Monday, November 20, 2017

Building a Seeburg Wall-O-Matic Interface (Part 6)

Developing the Software

This post is definitely not going to completely describe the software. The software is always going to be a work-in-progress, so follow the Git repository for more specific updates. Instead, its going to describe a bit about the development setup, and some basics about what the software is doing.

Wall-O-Matic Microcontroller Interface ESP8266 Code []

Preparing a development rig

The first step in this stage of the project was to actually have a development board to work with. While I could have simply used my real circuit board (as described in the previous post), doing so had a number of disadvantages:
  • Somewhat tedious to switch in-and-out of flash download mode
  • Required an extra adapter to connect to USB
  • Designed to be powered by a bulky transformer connected to AC mains
  • No way to test the coin switch outputs without a multimeter
  • No way to test the pulse decoding input without a real wallbox
In other words, using the real circuit board was not a solution well suited to an office desk environment that was outside of my electronics workbench.

To do the majority of the software development, I really only needed to test the ESP-12S itself and basic interactions on its I/O pins. Thankfully, this did not actually require the full circuit board. So, to make my life easier, I cobbled together a little breadboard development rig.

Development Rig Breadboard

For the ESP-12S part of this rig, I used an Adafruit Feather HUZZAH. Its a convenient breakout board that includes a USB port, reset button, and some nifty wiring that allows the USB serial controller to automatically switch the ESP-12S in and out of flash download startup mode.

To simulate the signal pulses of the wallbox, I used an Arduino Nano. This module also has its own USB port, and I/O pins that could easily be programmed to simulate any digital pulse stream I needed. Since I'm not testing the full filtering path, I really only need to simulate a clean series of logic-level pulses. The code that runs on this Arduino can be found here: [PulseSim.ino]

For the coin switches, three LEDs in different colors were enough. After all, I just needed to test that I could make them blink.

Since the Arduino is a 5V system, and the ESP8266 is a 3.3V system, I leveraged my existing bin of components to link these together. I used a spare opto-isolator and inverter, which did the trick. This had the benefit of making the two microcontroller modules completely isolated from each other, so they could be powered independently.

In schematic form, it looked like this:
Development Rig Schematic

Setting up the toolchain

The easiest way to get the development tools up and running was to leverage the "esp-open-sdk" project.  This project is essentially a Makefile and some scripts that downloads, compiles, and configures everything you need to write code for the ESP8266. This part was surprisingly painless, and decently covered in that project's README.

Decoding Pulses (For Real)

When I was originally figuring out the pulse protocol and filtering circuitry, I used an Arduino-based test decoder. The code for this basically used the "pulseIn()" function in a loop. Upon further investigation, I found that "pulseIn()" just busy-waits on the I/O pin. While perfectly fine for basic testing, this was definitely not a good solution for a complete system.

The approach I took here involved configuring an interrupt to trigger on any transition on the signal pulse's input pin. Inside the interrupt handler function, I then populated an array with the pulse durations and pulse gap widths and kicked off a timer.  If no pulses happened for a certain amount of time, the timer callback function would then inspect the array and determine what the song selection was. This approach ended up working quite well.

The code that implements this is here: user_wb_selection.c

Inserting Coins (For Real)

While precise timing is really not necessary for the wallbox's coin switch mechanism, I still went and measured how long a real coin would actually trip the switches for (40-60ms). I then wrote some simple timer functions that would trigger the selected coin switch for the appropriate amount of time.

The code that implements this is here: user_wb_credit.c

Interfacing with Sonos

This is the step where things started to get complicated. Some of the blog posts I read at the outset of this project had led me to believe it was just a matter of spitting the right HTTP POST at a Sonos speaker's IP address, but is anything actually that simple?  Especially if you want a robust and reliable solution?

As I worked through this process, I had decided that there were certain things I definitely wanted to have:
  • Sonos device configuration should be easy (i.e. no hard-coded IP addresses)
  • Can't make any assumptions as to the state of the Sonos speaker
  • The Sonos device's playlist should be treated as if it was a jukebox
The goal was not only to make setup easy, but to also ensure that I should never have to fiddle the Sonos app on my phone to get things into (or back into) a functional state. The Wall-O-Matic interface should function as standalone as technically possible.
Of course doing all of this meant that I needed to figure out quite a bit more of the Sonos protocol than a single enqueue request.

To figure all this out, I spent a lot of time staring at Wireshark while fiddling with both the official Sonos app and the node-sonos-http-api project's utility commands.

I'm not going to even attempt to document the Sonos protocols here, as that would take way too much time and effort to write up. I'll just be giving a short summary of the parts I used, then linking to my code that implements them.

Discovery Protocol

The Sonos discovery protocol is based on SSDP, and involves two kinds of device discovery: explicit searches and notifications.

Explicit searches begin by sending a UDP broadcast to IP address (port 1900) with the following payload:

MAN: ssdp:discover
MX: 1
ST: urn:schemas-upnp-org:device:ZonePlayer:1

All Sonos devices on the network will then reply to the sender, by sending something like this to the source port of the discovery request:

HTTP/1.1 200 OK
CACHE-CONTROL: max-age = 1800
SERVER: Linux UPnP/1.0 Sonos/37.12-45270 (ZP120)
ST: urn:schemas-upnp-org:device:ZonePlayer:1
USN: uuid:RINCON_D6E477CEC684A1400::urn-schemas-upnp-org:device:ZonePlayer:1
. . .

