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Drummer Wanted is a slide guitar playing automaton that will play as long as someone will play the drums for it, designed and built for the Fall 2012 Things That Think course taught at the University of Colorado -- Boulder.  There are two main mechanisms controlling the motions, one for the strumming arm and another to move the slide.  These are both driven from the same DC motor off different points from a gear box.  The input is taken from a hand drum with a piezoelectric mic affixed and sent to an Arduino.



We'll discuss the various components that made up this automaton and how it was all assembled.  Here's an overview of the pieces that make up the project.

Canjo
Frame
Slide Mechanism
Strumming Arm
Gearbox
Motor
Control 


Step 1: Canjo

The Canjo was built based on a simple cigar box guitar instructions found here.ri

The main modification made was to swap out a standard 1 gallon paint can for the cigar box guitar.  Since we knew we were going to use a slide this seemed like a cheap approximation of a resonator guitar.  We hoped it would provide more volume and richer sound.  

The neck is a piece of 1x2 pine from the hardware store. cut to about 24" in length for this project.  A notch about 4" is cut into one end which will become the headstock where the tuning pegs go.  These are installed by drilling three holes in the headstock and attaching with wood screws that came with them.  These tuning pegs from Amazon were used.

Two small holes were cut out on opposite sides of the paint can as close the bottom as possible.  This is where the 1"x2" is slide in.  Some wood screws hold it in place through the bottom of the paint can.  The nut and bridge can be made any number of ways, see the cigar box guitar website for ideas.  Here, metal plates which were cut down and bent to 90 degrees in a vice.  A similar technique was used for the saddle which holds the strings in place on the end of the guitar.  

Step 2: Slide Mechanism

The slide mechanism is largely unchanged from our original plans.  

Here are the parts used for the slide.

guitar slide
machine bolts
nuts/washers
wood screws
1/2" wooden dowel
set of drawer slides
1/4" plywood
metal plate (to lock the drawer slides motion together)

We used a set of cheap drawer slides from the hardware store to get the linear motion.  First cut a metal plate to roughly the size of the mounting portion of the slides and drill holes to match holes on the slides.  Then mount the plate to keep the two slides moving in parallel.  If this isn't used the guitar slide will be at an angle across the strings, and will rock as the slide is moved back and forth.

A wood dowel was cut into two pieces about a 3/4" wider than the slide on each side, enough to send a bolt through.  Drill holes in both dowels for the bolts.  Next attach one wood dowel to the slides with wood screws, finally attach the slides to the back of the neck of the instrument.  Notice how far the slide moves in each direction before running into the body or possibly the tuning pegs.  We used a 1/4" plywood cut to the size of the slides to shim it up off the neck so it would clear the tuning pegs.

Now feed a washer, followed by a spring and another washer onto the long bolts and feed it through the dowel attached to the drawer slides.  Place a nut on the other side of the wood dowel to hold it in place, the spring tension here will help keep the slide in contact with the strings. Put the guitar slide on the other dowel and attach it to the bolts across the strings with two more nuts.  Adjust the tension of the nuts until there is good contact with the slide on the strings but not too much pressure.

The guitar slide should now move back and forth up and down the neck of the instrument.  Next we automate the slide mechanism.


Step 3: Frame

A frame is needed to mount the instrument, cams, strumming arm and gearbox.  The frame is also designed to act as a presentation element--the lower portion of the frame supports the mechanical elements, while the upper portion can be designed to appear as a stage.  The frame made for this project utilized the following materials:

(2) 8' pine 2x2
(4) 8' pine 1x2

The pine should be cut to produce the following pieces:

(4) 2" x 2" x 24"
(2) 2" x 2" x 40"
(5) 1" x 2" x 36"
(2) 1" x 2" x 18"

A table base is built using the four 2" x 2" x 24" pieces as legs, two of the 1" x 2" x 36" pieces and the two 1" x 2" x 18" pieces as aprons.  For this project, the rails were attached to the legs using wood glue and screws.  With minimal modification, mortise and tenon joinery could be used for a more sturdy base.

