The making of the Magic Xylophone
It was required to build a radio controlled children xylophone for the opera "The Magic Flute". As a member of the props team, it was decided that I would be the person to attempt such a task. The prop was to light up on command and the hammers which were to be placed on the prop were to move up and down to give the illusion of the instrument playing itself.
I had never built anything of such complexity before, however having worked with basic electronics before I had a small understanding of what was required.
I am now going to explain how the magic was performed.
When I started the project I needed to understand exactly what it was going to do; how large it was going to be and what I was going to use to power it. With the xylophone being only 300mm in length, the space inside the prop was very limited, this however limited my choices of the following: A. Motor capacity, B. Power supply, C. Lighting capacity D. Radio Controller
After Talking to Martin Mallorie several times, and after much trial and error, I decided to build a box in the shape of the footprint of the xylophone. (wide at one end and gradually tapering in to the smallest bar) The first problem to solve was the motor to power the mechanism, After a trip to Maplins I purchased a 3v motor with built in gear box. I had to keep the motor small enough to hide inside the instrument but it had to be large enough to power the axle.
The lighting was the area of the prop I was most experienced with. Making models as a hobby, I regularly use LEDs as part of making the models. I decided to use 3v white LEDs in the prop as these would give a bright coverage of light even at low voltages. I normally use a 9volt battery to power LEDs as these make the lights last for a long time. However, due to the motor only running on 3v, I had to lower the voltage to the lights considerably as 9v would have burnt out the motor very quickly.
Next task to solve was the radio controller. This was the area of the prop that I had no idea how to even approach. After another trip to Maplins, I purchased a small PCB radio control unit of both transmitter and receiver. After examining it, I discovered that it was incorrect and worked on a much higher frequency which would have interfered with the radio mikes used by the stage crew.
Deciding that the garage door radio control unit was a bad idea and not worth the headaches, I then had the idea to use an already existing radio control unit. I removed the motors as I did not need them, along with the receiver from the car. With the car being a 2 channel system (forwards and backwards being one channel, then left and right being the second) I only needed one channel to work. I removed the motor and all wires for the steering gear and kept the wires for the rear wheels as these would be used to control the bigger motor which i purchased earlier.
I was a little concerned that the receiver was running on a higher voltage (4.5v) but i found that the motor can run within a tolerance of +/- 1v , which was good as it would not burn out, only run a little faster.
The mechanism which would make the hammer move up and down went through many designs and changes, and got gradually more complicated with each design. I took inspiration from several sources, such as a child's seesaw tied with elastic bands and a car engine cam shaft. How to incorporate them into such a small space was very complicated.
I settled on the idea of cutting two discs from MDF and off centring them on an axle which ran from the motor, down the full length of the box.
The designer had sketched on a design where she wanted the hammers to sit, using this as a reference i was able to measure between the bars on the top of the xylophone and this was where the MDF discs had to fasten onto the axle.
I then had to build the rest of the box around the motor and axle. with the rest of the LEDs installed i set to the task of making the push rods for the hammers to sit on. After thinking of various ways for the hammers to sit on the rods, i wanted them to be easily placed with no room for error or confusion when rehearsals, i used small but powerful magnets attached to both the hammer and rod so they would "snap" into place.
With the push rods in place it was now time to fasten the back panel onto the xylophone with a mounted switch. The bars on the top of the prop were to be screwed into place to allow for removal to give access to the lighting and motor.