Additionally, Sonos devices will periodically broadcast their presence on port 1900 without any discovery requests. When that happens, the payload looks similar:

CACHE-CONTROL: max-age = 1800
NT: upnp:rootdevice
NTS: ssdp:alive
SERVER: Linux UPnP/1.0 Sonos/37.12-45270 (ZP120)
USN: uuid:RINCON_D6E477CEC684A1400::upnp:rootdevice
. . .

Regardless of how it came in, I collected the IP address, port, and UUID from these payloads. Then, to get the zone name itself, I made an HTTP GET to the following URL:

The response to this was a big XML blob that contained the zone name, as well as other device information.

This info was then collected in a data structure, and later used as follows:
  • IP and Port - Used to actually connect to the device and to send it commands
  • UUID - Used as the reference to the device kept in the software configuration, and used as part of certain control requests. 
  • Zone Name - Used for display purposes, so the UI could give friendly information on which zone was selected.
The code that implements this is here: user_sonos_discovery.c

Control Requests

To instruct the Sonos device to actually do things (e.g. add song to queue, play, query status, etc), I had to implement the following control requests:
  • AddURIToQueue - Adds a file or stream URI to the playback queue
  • GetPositionInfo - Gets position info on the current track
  • SetAVTransportURI - Sets the transport URI to play (e.g. stream source or local file queue)
  • Seek - Select the queued track to play
  • Play - Start or resume playback
The code that implements this is here: user_sonos_request.c

Event Subscriptions

Unfortunately, the normal control interface provides no way to distinguish between the playing and paused states. The only way to do this, was to listen for event notifications. There's another protocol Sonos devices support where you can tell them to notify you whenever there's a state change. Thankfully, these notifications include the play/paused state.

The code that implements this is here: user_sonos_listener.c

Putting it all together

At boot, the device discovery listener is started and a discovery request is broadcast. As soon as the configured Sonos device UUID is detected, it is set as the active device and an event subscription request is made.

When a song selection is made on the wallbox, the following sequence of operations are initiated:
  • Construct a track URI for the selected song
  • Add the track URI to the Sonos device queue (AddURIToQueue)
  • Get the current position info (GetPositionInfo)
  • If the current transport is not a file URI, then switch to the local queue (SetAVTransportURI)
  • If the current transport is a file URI, and currently playing, then do nothing
  • Otherwise:
    • If not currently playing, start playing
    • If on a previous track, and not currently playing, then seek to the added track and start playing
    • If on a previous track, and paused, then seek to the next (or added) track and start playing.

The code that implements this is here: user_sonos_client.c

Providing a User Interface

At runtime, the wallbox itself kinda is the user interface for the project. Well, with one exception: inserting coins. I needed some way to instruct my board to "insert a nickel/dime/quarter" remotely, either via a manual interaction (i.e. click a button on an app/website) or via some sort of home automation setup. To accomplish this, I decided I'd simply expose some sort of basic HTTP API for triggering the coin switch relays.

At setup time, it was a different story. There was actually a lot to configure for this thing to work correctly:
  • Wi-Fi Network Setup
  • Selecting the Sonos device to play through
  • Selecting the type of the connected wallbox
  • Configuring which song file to play for each wallbox selection code
Some might simply bake all this into the firmware. I really didn't want to do this, since you'd never design a "real" product like that.

So the first thing I did, was to embed a web server into the device. To do this, I leveraged the libesphttpd project (and its sample esphttpd app). This project provides a simple webserver written against the ESP APIs, and a fair amount of sample/utility code to handle support functions (Wi-Fi setup and firmware updating) that I knew I was going to need.

As I began to explore this project, I soon noticed that it hadn't been updated in a while. However, there seemed to be a number of forks that had been picking up improvements and fixes. Ultimately, I settled on using Chris Morgan's fork of the project.

After several iterations, I ended up with a (mobile friendly) landing page that looked like this:
Local Web Interface

The setup and firmware pages were pretty much straight lifts from the esphttpd sample project, albeit with some minor style tweaks. The Sonos zone selection page was also fairly simple.  The most complex of these pages was the "Wallbox Configuration" page:

Wallbox Configuration Page
The "wallbox type" determines how many song selections there are, how they're numbered, and what the format of the wallbox's signal pulses are. (Contrary to a previous post, I did eventually get the model "200" mostly working and added code to support its signal pulse format.  The format is different from the "100", and actually slightly simpler.)

The (long) "base folder path" and the (short) "song" filenames are combined to form the playback URI for each song selection. Buttons help automate the otherwise-tedious entry process.

The layout of this page reflects the approach I took towards configuring the playback URIs. While I could have allowed a completely custom per-song-selection URI, doing so would have required quite a lot of configuration memory.