The remaining pieces are used to make a frame to support the canjo.  Attach the two longer 2" x 2" legs approximately 6" from the leg on the 18" rails.  Two rails are the added to this leg.  One supports the canjo (upper rail shown in the photo), while the other supports a lever arm, located below the top of the table.  The lever arm should be cut at 42", and mounted to the lower rail using a 1/4" x 2" bolt.  A 1/4" wood washer was made to provide spacing between the rail and the lever arm.  The pivot point should be located at the center of the lever arm and the lower rail.

Attach a nylon bearing at one end of the lever arm, and a spring to the other.  The spring provides tension to the arm, pushing the bearing end down.  Cut a plywood cam wheel with a radius approximately 12".  The profile of this cam wheel determines the motion of the slide, and ultimately the tune played.  Using a 1/4" dowel, attach the plywood cam to the leg nearest the bearing.  The cam should be spaced so that it is in plane with the bearing.

Step 4: Automating the Slide

In order to move the slide mechanism the long cam lever is used to pull it from both sides.  Nylon coated steel line was attached to each end of the lever at equal distances from the center.  It was then run up over nylon wheels which are sold for sliding doors, to reduce friction and allow the line to pull efficiently on the linear motion of the slide mechanism.  The wheels were mounted to the frame at the height of the slides with 1/4" bolts.  A spring is used to pull the far side of the cam lever up pushing the other side down into the cam.

The motion of the cam lever is controlled with the large cam.  The original diameter of the cam was over 2 feet.  A smooth shape was cut into the wheel to allow the cam to follow so that when pushing against the spring it would follow up smoothly and with some quick drops which make for nice quick movements on the slide.

Step 5: Strumming Arm

Once the cam wheel and lever arm is mounted, the strumming arm can be built.  The strumming arm is basically a lever attached to a crank wheel.  Cardboard prototypes are useful for determining the correct length of arm, crank wheel and push shaft.  Prototyping allows an ideal wheel size and arm length to be determined prior to implementing in wood.



A strumming arm is easily mounted to the frame using the bolt which holds the nylon bearings.  Attach a small pick to this arm, which lightly touches the strings of the canjo.

Cut a wheel out of plywood for the crank.  Cut two small wheels out of pine, and attach one to the end of the strumming arm, and one to the outer edge of the crank wheel using wood screws.  These two wheels should be able to spin freely.  Ultimately, a dowel will connect these two pine wheels, but do not attach the dowel until the final position of the crank wheel is determined.  The crank wheel will be friction driven by a wheel, which is driven by a gear connected to the gear box, and the final position needs to be near the position of one of the gears.

The crank wheel and driving wheel were cut using a laser cutter.  This precision is necessary, as these two wheels need to maintain close contact and consistent pressure to operate properly.  A printed gear is attached to the drive wheel. 


 

Step 6: Gear Box and Motor

We knew we were going to have rather large cams to move with our motor so we looked for the most powerful available for our budget. We found a $12 motor at a local electronic supply store.  The motor is in charge of moving both the slide mechanism back and forth and moving the strumming arm up and down with enough force to pluck the strings.  The DC motor we picked up was not geared and spun fast without load.  Stepper motors were available as well, which may have been stronger motors but involved more complicated actuation. 

We chose to build a gearbox to step the motor speed down to a rate more appropriate for our application.  This had the added advantage of increasing the available torque from the motor.  High torque is essential to begin rotation in the large cam.  The first thing we did was look for a set of gears we could nest in order to have multiple stages of step down.  The set we ended up using was downloaded from thingiverse.

This STL file was printed to scale using a 3D printer.  M8 bolts available at a local hardware store fit the hex shape inside the larger gear nicely.  Unfortunately the smaller gear was did not have the same diameter.  Since we needed a couple stages of step down we needed to be able to have the small gear drive a large gear with another small gear sharing the same shaft.  In order to accommodate this the small gear was carefully drilled out to fit the shaft of the bolt.

