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===== Raspberry Pi Project – A 1960s wallbox interfaced with Sonos =====
The inspiration for this project came from @nivnov pointing me to http://wallbox.weebly.com/. I had somewhat of a different take on the circuitry and software than it’s author, Steve Devlin. The pulse train of my Wallbox was also different. This is all explained below.

==== The Wallbox ====

A wallbox is a device dating from circa 1950s USA. It was placed on tables in diners to increase revenue from Jukeboxes. It contains no music, nor does it have a speaker. It could be considered a remote control for the diner’s main Jukebox.
I opted for a Seeburg Wall-O-Matic 100 Type 3W100. Wallboxes will be heavily mechanical with very basic circuitry. They will function such that a key combination will be output as a train of electrical pulses caused by an arm rotating over a set of electrical contacts.

{{:info:juke1.jpg|}}\\

==== Powering the Wallbox ====

The wallbox would have, traditionally, been powered from the jukebox. To use it standalone for this project, an AC-AC transformer was required. My Wallbox required 25VAC at a maximum of 3 amps. I used a Torodial transformer, mounted with its mounting washers, in a die-cast aluminium box. It kicks out a lot of heat so the box acts as a decent enough heatsink.
The wall boxes are traditionally unearthed. This seemed like a bad idea so I attached an earth to it via one of the unit’s screws.

{{:info:xfmr_photo2.jpg?400|}}\\

==== Interfacing with the RasPi ====

Steve’s circuit is floating and I managed to fairly impressively melt a RasPi by inadvertently forcing 25vAC through it via the gnd. In the end, I opted for a circuit that isolated the Pi from the Wallbox. A diagram is below:

{{:info:wallbox_if_schematic.png?400|}}\\

For the uninitiated, an Opto-isolator is an LED and a light activated transistor bundled into a single chip – it’s like a relay. In my case a DIP. The left of the circuit connects to the Wallbox’s 25vAC and Signal terminals; the right connects to the RasPi. I also hooked up 4 LEDs to the Pi to provide some kind of debug output in the event of software errors.

{{:info:wallbox-pi.jpg?400|}}\\

===== Decoding Pulses =====

As I eluded to above, the key combination is electrically pulsed by a rotating arm. Steve had the good fortune of having an oscilloscope to hand; I sadly didn’t. Rather, I started on the code that would decode the pulses and, initially, used it as a timer. It was clear, after filtering jitter, that the pulse train had a clear time gap in it. In Steve’s case, this gap represented a break between the letters and numbers. In my case, it represented the 20s in a base 20 numbering system.
The pulse train is sequential to represent A1 through to K0. If X represents the number of pulses before the gap and Y represents the number of pulses after the gap, X increments from 1 to 20 whilst Y stays at 1. Y then increments and X resets back to 1. The cycle repeats through to X=20, Y=4 for K0.

The maths was thus:\\
Decrement both the pre and post gap counts as 0 is represented by 1 pulse
The letter is (2 * post-gap-count) + (1 if pre-gap-count is > 10)
If letter was worked out to be > ‘H’, add 1 as the wallbox doesn’t have an I
The number is the pre-gap-count modulus 10 (i.e. pre-gap-count % 10)
Handling kernel level interrupts came with some problems of its own. Most notably, having to think sort-of-thread-safe when writing the code. The interrupt can… well… interrupt code processing at any time. If this is part way through a calculation, things get funky. This code is what I came up with.