The ESP SDK has APIs for "safe" persistent data work in triplets of 4KB flash pages. So 50KB of URI data (for a 200-selection wallbox) would inflate to 150KB of flash.  With a custom replacement for those built-in APIs, I could probably drop this down a bit, but it would still be more complexity than I wanted to deal with.

So instead, I decided that it made more sense to prepare a folder on the file server just for wallbox-triggered playback. Given that you'd have to organize your music by song codes anyways (to print title strips), it didn't seem like that big of a deal. This way you'd have a single "wallbox" folder with simple and short file names,

By doing this, on-device configuration was mostly automatic. In my own setup, I really only had to configure the base path. For the songs, I just went with the default names and only changed them if my files were something other than MP3s (like M4A or FLAC).

The code that implements this is here: user_webserver.c
The associated HTML and assets are here: html/


Combining everything above with some nasty looking Perl CGI on my home webserver, and fumbling my way through Google Assistant and IFTTT, I eventually ended up with something like this:

Monday, November 06, 2017

Building a Seeburg Wall-O-Matic Interface (Part 5)

Designing and Building the Circuit

This is where everything from Part 3 [Decoding the Pulses] and Part 4 [Inserting Coins] finally comes together into a complete circuit board design. Most of the work here actually took place simultaneously with what I've already described in those posts, so I won't rehash their respective elements. Basically, I did the overall design, then tested and proved out the individual elements on a breadboard, then put it all together into a final schematic and printed circuit board (PCB) layout.

Completing the Schematic

Early Iterations

It all began with a piece of scrap paper and a pencil... Starting with what I had seen on another blog posts about a similar projects, and adding in some of my own ideas about coin switch triggering, a rough schematic started to take shape:
The Pencil Sketch
At this point there were a lot of uncertainties and omissions. I hadn't yet figured out how I was going to filter the signal pulses, wasn't sure which relays I was going to use to toggle the coin switches, and apparently forgot that the relays would actually need a regulated DC voltage to trigger them. Interfacing a microcontroller was also something still far off in the future.

Not long after, I whipped out KiCad and started to layout a real schematic. I selected and tested components for filtering the signal pulses and driving relays to toggle the coin switches. As I proved things out on the breadboard in my lab, I tweaked the schematic until it ended up like this:
Early KiCad Schematic
Most of the components came straight from KiCad's libraries, but a few required some level of customization. KiCad didn't have any diode-protected relay symbols, so I had to make them. KiCad did have a ton of similar-looking rectifier bridge symbols, though, but it took a while to figure out which one best matched what I was using.

Of course, because of the constant changes, the wire routing quickly became a disorganized mess. At this point I decided to pull the various sections apart, and clean up the schematic.

I was also somewhat annoyed that the component for the 74HC14 (Schmitt trigger) didn't expose the power rails in any obvious way. This posed two problems for me: First, unlike many schematics, I had multiple power rails that needed to be distinct and separate. Second, I wanted an obvious place to put a bypass capacitor. Thankfully the newer KiCad libraries (via their Git repos) had redesigned the schematic symbols for this part, and now include a separate "unit" just for connecting power. So I kludged that library into my project, and forged ahead.

The microcontroller was still a bit of an unknown on the design, but I knew I was likely to need an isolated 3.3V supply for it. After some searching, I went ahead and threw in a DC-DC converter (NCS1S2403SC). It also required a custom symbol, albeit a rather easy one to create. The part was a tad pricey, and probably overkill, but it would do the job.

Design More Complete
(At the time I captured this iteration of the schematic, I had obviously forgotten to place a rectifier or any capacitors on the input of the DC-DC converter. I did remember to add those in later.)

Selecting a Microcontroller

Up until this point, I wasn't really sure what kind of microcontroller I wanted to use for this project. Other people have used Raspberry Pi boards, but I was fairly sure I didn't want to do that if it could be avoided. I really wanted something I could more tightly integrate, and that would be more purpose-built. Having a whole Linux system, complete with OS maintenance and startup/shutdown concerns seemed like overkill.

After watching a few too many EEVblog videos about a Nixie Tube Display Project, I became aware of this wonderful little Wi-Fi enabled microcontroller module known as the ESP8266. These modules are typically packaged onto surface mount carrier modules (which package the antenna, memory, and FCC certifications, etc). In Dave's project, he used a WEMOS D1 mini carrier board. This board packaged an ESP-12S module with a few necessary support components (USB port, reset button, etc) and a place to put pin headers.

Since I was unable to directly order that specific module when I was at this stage of design, I went ahead and instead ordered an Adafruit Feather HUZZAH. Its slightly larger, and has a few more support components I didn't really need, but is essentially the same thing. My intent was to use this module as a breadboard-friendly option for development and testing of the software, which I'll go into detail on in a future blog post.

For the actual board, however, I decided to just use an ESP-12S module directly.

Finalizing Revision A

The final step in getting the design ready was to include the ESP-12S into my schematic. Thankfully I found an existing KiCad part library that included the ESP-12E, which I could trivially edit into the part I needed. For the rest of it, I just looked at a variety of "getting started with the ESP8266" blog posts and various schematics for existing carrier boards. All I really needed was a few pull-up resistors, a reset button, a jumper for putting the thing into the flash download startup mode, and a pin header for connecting to the UART.