The design we chose required the large gear drive the smaller gear on the same shaft which meant they had to be fixed on the shaft.  This was done using set screws.  The smaller gears had holes in their side, while for the larger gears we had to drill a hole through the side of the bolt and the gear to lock it to the shaft.  For both of the gear sizes a #6-32 machine bolt was used as a set screw.  The bolts and small gear were tapped using the appropriate tap.  

More 1x2 lumber was used to build a small frame to house the gearbox.  The frame was sized to the length of the bolts used as the gear shafts.  The longest M8 bolt available was 100mm which proved to be large enough for the stack of gears.  

Two adjacent gears were lined up on the table and it was 3 inches from center to center.  Holes were drilled in the gearbox frame 3" apart.  Brass grommets were used in the holes to reduce friction and wear on the wood.

The gearbox was driven by the motor using a v-belt.  This required attaching a pulley to the motor shaft, and a larger pulley to the input shaft of the gearbox.  Pulleys are available in the HVAC section of a local hardware store. Unfortunately, these pulleys are designed for slightly larger motor shafts, which required a spacer to be constructed from a piece of sheet metal.

Assembly
Nuts were used to keep the shafts from sliding out of the frame.  In some cases, the gears themselves provided this function.  The gear box assembly was done in several stages.  On the first stage, the position of the wheel and first set of gears was determined and nut locations were marked on the shaft.  The bolts were then removed and holes were drilled through the bolts at the nut position.  This allowed the set screws to actually thread into the shaft to hold their position.  In order to access the hole in the nuts to set their position we had to drill holes in the sides of the large gears for the 6-32 screw.

With these mounted in place the second shaft was inserted with the gears and the large gear for the second stage was aligned with the teeth of the small gear for the first stage.  New markings were made, and the nuts and bolt were similarly drilled and tapped.  This process was repeated for all four stages.  

The output stage came out on the opposite side of the gearbox frame as the input wheel.  This was used to directly drive the large cam which moves the slide mechanism.  The head of the bold was fit into a hole drilled into a 1" x 2" glued to the cam.  A through hole was drilled into the side of the 1" x 2" and through the head of the bolt.  A wood screw was then driven through this hole, allowing the bolt to drive the wheel.



The strumming arm was driven off an early stage in the gear box which made it strum faster.  A gear was glued to a laser cut circle of plywood.  A rubber band was glued to the outer rim of this circle, which provided a high-friction surface to drive the crank wheel.  This wheel was mounted so that it was in contact with the crank wheel, which moved the strumming arm.  



Challenges
The gear box provided a lot of torque to spin the large cam and the strumming arm.  Unfortunately, the 3D printed gears were not quite strong enough to handle the torque.  The final stage gear sheared in half a number of times.  We originally used a sparse build on the printer, but the full build was also not able to stand up to the force.  
 
In the end we got some good footage proving it works but after a little while the final gear would inevitably break.  Given more time we could have the final gear made out of metal, which would be more suited to the high-torque application.

Step 7: Arduino I/O

An Arduino Uno was used as the brain for this project.  A simple motor driver board was used based off this Instructable.  This is shown for a solenoid circuit, but it works to send a higher voltage to the motor than the Arduino can driver.  The motor required a lot of current when loaded by the gear box and the cams.  Originally we tried with a DC adapter which provided 12V of power at 300 mA.  This adapter was not powerful enough.  In the end we had to drive it with a large power supply which could provide 3 Amps at about 15 V.

A piezo-electric transducer is used as input for the Arduino.  This was attached underneath a drum head and sent directly into Analog input A0 on the Arduino.  

Originally we hoped to be able to control the rate that the automaton played based on the rate that a player was playing the drum. In practice, we found that we could only realistically turn the motor on or off, due to the high load demands.  This simplified the original program.  We used a stopwatch library found here.  This library made it simple to keep track of how long it had been since an input was heard.  As long as there was someone playing the drums it would stay on, but after not hearing an input for 3 seconds the guitarist will wind down.
This sounds unbelievable! So you hit the drums and the automaton knows to play the canjo/guitare?
That's pretty much the idea. We're pretty happy with how it turned out, given the time constraints. Glad you liked it!
Pretty dang sweet! What kind of time frame were you working under? <br>

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