Originally built in the 1970s by the communications manufacturing company TekTone, this intercom unit had last been used in the American Felt building next to Union Square in New York City. After some number of decades of use, and who knows how many years of collecting dust in the basement of this iconic building, it was eventually salvaged by a designer who couldn’t figure out how to get it working. I purchased it off of him and spent the next several months restoring this machine into a retro-futuristic device designed to retain the beauty and characteristics of it’s original ’70s design.
Stage 1: The Tear-down
In order to understand how it worked, I first need to reverse engineering the circuit board as best as I could. The machine features a CRT monitor that would have presumably been connected to an entryway camera, a sliding switch for controlling the screen brightness, a push-button switch for turning it on and off, and a telephone (again, presumably for communicating with an entryway speaker in a building).
Connected to the telecom was a large, 12-pin connector that powered and controlled all the functionality. Tracing these pins to the circuit board we could see what triggered power, where the video and audio inputs were, and what pins controlled the speaker and mic for the phone. I also had to repair a few damaged wires and re-solder some damaged connections (including the wire that drove power, which had been severed).
Stage 2: CRT Power-Up and Signal Testing
I supplied 10.7V and GND using a tabletop power supply, initially only resulting in a high-pitched hum (typical of an older CRT where electromagnetic forces cause real mechanical vibration). With some tinkering to the power supply I was able to get a single dot to appear without supplying a video input. Good news! If the screen was working, then in theory all I needed to do was supply some video input video the video pin.
Stage 3: Interface with Raspberry Pi
To send a video signal to the CRT I wired a raspberry pi to the video pin via a 3.5mm TRRS to RCA cable. The Pi was configured to output composite video through /boot/config.txt edits, using the fbi and framebuffer libraries to display PNGs. Again, with a lot of tinkering and configuration changes, I was able to get the screen to display an image, and eventually video.
Stage 4: Handset and Audio Integration
A 3.5MM plug to USB sound card was used to integrate the speaker and mic from the phone handset. Most of the work came when trying to get the button on the handset to work. This involved wiring the handset to the GPIO of the raspberry pi in order to detect the signal, later using polling and edge detection to fine tune the pressing signal. I also added 1kΩ pull-up resistor to stabilize GPIO reading.
Stage 5: Camera Integration
The last stage of the initial testing and restoration was to integrate a camera. To do this I used a mini camera video module compatible with raspberry pis. This camera interfaces using the dedicated CSI interface, via the CSI bus, making setup extremely easy.
Stage 6: Designing and building the enclosure
With the telecom functionality (mostly) fully tested, the next step was to design an enclosure for all the new components we’ve introduced into the system (raspberry pi, camera module, usb sound card, step down converter). This would serve as the brains and powering hub for the telecom – how the user and the telecom would communicate and interact with each other. And for this reason, it needed to look the part.
The design was meant to mirror that of the telecom, mimicking the angularity and color scheme. A rotary knob was added in order to interface the OS I was designing, as well as a camera port in the top right for the camera module.
Next I printed and test fit all the internal components.
Stage 7: Designing the OS
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