I didn't bother including a USB controller directly on my board, since it wasn't something I was going to need at runtime. Instead, I opted to just buy an external CP2102 adapter that I could plug in whenever I wanted to download updated firmware.

Eventually I ended up with this schematic:
Ready For Layout

Laying out the PCB

Finding all the Footprints

The first step towards laying out a PCB is making sure you have accurate footprints assigned to all of your components. Depending on what you're building, this is often a mixture of trivial and frustrating.

For the resistors, I knew I was going to use standard-sized "jellybean" 1/4 watt parts from my collection, so I selected reasonable footprints that made sense. (Of course KiCad calls this footprint "R_Axial_DIN0207_L6.3mm_D2.5mm_P10.16mm_Horizontal", so it did take a little bit of work to decide that was the best choice from the library.)

For the capacitors, it was a little bit more of a challenge. I had a collection of usable parts for many of them, but not necessarily of known dimensions. Just to play it safe here, I decided to source all of my capacitors from fresh Digi-Key orders using quality brands. That way I would actually know the specific dimensions of the footprints for each one.

The rectifier bridge used an unusual package type (WOG) that wasn't in the KiCad library. I managed to track down someone's library that contained the right footprint. Making sure the pins all matched the schematic symbol still took some effort, as this part doesn't actually have numbered pins.

The relays were the first fully "custom" footprint that I had to create myself. Fortunately, they were just trivial modification of a common package. I took a DIP-14, deleted the 3 middle pins on either side, and made sure the pin numbering matched up with my schematic symbol.

The DC-DC converter was also a custom footprint, and one that required a bit more work. I found an existing footprint for a similar component from the same manufacturer, and tweaked its footprint based on the datasheet for my component.

Thankfully the component library I based my ESP-12S off of also contained a footprint. I just had to make a few minor modifications to remove certain pins, and I was all set.

Getting it all routed

Now that I had all of my component footprint data sorted out, it was time to actually layout and route the printed circuit board (PCB). I soon discovered that KiCad's PCB designer is actually pretty bare in its choices for things like trace sizes, drill sizes, via sizes, design rules, and the like. Thankfully, I found a few excellent resources that helped me configure it for this project:
After spending an afternoon laying out all of my components, and routing all the traces between them, I finally had a PCB design!
PCB Layout (Revision A)

KiCad's 3D view was also pretty cool, and helped with keeping the actual look of the board in mind. I didn't bother creating or fixing any missing/incorrect 3D models, but its still worth showing off:
PCB 3D View

Fabrication Time!

Sending off to OSH Park

I had decided to use OSH Park for making my PCB, mostly because of how easy they make the whole process. I basically just had to upload my ".kicad_pcb" file on their website, and they did the rest.

OSH Park PCB Preview

Finding Design Issues

While I was waiting for the PCB to come back from OSH Park, I decided to spend some more time testing and analyzing my design. During this process, a few initial issues jumped out at me:
  • The Zener diode I was considering as a voltage clamp on the output of the signal rectifier was a bad idea. It was based on a misunderstanding of a suggestion I saw on a website, and didn't actually work for my use case. This is the one element of the schematic I did not test before doing the PCB layout, and I really should have.
  • The relays were connecting the coin switches to the Common terminal, rather than to 25VAC. This mistake can apparently be traced all the way back through every one of my schematics, even though my bench test breadboard was doing it right.
  • The ESP-12S could use another button so I'd have a way of causing my software to start in a "setup" mode. Without doing this, reflashing would be the only way to recover from a broken Wi-Fi configuration.
The first two issues thankfully could be "patched" as part of the board assembly process. The third issue was something I could live with, but would address in a future revision.

Building the Board

After an uncomfortably long (but not really that long) wait, I got the manufactured board back from OSH Park:
Actual PCB (Revision A)
While assembling the board, I took copious notes. There were many things I wanted to verify due to upfront uncertainty, and many other things I discovered during the assembly process. If I was going to do another revision, I wanted to make sure I got all the kinks out.

Fully Assembled PCB (Revision A)

Issues discovered during assembly:
  • Diode bridge (WOG) footprints had holes and pads that were almost too small. (I managed to make the parts fit, but it was extremely tight.)
  • DC-DC converter's silkscreen was all wrong. (I expected this when I first figured out the footprint. Thankfully this one was just cosmetic.)
  • DC-DC converter probably should have filter capacitors on its output. (Just in case.)
  • Opto-isolator pins were a bit too wide, and took extra bending to fit. (It turned out I had ordered "DIP-4M" parts, whereas I used "DIP-4" footprints.)
  • UART pin header (J3) had the +V and Gnd silkscreen labels backwards (Big problem if I hadn't caught it, trivial to work around now that I know about it.
  • The board had tons of dead space, and was way larger than it needed to be.
After assembly, once I got the board working, I made another discovery. Apparently, my ESP-12S (ordered from Electrodragon) had only 1MB of flash. I had been led to believe that the ESP-12S comes with 4MB of flash, so this was quite annoying. Just to be safe, I decided that any future ESP-12S parts would come from another supplier that actually said 4MB on their product page (e.g. Adafruit).

