Foot-Action Guitar Strummer

Designers: Jonathan Lee, Jason Leung, and Matthew Topel
Client Coordinator: Shiela Tayrose, Occupational Therapist
Supervising Professor: Larry Bohs

INTRODUCTION

The client is a 17 year-old man who had a left hemispheric stroke at the age of three that affected the right side of his body. Because of the stroke, he does not have the dexterity and fine motor control necessary in his right arm to strum a guitar. A device was built to allow him to strum the guitar using his left foot. The strumming mechanism is composed of a pick rod, which holds six individual guitar picks, and two complementary-action solenoids that move the rod across the strings. This mechanism is mounted to the guitar face and controlled by a momentary switch within a foot pedal. The system is powered by an AC/DC converter that plugs into a wall socket. The device is portable, produces good sound quality, and requires minimal user effort.

SUMMARY OF IMPACT
This device has enabled our client to start taking guitar lessons. The client’s mother commented, “Given that I had to make him put down the guitar and leave for school this morning, I would say the project has been quite a success. (He) is progressing in his lessons and last night he was practicing the chords for “Redemption Songs” by Bob Marley. He says it’s pretty hard but he is confident he’ll get it. This whole experience has been good for his self-esteem, in addition to giving him a hobby.”

TECHNICAL DESCRIPTION
The Foot-Actuated Guitar Strummer (Figure 1) consists of a pick rod, two solenoids, a guitar mount, and a foot pedal. When the user pushes on the pedal with his foot, the device strums downward, and when he releases the pedal, the device strums upward.

The pick rod is constructed from a 1/2″ square Delrin rod. Six slits, cut 1/8″ into the bottom of the rod, hold six standard guitar flat picks so that the separation between picks is the same as that between the guitar strings.

One push and one pull solenoid are connected together and to the pick rod, allowing independent timing for the up-strum and down-strum. A hole drilled along the axis of the pick rod accepts the push-solenoid plunger, which is secured to the pick rod with a setscrew. Both solenoids are powered with a 12V, 5A AC/DC wall plug, which allows the strummer to be activated indefinitely. The surfaces of the push solenoid plunger are padded with neoprene to mitigate excess noise from the solenoids stopping and starting during operation. A Plexiglas enclosure with air holes prevents user contact with the hot surfaces of the solenoids.

The solenoids and pick rod are attached to a base plate with a machined bottom, which slides into a track mounted on the guitar. Ball detents on the track allow for easy and consistent positioning, and a wing screw secures the strumming components to the sliding track.

The foot pedal uses a commercial footswitch housing. The commercially supplied switch is replaced with a SPDT momentary switch connected to the power supply such that one solenoid is activated at all times: pressing the pedal activates one solenoid while releasing the pedal activates the other. This method of control allows for a playing speed comparable to hand strumming. A standard 1/4″ stereo guitar cable connects between the SPDT switch in the pedal and the solenoids on the guitar. Each connection in the system uses a different sized plug and jack to ensure correct connection of components.

The device was tested to determine the maximum strumming rate and the temperature escalation over time. The maximum strumming rate was 17 strums per second corresponding to 9 pedal presses and 8 releases per second. This high frequency should prevent strumming rate from being a limiting factor in any musical composition. The surface temperature of the solenoids rose sharply to a temperature of 140F. However, the enclosure attained a maximum temperature of 89.6F degrees, low enough to protect the user from potential injury.

Figure 2 shows the client using the Foot-Action Guitar Strummer. Cost of parts for the device was approximately $525.

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