Connecting multiple boards together

 One of the big hangovers from trying to produce a retail-ready product, was the idea of making sure all the electronics were safe. To date, we'd used pin headers as edge connectors on the sides of each board edge - just plug in some pins into a recessed socket and connect the next board.

The idea was for everything to "just work" without needing any configuration.
This proved trickier than first imagined.

For example, it introduced the concept of "polarity" for the connectors.
If we had a connector with power on the left, and ground on the right, on the top edge of the board, it meant that the connector on the bottom edge also had to have power on the left and ground on the right.
Which is totally fine. Until someone unwittingly rotates a board 180 degrees (they look the same whichever way around they go!) and tries to plug it in.

Now you'd have power going straight into the ground pin on another panel, and likewise ground into power. In short, it had a tendency to go "poof!"

We tried playing with off-setting the connectors from centre (so each board would only connect one way around). And by using multiple pole connectors, that were symmetrical from left-to-right:

but then we introduced the problem of users accidentally "off-setting" the pins - if boards were connected pin1 to pin2, pin2 to pin3, pin3 to pin4 and so on, we'd not only end up with a power-to-ground short, but we'd also run the risk of driving the data line to ground (or power if the pins were offset by two).

However much we played with the idea, it felt like three pins was "safer" - power and ground, and a single data wire (using a crude PWM to transfer tiny packets of data).

This led to us investigating using a bridge rectifier on each set of power supply pins. This way, so long as we kept the data pin in the middle, it wouldn't matter if the pins were flipped - data would still be in the middle and the power-to-ground problem would go away (since the rectifier would always ensure the voltage "came out" the right way around!)

The only problem with adding a rectifier is the voltage drop - passing through two diodes means losing around 1.4V, so with a 5v supply, we'd be operating at around 3.6V.

This, in itself, wasn't a major problem - we could set our Arduino Pro boards to run at 8Mhz and from a 3.3V supply. But then by the time we were powering 8 x hall sensors as well, we were on the border of having a reliable power supply.

If we only ever connected one or two boards together, things just about worked.
But voltage drop also inherent in the wires and the copper tape tracks we used to form a sensor matrix meant that if you connected three or four boards together, those "furthest away" from the source of power sometimes misbehaved.

So then we had the brilliant idea of running our boards off a 9v supply, and adding a step-down voltage converter (module) to each "panel". So from our connecting socket, we'd take 9v, run it through a bridge rectifier (to solve any reverse polarity issues) then take this (reduced) output (of around 7.6V) and put that into a step-down voltage converter module.

The end result was a reliable board, no polarity problems, an entirely safe connector (if you flipped the supply polarity, it still worked and if you offset the pins, you'd connect either a power or a ground pin and with no reference, the electronics remained undamaged).

It felt like a great solution.
Except it suddenly meant the the electronics (and how to solder a board up) had got really complex again!