Revision B

Okay, I'll admit that a second PCB revision is kinda silly for a hobby project like this. However, I was taking the design process for this project far more seriously than your average one-off hobby build. So what the heck...

Small Schematic Changes

I fed all the design changes from the previous sections back into the schematic, and arrived at a more "final" revision B:
Schematic (Revision B)

Compacting the Layout

I then spent some more time moving components around, and got the layout quite a bit more compact:
PCB Layout (Revision B)

Running the numbers, this turned out to be more than 8.5 square inches smaller than the Revision A layout!

Back to OSH Park

This time it was simply a matter of uploading and ordering. Since I has gotten the kinks out, I also felt confident enough to actually share the design:
OSH Park ~ wallbox.kicad_pcb

OSH Park PCB Preview (Revision B)

Building the Board

This time assembly was very straightforward. All the issues I had discovered in the last revision were resolved, so it was simply a matter of soldering in components.
Assembled PCB (Revision B)
Well, it was mostly simple. At least until I got to the ESP-12S module itself. This time I had the bright idea to attempt "proper" SMD soldering with solder paste. Everything seemed to go fine until I was actually ready to switch the power on... And it didn't work...

During the painful troubleshooting process, I discovered that all the pins on one side of the module were apparently shorted to ground. I tried reheating them with my soldering iron, using solder wick to clean things up, and even using a desoldering gun to suck away excess solder. Nothing seemed to fix the shorts.

I then had this crazy idea to point my ridiculously oversized hot air gun at the thing, in an attempt to reflow the solder. For a long time, nothing seemed to be happening. Then, out of nowhere, the whole ESP-12S module popped right off the PCB!

At this point I tested both the PCB and the module, and couldn't find any evidence of shorts. So I resoldered it the "old fashioned way," and it started working. Phew!

Since the ESP-12S module is not a "real" SMD part, but rather a surface-mountable carrier PCB, I suspect the PCB "sandwich" prevented the solder paste from correctly reflowing around some of the pins.


One thing I made sure to do throughout this entire process, was to keep a detailed and accurate bill-of-materials (BOM) for the project. Initially I did this with a plain text file, but eventually I did the grunt work to move all the data into the KiCad schematic itself. The BOM included part descriptions, datasheet links, part numbers, and vendor information. This made it easy to use various companion tools to generate HTML or CSV versions for easy sharing and part ordering, and provided a handy reference during assembly and testing.

Bill-Of-Materials Snippet

Project Resources

Tuesday, October 31, 2017

Building a Seeburg Wall-O-Matic Interface (Part 4)

Inserting Coins

Credit Mechanism

The whole process of accepting coins into the wallbox is comprised of at least three major components:

Slug Rejector

The slug rejector is a mechanism that uses magnetic fields and mechanical components to determine whether an actual coin has been inserted, or simply a "slug" of similar dimensions. Its operation is perhaps best explained by this excerpt:

Here's what my wallbox's slug rejector actually looks like:
Wall-O-Matic 100 Slug Rejector

Coin Switch Assembly

Once a coin has successfully passed through the slug rejector, it hits one of the three coin switches within the coin switch assembly:
These switches toggle different components of the credit assembly, depending on what kind of coin was inserted.

Credit Assembly

When the coin switches are toggled, the credit assembly electromechanically counts the "credits" using gears and solenoids:
Credit Solenoid & Switch Assembly
This is one area where the model 200 is significantly more complicated, as it uses a "dual credit assembly" that is capable of assigning different credit amounts to different songs. The mechanism in the model 100 (shown above) is much simpler.

Once sufficient credit has been recorded, the program light is illuminated and the selector buttons are unlocked.

Relay Circuit

Fairly early on in this project, it was clear that I didn't actually want family and friends to have to put actual coins into the wallbox. Sure, its kinda neat to do every once in a while, but I'd rather not have keep a pile of coins nearby and constantly open it to empty the coin tray. Thankfully, simulating a coin drop is actually quite easy. All you have to do is solder some wires to the coin switch terminals, and momentarily short them as if a switch had been toggled.

Coin Switch Assembly /w Control Wires

Now the easy way to do this would simply be to install a button somewhere. But what's the fun in that? If I'm going to be connecting a microcontroller to this thing anyways, I might as well incorporate the "coin drop" into its functionality.

Circuit Design

In theory, this is actually quite easy to do. You just need to bring wires from the coin switches to outside the wallbox, and connect them to a few relays (one for each switch).

In practice, there were some additional considerations:
  • I wanted to maintain complete isolation from the microcontroller side to the wallbox side, just like I was doing for the signal pulses
  • I needed to drive the relays with something like "logic level" signals
  • Relays capable of doing the job needed at least 5VDC to function
The first thing I needed was a 5VDC power supply that could exist on the "wallbox side" of the system. So I built this fairly standard over-engineered LM7805 setup:
Relay Power Supply

The next thing I needed were three instances of this opto-isolated relay driving circuit:
Relay Circuit (x3)
Its important to note that the +5V and Ground rails from these two schematics exist entirely within the confines of the relay portions of the system. They don't connect to anything else.

