoxy M4 screws to washers, and washers to the angle bracket such that they point out of the bottom
  8. Slip the gasket and bottom onto the screws and screw down to make sure everything lines up, you may have to expand your gasket holes a little

Step 5: Build Sensor Enclosure

    1. For this you will need to cut some PVC tube to length, which will depend on your sensors, but was 6cm for me.
    2. Drill a hole for your eye-hook in the bottom of end cap, putty it, and screw it in (this will be the anchor connection so make sure it's firm)
    3. Drill a hole in the end cap for the size of the sensor you have. Mine was 1/8".
    4. Apply putty or epoxy and slip through hole
    5. Epoxy or otherwise affix your sensor in place
    6. Drill a M6-7 (or large enough for all your wires to fit through) hole in the center of the screw cap (through the nub), the positioning is important for stability
    7. Take communication and power enclosure before and remove the bottom
    8. Drill a corresponding hole in the center of the bottom of the communication and power enclosure
    9. Drill and tap four M3-4 holes in the screw cap corners (make sure to leave at least 2.5 mm around the edges)
    10. Drill four corresponding holes in the bottom of the communication and power enclosure
    11. Cut a rubber gasket to the size of the nub on the screw end cap, and cut holes out to match the screw cap
    12. Screw in the bottom of the enclosure through gasket to the screw cap
    13. Screw the bottom of the power enclosure into the screw cap and run your solar power and bluetooth wires through the hole
    14. Place the solar panel and bluetooth in the power and communication enclosure. Fix them if necessary.
    15. Return the bottom of the power & communication enclosure, by screwing it in.
    16. Screw the end cap into the female screw section
    17. Plug the female screw section into the female - female adapter (use putty to ensure seal)
    18. Plug the adapter onto the PVC pipe (use putty to ensure seal)
    19. Connect up the power and bluetooth as before.
    20. Plug in the sensor wires
    21. Make sure to strain releive or tape down any loose wires, and then set it into the pipe
    22. Plug the PVC into the end cap (use putty to ensure seal)

    Step 6: Testing

    Get a bucket capable of submerging your whole system and place the buoy in the bucket. You're looking for:

    • Bubbles
    • How much ballast you need

    When checking for leaks, make sure you rough it up a bit, as there will likely be plenty of debris in the water, and you're better off having a break in the lab than in the environment. Then get something dry (I used sand in plastic baggies) and slowly add ballast to the bottom of the PVC section until only the power & communication section is exposed and the whole system is stable. You may also want to check:

    • Power consumption
    • Communication

    When you're done, unplug the power, it's time to hit the water. Don't plug in the power until you are in position, then go for it and screw it all together. Lower the anchor in first, then place your bouy in, and tie it off. You're aiming to have no excess slack, but not too taught.

    What I found was pretty positive. However, depending on cloud cover you can run out of power pretty easily. The next version of this will use less power, and have a better seal along the top enclosure, because it took forever to get that to be properly water tight. Improvements to the program are also in order.