The Los Angeles Metro Gold Line train runs 20 feet behind my back patio and its noise pollutes my space a hundred times a day. Since I can’t get rid of it, I figured why not do something fun with it!

The idea is to install a microphone outside and feed the sound into a system that will apply effects to the sound and broadcast the altered waveforms within the loft from a dedicated set of speakers. After telling a few folks about this idea I got turned onto the wonderful H__R iPhone app and thats exactly the sort of audio mangling I had in mind. If it had an API I would just dedicate a used iPhone to the project but I guess I’ll use PureData or Max/MSP to do the job instead.

I didn’t want to have an obvious computer interface laying around the loft just for this but the project requires some kind of control system to provide a means to fiddle with the audio alteration. Something that will allow me or guests to change the major program, and adjust its parameters.

I’m a big fan of repurposing deeply obsolete tech or industrial parts and decided that this project would be well served by some kind of vintage test equipment with lots of knobs and switches on it. With this in mind I rolled out to Apex Electronics in Van Nuys. The salvage yard there is full of old aerospace junk where I found this Honeywell Model 2733 Potentiometer. This box is perfect with lots of knobs and switches and a great retro feel.

I read somewhere that these were used to calibrate guided missiles during the cold war but have no idea if that is in fact true. I do know that when I took it apart I found this curious looking glass tube:

A little research revealed this to be a Weston cell. I didn’t like the idea of having a bunch of cadmium and mercury hanging out in a fragile glass tube so I removed that nastiness and took it to the hazardous waste disposal facility rather than risk it ever breaking in the shop!

Major Design Goals

When I started this project I was going to base it on an Intel Edison microcontroller but since Intel has discontinued the product I decided to use a Raspberry Pi. The Edison had built in analog I/O which I need to measure the position of the various knobs but the Edison toolchain kinda sucked and the Pi is much more fun to work with, at the expense of needing to integrate an external ADC, but thats cool. The ease of dealing with the Pi more than makes up for that minor hassle.

The Pi will run a socket.io server that will transmit control change messages for each knob and switch on the front panel, allowing me to interface with it from whatever system I eventually build to do the actual audio mangling.

There are several challenges in store as the main dial (that big black wheel) is a huge wire-wound 1Ω potentiometer, and its busted. So I have to figure out the right kind of sensor and a way to mount it on the center axis of the wheel such that i can read the full range of motion it allows. Typical pots have their range of motion limited to an arc that is less than that allowed by the stops built into the wheel so that’ll be interesting.

The other pots are old and noisy as this box had been sitting in the Apex yard exposed to weather for who knows how long so they will need replacement.

I forgot to take a picture of the original wiring harness before I tore it out and started to wire the main selector switch, but here are the lovely guts of the box after I ripped most of its original wiring out and had already begun to wire the main selector switch:

Another major issue is that module on the right side. That was some very odd optical indicator with mirrors and lights in it that seems to have reflected a beam on light onto the lower window. I’m hoping to mount a small e-ink or OLED display in that location that will display some kind of status information but so far have not found any displays that fit right. I also need a solution for that curved scale on the top right. I’ve looked for round OLEDs but all the round displays I’ve seen are too small so not sure what to do about that just yet.