While this all looks fairly straightforward, I did run into some interesting problems figuring all of this out.

Making it work

The first problem was getting the relays to actually trigger. These relays were designed with diode protection, something I'd never used before, and thus only worked if connected in a single direction. They are also reed-switch relays, and don't actually make any noise when switching. Throughout most of this process, I had a multimeter in continuity-beep mode with its probes across the relay's load pins. It was quite satisfying when I finally got it to beep.

The second, and perhaps larger problem, was actually getting the relays to trigger when running off my own driver circuit. I could get them to work off my bench power supply, but every time I used my own circuit it was a complete fail. It seemed as though I was having issues getting the 5V supply to work correctly.

During my initial frustrating attempts, I somehow managed to short something and blow up or fry a couple regulators and relays. When it still didn't work, I thought the regulator must not like the oscillations of rectified AC and needed a bigger input capacitor. So I put the biggest capacitor I could find across the inputs... A 6600uF 25V monster that was in my bin... All of a sudden, everything seemed to be working... And then it exploded...

After picking up most of the larger pieces, my breadboard looked like this:
Breadboard After Capacitor Explosion

It then occurred to me that I was probably over-driving that capacitor. The output of my diode bridge was supposed to be a rectified 25VAC wave, which should have been right on the edge. Of course AC voltage is actually measured by RMS, not peak. So doing the math, my peak was actually closer to 35V.

Of course once I measured things, it was actually even worse than that. Everything up until this point had been assuming my main transformer was getting 115VAC and outputting 25VAC. Close, but not quite:

Mains AC
Transformer Output AC
Yep, I was actually driving the capacitor (and regulator) with almost 40V peak. No wonder it went boom a little while after it charged up.

The next thing I did was try again with a capacitor rated for 50V. This time nothing blew up, thankfully. However, the relays also didn't work. I then began to notice a very odd problem. Apparently the regulator consistently produced a 5V output whenever I measured it at idle, but instantly dropped to around 1V whenever I put any load (including a simple resistor) across it.

Time to look at the datasheet:
LM7805CT Absolute Maximum Ratings
Okay, looks like I was driving this thing just a few volts past its maximum rating. Thankfully not enough to release any more magic smoke, but apparently enough to make the regulator malfunction.

Fortunately, I had one more option. Like most transformers, mine had a center tap. I didn't originally think I was going to need it, but now it seemed like a useful option. Measuring from the center tap, I had a more reasonable input voltage to work with:
Transformer Center Tap AC
Once I used this as my regulator input, everything started to work just fine.

Next Steps

Now that I have major components of the actual wallbox interface circuitry all figured out, the next step is to put them together into a complete circuit and system design. Of course that's a topic for another blog post.

Monday, October 30, 2017

Building a Seeburg Wall-O-Matic Interface (Part 3)

Decoding the Pulses

What the signal looks like

The way these wallboxes signal a song selection might seem a little weird from a modern perspective. When you press the buttons on the front, a collection of contacts are closed and a motor wipes a metal contactor across a studded disk:

Wall-O-Matic 100 Contact Wiper Mechanism

On the other side of this is a signal wire coming out of the wallbox. On that signal wire, you basically get a stream of pulses corresponding to the selection. Of course, those pulses aren't really a clean square wave. Rather, they're slightly noisy 25VAC. If you hook the signal wire up to an oscilloscope, it looks something like this:

Raw pulsed AC waveform

What the other projects did

Each of the other projects I looked at did things a little bit differently, but they all had a common theme: Rectify the AC, make sure its levels were brought in line with something a microcontroller could handle, and figure out the rest in software. Some of these projects also used an opto-isolator, so that sensitive electronics couldn't be damaged by crap coming from the wallbox.

The basic schematic looked something like this:

Rectifier, Regulator, Resistors, and Opto-Isolator
While this approach can be made to work, there's a fair amount of noise you have to account for in software. The pulses are not contiguous, and they are coming from a mechanism that is fundamentally prone to contact bounce.

Preprocessing the signal

Most of the elements of the basic design seemed good to me. I liked using a voltage regulator to bring down the levels, and I liked the idea of isolating the wallbox from the microcontroller. However, I didn't like the idea of having to reliably decode a pulse stream out of noisy rectified AC. With some additional circuitry, I figured that I could get to a significantly cleaner signal.

Circuit diagram

It took a fair amount of research and experimentation to come up with this, but here's the circuit I ended up with. On the input side, it takes pulsed AC from the wallbox's mechanism. On the output side, you get a clean digital pulse stream that is suitable for triggering interrupts and counting with minimal fuss.
Signal Processing Circuit

This circuit adds two main elements on top of the previous designs. On the input side, it adds an an appropriately sized capacitor. This capacitor's purpose is to smooth out the rectified wave just enough that it is invisible on the other side of the rectifier, but not so much that it obscures the pulse gaps. On the output side, it adds an RC debouncer designed to make sure the pulses are stable and have clean transitions. (I have to give credit to Jack Ganssle's page on the topic, for providing one of the most useful explanations and examples for figuring out this part.)

