Power Control Panel

Last Updated: 2026-09-09 Cold-war inspired power distribution unit for the workbench.

> Background

For a long time, I've had one of those computer power strip things that fits under your monitor and has switches labeled for typical items like "computer" or "monitor" or "printer." It worked OK, although over it's 30+ year life, the switches started failing (repairable though!) and having just four outlets still resulted in a lot of plug switching for many tasks. Then last year, I happened to stop by a surplus electronics shop (Skycraft Surplus in Orlando FL, USA - a must-stop location if you're near) and picked up about $250 of random parts from every corner of the building. One item was a paddle switch like those used in the IMSAI 8080 front panel. Naturally, I took to pulling every dimension off of it and creating a CAD model that would fit over currently available toggle switches (link at bottom of page!). Then the search for a project in need of a control panel began...

> Design

The power control panel is broken into two components. The first is the switch panel. It includes two banks of four switches plus a separate "master" switch to override the others and shut off all circuits. Since I wanted the outlets to be at the back of the workbench but the panel to be at the front, I designed the front panel to connect over a serial link to the second component: a bank of relays. This separation allows the control and high voltage operation to be physically separated and placed where convenient. The relay board produces all the timing and control signals for the serial link. On board is an optoisolator with a CMOS output that generates a 60 Hz, almost-square wave from the AC line. This is fed into a hardware state machine constructed of SMT 74xx-series logic chips (no software required!). Two pairs of RS422 transceivers communicate over an RJ45 cable to the front panel, where another set of transceivers receives the control signals and controls a parallel-in, serial-out shift register. This shift register is the particular 74HCS166 and helpfully has built-in schmitt-trigger inputs, which are nice when dealing with mechanical inputs like switches. (As I'm writing this, I've realized that I did not add a capacitor to the inputs, meaning there's no debouncing! It's probably fine though since the sample rate is about 9/60 Hz.) The master override switch is implemented with a simple AND gate between the override switch and the shift output of the input shift register. On the relay board side, there is a serial-in, parallel-out shift register that drives four dual-channel inductive load drivers. These drive the relay coils that end up switching the HV AC outputs. On a side note, the SZNUD3124 drivers are a handy little device, including a CMOS-compatible input with pull-down and a built-in inductive flyback protection zener to clamp the relay coil's voltage while still allowing it to raise some, keeping the contact opening relatively quick. There is also a power-on reset generator to clear the outputs of the relay board shift register so that none of the circuits temporarily switch on before the first full reading of the input panel state can be shifted in. The AC and low voltage DC sections of the relay board are isolated via both physical gapping of the traces on the board and isolation slots cut into the substrate. A major part of designing high voltage circuitry is ensuring that both your clearance and creepage distances are obeyed for the environemnt and voltages that the circuit might experience. Briefly, clearance is the straight-line distance between two conductors at different potentials, while creepage is the shortest distance along a surface between the two conductors. The minimum creepage distance is greater than or equal to the clearance to meet the same maximum voltage potential rating. Creepage depends on a host of factors including the working voltage, pollution degree, and the materials involved. I'd highly recommend reading up on this! (e.g. IEC 62368-1) Other devices on the AC side of things include a 15A panel mount thermal breaker and a surge supressor with indicator output.

> Front Panel Construction

The front panel is designed out of two separate layers of aluminum plate. The outer plate is mainly asthetic and painted in a textured gray powder coat. It mounts to the rear plate via four square U-bolts which also act as switch guards. The rear plate is the mount point for the switches and input PCB. This dual plate design allows me to hide all the mounting holes for the switches and PCB to keep the front panel looking clean. All connections are made point-to-point and I opted to do some wire lacing for this (pictured below). If you've skipped ahead to the photos and are now back here wondering why there's a large capacitor and two resistors on each of the LEDs, they're astehtic! Each of the switches are wired with a pull-down resistor, so when the switch is "ON," the switch output is connected to +5V. In addition to feeding this back to the input PCB, the output contact goes through a 510 ohm resistor into a 100 uF capacitor to ground. On the other side of this is an amber LED, also connected to the capacitor's positive terminal through a second 510 ohm resistor. The capacitor-resistor combination adds a slight time constant to the LEDs, giving a bit of an incandescent bulb warmup and cooldown effect, giving more of that "retro" feel. (And as a fun exercise, figure out why I didn't just put the LED directly in parallel with the capacitor!) The LEDs fit within small mushroom-shaped pieces I 3D printed from a translucent plastic with a concentric infil and very fine layer height. This acts as a very nice diffuser to the LEDs and gives them that large diameter look of the old incandescent panel mount holders. There's a small black ring to clean up the edges. An interesting component to the construction of the paddle switch I acquired is that at its core, it is a standard toggle switch that you'd find with a metal bat-shapped lever. It had been modified slightly to have a cylindrical level that was a bit shorter than typical, but otherwise it looks like a standard toggle switch. It also has a metal frame around it to give the paddle a rocker point. When I transcribed the dimensions from the paddle into 3D CAD, I just changed the size of the hole for the lever to fit an off-the-shelf switch. Then a sheet metal part was designed to hold the paddle pivot at the same location as the pivot on the switch. In prototyping, I even managed to have the pivot 3D printed from PLA and it survived quite a bit of cycle testing (>1k cycles). The pivot is bent by 3D printing a small die and screwing the flat component to the die. Then the metal can be pressed (or hammered) against the die. I used this to make 20 of these, and did not see a degredation of the die piece, despite being PETG.

> Photos!

Ain't that sick? Wire lacing! (rear of control panel) Inside the desk Relay board wiring "Covered" relay panel Outside of the relay panel Oh, and the v1.0A relay board bodge ;)

> The Repo

And if you've finally made it all the way down here, congrats, you now have access to all the design files and can build your own! ElectronicsTinkerer/Power-Control-Panel