Here’s another plug which didn’t work initially, but as always it was a simple mistake in the software (and a not-so-clear datasheet) which prevented me from using this thing:

The Heading Board is based on a somewhat unusual HDPM01 combination sensor by HopeRF, containing a 2-axis compass / magnetometer and a … temperature + pressure sensor. My interest in this thing was the compass, which is relatively low-cost compared to most other options out there.

There’s now a HeadingBoard class in the Ports library to make this thing sing. Note that it’s called a “board” and not a “plug” because it requires two ports and sits as a mini-board on top of a JeeNode (same as the Room Board):

This board is a somewhat awkward match for the ports concept, because it needs a 32 KHz clock to function. I’ve hooked that up to the IRQ pin, which is reprogrammed by HeadingBoard::begin() to generate that clock using Timer 2, so you may have to jump through some hoops to use this if other ports use the IRQ pin for a different purpose. A more general approach would be to generate the required 32 KHz on-board with an NE555, as is done in an upcoming Infrared Plug – but that requires extra board space (or components on both sides of the pcb).

Here is the “heading_demo.pde” sketch, now added to the Ports library as example:

Sample output:

I haven’t figured out the conversion from X-/Y-axis values to compass heading yet. There is some sensitivity to how the board is positioned horizontally – this could be compensated by detecting the angular position using a Gravity Plug. But as you can see, there is a clear variation in readings as I turned the JeeNode + Heading Board around the Z axis.

So if you’re into robots: a Heading Board plus a Gravity Plug is all you need to determine your absolute orientation in all the 3 axes in space, i.e. X, Y, and Z.

Onwards!

PS. Here’s a crazy idea: we could use the Heading Board to create a door-open sensor. Simply attach this thing to a door, and track the Z-axis orientation!