Tour of oscilloscope screenshots

Probably the clearest way to explain what this circuit does, is to actually show what the pulses look like across its elements. So here goes, with a sequence of oscilloscope screenshots:

Output of full wave bridge rectifier

Rectifier output with 2.2uF capacitor
Output of KA78R33 voltage regulator
Output of TPC817C opto-isolator
Input of 74HC14 Schmitt trigger
Output of 74HC14 Schmitt trigger

Figuring out the protocol

Reverse engineering

Once I had a clean digital-friendly output, it was time to document the actual protocol. I began by capturing traces like the ones shown below, for a wide range of song selections, while taking notes. Keep in mind that this is specific to the model 100 unit I was working with, and that its entirely possible that other units generate different looking pulse streams.

Pulse stream (Song A-6)
Pulse stream (Song B-6)

I made the following observations:
  • Pulses appear to be in two groups
  • Each pulse is ~50ms wide
  • If the first group has 10 pulses or less, the groups are separated by a long (~814ms) pulse
  • If there are more than than 10 pulses in the first group, then the groups are separated by a medium (~174ms) gap
  • The full pulse sequence is ~2.1 seconds in duration
  • The first group has 1-10, 12-21 pulses, and appears to be the least-significant figure
  • The second group has 1-5 pulses, and appears to be the most significant figure
If I chart this out to see how it maps to the song selection buttons, I end up with a sequence like this:
    A1 ( 1, 1), A2 ( 2, 1), ..., A10 (10, 1)
    B1 (12, 1), B2 (13, 1), ..., B10 (21, 1)
    C1 ( 1, 2), C2 ( 2, 2), ..., C10 (10, 2)
    D1 (12, 2), D2 (13, 2), ..., D10 (21, 2)
    (Note: The letter 'I' is skipped.)

From this information, a pulse decoding function can be written!

Reading the manual

I later discovered that the service manual actually did contain an excerpt explaining how these pulses work. In case anyone is curious, I've reflowed and pasted it below:

From this I gather that they were only really paying attention to the rising edge of the pulses. I'm glad I analyzed the full details, however, as it makes it easier for a robust decoder that can reject invalid/stalled/flaky selection sequences. It also makes it possible to implement short-but-effective timeouts depending on where in the pulse sequence we are.

Test decoding

Many years ago at an unrelated tech conference, I managed to acquire an Arduino Uno. It basically sat in its box until a few months ago, when I realized it could be useful as a "bench tinkering" microcontroller.
Arduino Uno
Despite never having used an Arduino before, this little device turned out to be the perfect way of testing my pulse decoding logic. I plugged it into the output of the signal processing circuit (built on a breadboard) from above, and whipped up a quick-and-dirty sketch that can successfully decode the pulses.

   Seeburg 3WA Wall-O-Matic 100
   Test sketch

const unsigned long USEC_PER_SEC = 1000000;
const int pin = 7;

void setup() {
  pinMode(pin, INPUT);
  Serial.println("Wall-O-Matic Pulse Tester");

void loop() {
  unsigned long lastTimeMs = millis();
  unsigned long durationUs;
  durationUs = pulseIn(pin, HIGH, 5 * USEC_PER_SEC);
  unsigned long pulseTimeMs = millis();
  if (durationUs == 0) {

  int p1 = 0;
  int p2 = 0;
  bool delimiter = false;
  Serial.println("Start of pulses...");

  do {
    unsigned long elapsed = (pulseTimeMs - lastTimeMs) - (durationUs / 1000);
    lastTimeMs = pulseTimeMs;
    Serial.print("Pulse: ");
    if (durationUs < 1000) {
      Serial.print(durationUs, DEC);
    } else {
      Serial.print(durationUs / 1000, DEC);
    Serial.print(", elapsed: ");
    Serial.print(elapsed, DEC);

    if (p1 > 0 && !delimiter && (durationUs / 1000) > 500) {
      delimiter = true;
      Serial.println("----DELIMITER (PULSE)----");
    else {
      if (p1 > 0 && !delimiter && elapsed > 100) {
        delimiter = true;
        Serial.println("----DELIMITER (GAP)----");
      if (!delimiter) {
      else {
    durationUs = pulseIn(pin, HIGH, (delimiter ? 1 : 3) * USEC_PER_SEC);
    pulseTimeMs = millis();
  } while (durationUs > 0);

  Serial.print("-> Signal: ");
  Serial.print(p1, DEC);
  Serial.print(", ");
  Serial.print(p2, DEC);

  if (p2 < 1 || p2 > 5) {
    Serial.println("Pulse 2 invalid value");

  char letter;
  int number;
  if (p1 >= 1 && p1 <= 10) {
    number = p1;
    letter = 'A' + (p2 - 1) * 2;
  else if (p1 >= 12 && p1 <= 21) {
    number = p1 - 11;
    letter = 'A' + ((p2 - 1) * 2) + 1;
  else {
    Serial.println("Pulse 1 invalid value");

  // Skipping 'I' for some reason
  if (letter > 'H') { letter++; }

  Serial.print("-> Song: ");
  Serial.print(number, DEC);

Concluding thoughts

Even though this post flows from start to finish, there was actually a lot of back-and-forth as I figured everything out. Part-way through the process, I upgraded from an ancient low-end analog oscilloscope to a modern digital storage oscilloscope. This tooling upgrade made a huge difference in my ability to experiment and refine this design. It enabled me to actually see all the signal transitions and glitches, and to determine all the necessary components to get to a clean pulse train. Early on, the Arduino code was actually capturing (and attempting to overcome) a lot of signal noise. The final version, however, can pretty much ignore it as a factor.

