Building a Prototype

Dragino Unit

HopeRF Radio Unit

Source a HopeRF RFM23B or RFM23BP from http://www.hoperf.com/rf/fsk/RFM23BP.htm or one of many resellers. HopeRF sell sample quantities at fair prices, e.g., US$100 for 10 of the BP (Boosted Power) units, which is substantially cheaper than from distributors.

Make sure you get the correct frequency version for your country. For Australia, NZ, South Africa, Israel or New Zealand, you probably want the 915MHz one. For Europe you probably want the 868MHz or 433MHz one. All our development is using the 915MHz ones for now.

RFM22B Wiring Diagram

NOTE: The RFM22B is a 3.3v only device (the RFM23BP on the other hand can take a 5v supply).

  • THE RFM22B WILL BE DESTROYED WITH A 5V SUPPLY!

The RFM devices use SPI, which the Arduino supports natively: http://arduino.cc/en/Reference/SPI The RFM device supports 10MHz SPI, which is faster than the 4MHz provided by the default Arduino SPI library, so we do not need to slow the Arduino down.

But we also want to monitor a few lines from the RFM boards for interrupt notification, and control device selection and master/slave mode.

  • SDO (output) SPI data
  • SDI (input) SPI data
  • SCK (input) SPI clock
  • VCC
  • GND
  • SDN (input) (shutdown; reduce power consumption to minimum)
  • nIRQ (output) Interrupt notify
  • nSEL (input) SPI select

This is complicated by the Dragino and daughterboard talking by SPI already. Need to find out what we need to do to allow the daughterboard to talk SPI to a slave device.

According to http://www.dragino.com/downloads/downloads/MS12/Dragon%20Board%20v0.6.pdf and http://www.dragino.com/wiki/index.php?title=GPIO suggests that the dragino talks SPI to the Arduino-compatible daughterboard using the GPIO lines. Switching those to input should allow the SPI interface to be free for talking to the HopeRF module.

However, probing with a multi-meter suggests that this is not the case, and that the ISP port for SPI on the daughter board is NOT connected in that way, and may be independent.

Anyway, the bottom line is that the ISP interface, plus a few IOs should do the trick. SDN (output from daughter board), nIRQ (input to daughter board) and nSEL (output from daughter board) are the ones that would need to come from somewhere other than the ISP port. Thus we need two wires with level conversion. These are not lines that should require high-speed switching, so a fairly crude solution can be used on those.

See for http://tae09.blogspot.com.au/2012/04/dragrove-daughter-board.html Dragrove daughter board schematic (although annoyingly not as an image).

In addition we need a 3.3v supply line (which is available one the daughter board), and to voltage convert the SPI interface itself. All rather annoying, and good reasons to use the RFM23BP module which is natively 5v.

RFM23BP Wiring Diagram

  • TODO: Provide wiring diagram.

Power Supply

There are various options, as described below.

Mains

Operating a prototype from mains is straight-forward, as the mains adapter in the Dragino unit can be used.

9v/12v/24v Battery

For this option, we are using a RedArc BCDC-1206 as the quality solution. Basically it takes 9v - 32v and turns it into 12v that can run the Dragino (or be fed into something to charge a battery).

See also solar operation for more information on this unit, and some low-cost alternatives that we are looking at. We will work with some of the VillageTelco crew who have been looking at similar battery and solar operation options for the very similar Mesh Potato.

Solar+Battery

See solar_operation for information on sourcing the components required to create a solar powered prototype.

  • Connect the solar panel to the RedArc SRP0120 unit.
  • Connect the SRP0120 battery 1 outlet to the battery.
  • Connect a power connector suitable for the Dragino to the battery.
  • Make sure you have all polarities correct when doing the above!
  • We recommend you put an in-line fuse between the Dragino power connector and the battery.
  • Test that +12v is on the centre pin of the Dragino power connector.
  • Test that it actually charges when the panel is in the sun.

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