I have done it! I've created the ultimate lazy musicians hands free kit - the self strumming guitar. No need to put down your beer or your sandwich anymore just because its gig time, get yourself one of these and let the guitar do the hard work for you.
I should point out that I am writing this instructable as a guide to building the end product, however if you watch the video you will see its development was a lot more organic with various design changes made along the way. I want to show you how to make one for yourself without necessarily taking you down all the dead ends and mistakes I made. If I were to build it again there are quite a lot of things I would do differently but you're going to have to wait for Mk2 for that.
Step 1: Preparing the Guitars Body
The first job to do once you have sourced a suitable guitar that you don't mind ruining for this project is to prepare its body. There's going to be a load of extra bits and pieces squeezed in there and you will need to make some extra space. So if you have a strat style guitar like mine whip the strings off, remove the scratchplate by undoing all the little screws and desolder the pickups and pots from the jack plug. Also remove the neck by taking out the 4 screws and plate on the back of the neck.
Pick a nice spot for the strumming bar to go clear of obstructions and avoiding body contours where possible (because these are a pain for routing) and draw a straight line in Sharpie to work to. Use a straight edge with a router to cut out a slot in the body, making sure it is wide enough for whatever you are going to use as a strumming bar ( I used a carpenters pencil) plus fixings either side. You should plan your bar to be long enough to go under all the strings and leave space for the mechanism at either end. In my original design my strumming bar was driven by a powered cog at one end connected to a free turning cog at the other end however I ended up replacing the free turning cog with a captive bolt. You need to allow room for all these components and fixings so it is a good idea to have sourced your parts before starting to cut.
To mount your cog you will most likely need to cut all the way through the body yet your router might not be long enough to plunge all the way through. To get around this I routed my slot on top and drilled a hole at either end, flipped the guitar over and connected the drill holes underneath with the router. This left me about 1mm of wood in the middle which I quickly knocked out with a rasp.
Step 2: Constructing the Strumming Bar
To construct my strumming bar I used a carpenters pencil and some parts I salvaged from an old printer, primarily a DC motor with a small cog fitted to it and a larger cog about 40mm in diameter.
The first task was to find an effective way to mount the large cog securely to the guitars body and it is imperative there be no play in the cog whatsoever. I mounted the cog on a roll pin followed by a washer, a small plastic spacer, and another washer that had a flat edge cut into it so it could rest on a flat surface. I then used my bench vice to force a pair of nuts onto the other end of the roll pin far enough so that all the parts sat snugly together but not so tight as to stop the large cog turning freely. This assembly fitted into a special cavity I routed out (see pictures) which would eventually be filled in with a mixture of wood glue and sawdust to lock the cog firmly into position.
Onto the cog was bolted a carpenters pencil aka the strumming bar, and at the other end of the strumming bar was fitted a bolt with a long piece of thread exposed which would later run back and forth in a slot created between two pieces of wood fixed together and glued into the cavity.
Step 3: Strengthening the Body
Once I had cut enough chunks out of the body to allow me to fit in all the extra bits I had, there was a large hole cut all the way through the guitar and almost its entire width and I was concerned that it was likely to break in two once I restrung it and brought it up to tension.
To counter this and return strength to the body I bolted two pieces of scrap steel to the back of the guitar.
Step 4: Wiring It All Together
There are two electrical systems working side by side in this guitar, a circuit to collect audio signals from the pickup, and a circuit to drive the self strumming mechanism.
1. The audio circuit
Originally this was a 3 pickup guitar with volume and tone, and 5 way selector switch. Due to space requirements I reduced this circuit to a single pickup and volume control which is a very simple circuit to construct. Essentially it is a pickup and volume control wired in series, the only pitfalls to watch for are making sure you get the polarity of pickup and jack the right way around and grounding. With all the electromagnetic noise thrown out by the motor it is essential to get the grounding right, all metal parts need to connect to the ground side of the jack which in this case meant the ground side of the pickup connects to the ground side of the jack via the metal casing of the volume pot. The bridge is already pre wired to the jack so nothing additional to do there but worth mentioning in case you have to take the bridge off during the build for any reason.
2. The Motor Driver
My first attempt at a motor circuit was using the spare tone pot to control the speed of the motor - this was an abject failure. After a little research I purchased a PWM (pulse width controller) circuit for about £3 which was incredibly easy to fit, two screw terminals for the motor and two screw terminals for my 9V battery. Done!
Step 5: Put It All Back Together
Once all the separate parts were built and working it was time to assemble everything. The strum mechanism could be fixed in place with the piece of wood with a slot to guide the captive bolt at one end, and the cog fixed into position at the other end with my wood glue and sawdust mix (which took about 4 days to fully harden but is now like kryptonite!). I opted for the low tech option to mount the motor with a large cable tie, the advantage of this is that I could experiment with different sized shims between the motor and guitar body to keep the two cogs close enough to mesh well but not so close that they created resistance.
The only other jobs were to bolt the neck back on, cut down the scratch plate to accomodate my modifications, fit the volume control, speed control and pick up back into the scratch plate and screw it back to the body. Oh and not forgetting to replace the strings.
Step 6: Adding Makeshift Plectrums
The last job was to add plectrums to the strumming bar. My initial plan was to cut slots aligned with the strings into which I would glue cut down plectrums, however after spotting a bag of small cable ties in the corner I decided this was a silly idea. Instead I fixed 6 cable ties (one for each string) to the strumming bar with the long ends pointing up. I then cut all the cable ties down and at an angle so the ends just brushed the strings without getting snagged. After a few adjustments I was ready to play!
Step 7: Play It!!!
The advantages of a self strumming guitar are many and varied but for me the primary attraction is I can play guitar while drinking beer, something musicians have been struggling with since the birth of rock 'n' roll.
There is also the potential for developing entirely new playing techniques using both hands on the fret board, I attempt this with limited success in the video.
Finally there is the potential to provide people who have lost or lost the use of an arm the chance to play a guitar using their one working arm. This is something I will keep in mind when I go about developing Mk2.
Step 8: Parts and Tools
1 x Cheap guitar you don't mind ruining
1 x small DC motor 9 or 12V
1 x 9V battery
1 x PWM speed controller
1 x Carpenters Pencil
Assorted pins and cogs salvaged fom whatever machines you can get your hands on
Nuts and Bolts
Wood glue and scrap pieces of wood