Introduction: Rotary USB Selector

The purpose of this project is to create an easy way to select music in a car without needing to look at the stereo or media sources. I typically only listen to a single album on shorter journeys, and my previous solution to changing albums was to judge the music I was looking for by the feel of the flash drives in a small cup in my car's cup holder. I use separate drives for each album because navigating through anything more than 30 songs using just "next" and "previous" buttons becomes very tedious.

This temporarily evolved into a wooden USB holder that fitted better in the cup holder so that I could identify drives based on their position instead (I ordered a batch of low profile micro SD adapters for $1 because micro SD cards are typically cheaper than low capacity flash drives), but it still wasn't the most elegant solution.

Ideally a hands-free option that would allow you to store all your music on a phone and select songs using voice recognition would eliminate clutter, but car stereos with Bluetooth connectivity are much more expensive than ones with a simple USB interface, so this is a workaround for my existing setup that cost well under $10 in parts from eBay.

Step 1: Required Parts

The parts needed are as follows:

4P5T Rotary switch
(Cost $2.44) Higher numbers of "Throws" are available, but are more expensive and bulkier.

20x Female Type-A USB Ports
(Cost $0.99) Overkill quantity for this project, but the spares can be used to make a holder for additional flash drives!

You will also need thin wires and something to hold it all together. I took a short section of Cat 5 network cable because it's easy to solder and the colour coded strands make wiring to the correct pins easier. The top and base of the selector are made from scraps of Perspex, though there's nothing to say you can't use a load of hot glue as long as the moving parts of the switch are walled off first!


Male Type-A to Female Type-C Adapter
(Cost $1.88) Allows to connect the selector to the stereo using your phone charging cable. Substitute the Type-C port for Micro / Mini USB or lightning based on whichever cable you are most likely to have available.

10x Micro SD to USB Adapters
(Cost $1.55) Simply use your own flash drives if you want, but these only protrude about 1cm from the stereo, so there's less chance of knocking them when using the indicator stalk.

Step 2: The Switch

Familiarise yourself with the switch before you begin. I'd have preferred a switch that used 4 or 5 separate decks which would avoid the need to cross wires because all the pins would be in vertical alignment and there would be more travel between each click, but this is a cheaper and more compact design.

Each deck is split into two halves, each having a single pin that always makes contact with the curved strip. As the dial is rotated, the leading edge of the metal strip makes contact with successive pins. The dial is actually able to rotate through 11 positions, but there is small limiter that can be placed in the 5th hole to ensure only 5 positions are selectable before it can no longer rotate.

Step 3: Top & Bottom Plates

The 5mm Perspex was slightly too thick for the switch's hex nut to fit on with the washer and limiting pin on either side, so I used a blowtorch to heat them for a few seconds to sink them into the Perspex until they sat flush with it.

How you space the USB ports around the top disk is up to you, but I decided to put them directly in line with the rotary knob to identify the selected USB drive more easily and differentiate the receptacle ports from the output cable port. Since the limiting pin has an offset tab, you'll have to take that into account when drilling the USB ports out, as the alignment can't be changed later.

After tracing a rectangle for each port directly in front of each of the switch's 5 positions, I used a 5mm drill bit and a mini file set to make holes barely large enough for the USB ports. It's better to file out too much and have to fill in the gap with epoxy than to have it too tight and deform the metal of the port. After filing the holes out, I realised the shielding tabs that help hold the flash drives in place actually protruded further than the form of the port itself, so I couldn't bring the top edge in line with the Perspex without either bending the tabs in or filing the Perspex more. fortunately, having the USB ports recessed by another 3mm made the flash drives even lower in profile, and prevented the usual gap between the Perspex and the output cable.

The hex nuts for the bottom of the switch are recessed into the base plate using the same blowtorching method, allowing the top ones to be tightened without an adjustable spanner.

Step 4: Wiring

The position of the ports relative to the switch terminals determines the length of the wire needed, but it will likely be between 2 and 6 cm.

To avoid the possibility of short circuiting the 5v and ground pins, I soldered them on separate decks. In theory it shouldn't be possible to short circuit them since the semi-circular runner tabs are only long enough to reach half way around the switch, but if the limiting pin was in the wrong place and you went through more than 180°, it could end up reversing the polarity. If in any doubt, it may be worth sourcing a 4-deck switch so the runner strip is limited to a single set of contacts.

Cat5's colour coding is convenient for cable managing. I used the green and white twisted pair for data (Green=Data+ and White=Data-), the orange for the +5v and the brown for the ground. See the photo for the correct order of the wires.

To strengthen the array of USB ports, all the adjacent shield pins are soldered together too.

Step 5: Side Braces

As a simpler alternative to holding the USB ports in place with small chunks of Perspex, or hot glue, I used a small amount of PCL plastic pellets (Often known as Poly Morph). By pushing it into the gaps between the USB ports and below them, it locks them in place so flash drives can't push them any further in. It also acts as a barrier to protect the wiring within.

While it's a relatively easy project to complete, it's worth testing it on a host device and flash drive that you aren't overly concerned about damaging, just in case there are any issues.

Mine worked flawlessly as a USB selector in my car and on a laptop!

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