While this was a lengthy post, there's definitely more to come.

Tuesday, October 24, 2017

Building a Seeburg Wall-O-Matic Interface (Part 2)

Procuring a Functional Wallbox

Providing Power

Before you can do very much with a project like this, you need a way of powering up the wallboxes. These things don't run off the mains, nor do they use DC. Rather, these devices run off 25VAC. While you likely don't have a power supply just laying around that can provide this, its pretty simple to assemble one. You really just need an appropriate transformer and a few (optional) support components. This then connects up to a terminal strip right inside the wallbox itself.

In my setup, I used the following components:
I just wired these together as shown in the above schematic, using some heat shrink tubing and electrical tape to cover up the exposed contacts. Specifics aren't all that important here. All that really matters is that you get approximately 25VAC on the output, and won't fry something if there is a short.

 (Note: I don't actually have the rocker switch in this version, as I just used a switchable power strip. I do plan to add one on the final version.)

Okay, now to get started...

Seeburg Wall-O-Matic 200

This journey began with the acquisition of a Seeburg Wall-O-Matic (V-3WA) 200 from a rather nice eBay listing. It appeared clean, in good condition, chrome intact, and with the key. On the surface, it seemed like everything I needed to get started:

Unfortunately this wallbox basically sat untouched for a few weeks, since I still had to buy the necessary components (shown above) to power it up and I was preoccupied with other things at the time.

When I finally powered it up, the first thing I discovered was that multiple light bulbs needed replacing. That much was no big deal, so I just ordered them off Amazon (#51 and #55 bayonet mount light bulbs). I then discovered that the electrical contacts were dirty, the mechanics needed a little fiddling, and it didn't seem to be working correctly.

Thankfully it wasn't too hard to find the repair manual online. Of course it was written in 50's speak, and it was sometimes hard to match the terms and illustrations to what I was seeing inside the actual device.

I spent the next week or so in a state of constant frustration. I replaced the bulbs, cleaned all the contacts, tried to adjust and/or understand what parts of the mechanism I could, kept cursing at the DCU ("dual credit unit") that I was afraid to disassemble, and eventually sorta got it half-working. I got it to the point where I could manually toggle the coin switches and punch in a selection. Of course it would get stuck part-way through the signaling cycle half the time, and I'm not sure if it worked consistently with actual coins. (It was also dirty enough that I felt the need to wash my hands every time I was done fussing with it.)

From all of this, at least I learned quite a bit about how these devices operate. These things were designed in an era that pre-dates "electronics" as we know them, and are electro-mechanical in nature. They use a complex assortment of gears, cams, metal strip contactor switches, motors, and solenoids to accomplish what you'd do today in a single $0.50 microcontroller. (Even if it was only the 1970's, chances are you'd do this with a small assortment of transistors and logic chips.)

Eventually, I decided it was in my best interest to give up for now. I didn't feel comfortable disassembling the parts that needed the most attention, and I really didn't want to focus all of my energy on this stage of the project. So I decided to just go ahead and actually order a known clean/functional unit, from a dealer that actually specializes in this sort of thing. I can always return to this unit later, and it'll make a nice display piece regardless.

Seeburg Wall-O-Matic 100

This journey continued with me ordering a Seeburg Wall-O-Matic (3W-1) 100 from an actual retro equipment dealer. This time at least I knew I was getting something that had been cleaned and lubricated on the inside, in addition to being in good condition on the outside.

Okay, the buttons could probably use some restoration or replacement, but the rest of it looked excellent. Especially on the inside...

When I powered this unit up, everything magically worked. Okay, I might have had to fiddle with the coin switches a little bit, but those are easy to knock out of place simply by removing the title strip and coin rejector assemblies. Regardless, I was quite happy. I now had a fully functional and reliable wallbox I could use as a foundation for the next stage of the project.

Besides simply being clean, lubricated, and functional, this model had another big advantage over the 200. Its mechanism is a lot simpler. It doesn't have an overly complex "credit unit" in the middle, and I don't think I'd be afraid to try disassembling any of its mechanism if I needed to.

I did later discover this unit had a few modifications done to its coin mechanism, however. It doesn't accept dimes (only nickels and quarters), two of the coin switches were tied together, and one of the coin solenoids was disconnected. I wish these modifications hadn't been done, but they're not a showstopper. I can easily live with them. They basically mean that the device now has only two credit states: A dime adds one song credit, and a quarter adds two song credits.


Repair manual for the 100
Repair manual for the 200