I recently found myself in need of a MIDI thru box for my desktop synth setup and decided to build one from scratch. A MIDI thru box is a pretty simple device and at a total cost 15-20 euros for parts (including a home made circuit board), it’s a great choice if you are looking for a simple DIY electronics project.
Complete schematics, circuit board layout and part list for this build are available under the Creative Commons license on GitHub.
Circuit board designed in Eagle CAD
A bit of research revealed that most MIDI thru box designs posted online are strikingly similar. So after a brief stop at my local electronics store to pick up some parts, I quickly prototyped the circuit on a breadboard and settled on a design.
Although the Hex Schmitt trigger I use can in fact drive 5 MIDI outputs, I chose to sacrifice one of them for an activity LED. Life is too short to troubleshoot MIDI connections, so this was an easy decision. If you have the need, another Hex Schmitt trigger could easy be added to the circuit for a total of 10 outputs (plus the activity LED).
Next step was documenting the schematic in Eagle. I have been wanting to switch to KiCad for a while but this project was simple enough for the free version of Eagle, so I put off learning a new CAD package once again. With the schematic in place, I moved on to the circuit board layout. I opted for a single-layered design because it’s considerably easier and cheaper to make single-sided circuit boards on your own.
Etching the circuit board is by far the trickiest part of the process if you haven’t done it before. Unless you are willing to spend hundreds of euros on a specialized equipment such as an UV light box and an etching tank, you may find that quite a few failed attempts are needed before you get it right… but it can be done! Personally, I use the “photo resist” method with a regular 15 watt CFL bulb, a photo frame and a couple of plastic containers, all of which I picked up at IKEA for next to nothing. It took me an hour or so to make a near-perfect circuit board for this project, including holes for all the components and a makeshift silkscreen.
Making cheap circuit boards on the kitchen table
After completing the circuit board, populating and soldering the components was a simple matter. I like to build and test individual sections of the circuit in isolation, so I started with the voltage regulator, then moved on to the MIDI input (including activity LED) and finally completed the MIDI outputs one by one – testing each section carefully along the way. It may take a bit longer to do it this way, but it eliminates the frustration of having to sort out multiple issues at the same time, which I often find myself doing if I populate and solder an entire board in one go.
Having built a working circuit, the only reaming task was to build an enclosure to protect it. I like to design pretty acrylic boxes for my DIY projects and have them manufactured by Ponoko, but since this box will be stowed away under my desk I opted for a more basic solution and simply cut two small pieces from a 3 mm plywood board I had lying around – one for the top and one for the bottom of the circuit board.
Finished MIDI thru box
If someone would like to contribute an acrylic box design to this project I would be happy to add the drawings to the GitHub repository.
I have used the box for around 20 hours now and have not experienced any issues. There is no noticeable latency when playing notes and MIDI clock is tight.
If you are a green DIY enthusiasts who have put together a few kits and are looking to take it to the next level, this project is a good choice. You could use stripboard but I really recommend that you bite the bullet and learn how to etch your own circuit boards. I find that using “proper” circuit boards helps me avoid mistakes and makes my builds much more enjoyable.
Feel free to post a comment if you have questions or need help.