Solenoid woes. Xylophone player construction.

So, I had this idea to build a solenoid operated xylophone player, controlled from a Raspberry Pi. I had bought about 20 mini solenoids, and need 12 for this project. After a few tests I hit on the idea of using plastic wire ties as the hammers, and just using them hand-held they seemed to work well (-enough).The solenoids were chosen some time ago, and these ones were the cheapest I could find. They are surprisingly expensive when you want a lot! My usual suppliers for electronic/electromechanical parts are Farnell/CPC, RS, and Rapid Electronics. I can't recall which one supplied these.

I set to, and built a simple wooden frame that would match up to the xylophone (which was a christmas present from when I was about 6 (yrs old)! First problem was the armatures were very small and had a 1mm hole which was much too small to thread a wire tie through. So I sawed through and filed out so there was now a 'C' shaped notch that could grip the tie.
Solenoids in place, armature gripping the wire tie.

So after 12 armatures I was ready to fix them to the bracket. By just holding a solenoid and wire tie in place it was clear this would not work! For the solenoid to pull the tie down like a hammer it needed to be at a slight angle, because the stiffness of the ties was too great.
Among my bits and pieces was some aluminium extrusion which had a narrow ledge running along its length.
Aluminium extrusion screwed to the wooden frame to hold solenoids at an angle.

 By screwing this to the frame it meant I could fasten the solenoids (hot melt glue was my first choice!) at a suitable shallow angle and the ties could be pulled, at least, that was the theory.
Nicely painted base board to fix the frame to.

The base board was just an old melamine coated shelf which happened to be a convenient size. I fixed a piece of wooden batten with four screws, that the frame could be screwed to.
Underside of the base board showing the fixing screws.
The xylophone in position; ties glued down; solenoids glued in position

Putting the frame in place on the baseboard, I could see where to position the wire ties and so they were hot glued in place. All seemed to be going well. The next bit was slightly tricky. I applied hot glue to the first solenoid, and holding the armature (with its tiny return spring) in place, I got the armature clipped on to the tie and pressed the solenoid to the aluminium extrusion and --- whoa! The aluminium cools the glue very fast and it sticks fast. Luckily I didn't need to re-position the solenoid, but now it was clear that I was going to have to be very careful glueing them in place.
As you can see from the pictures, I did manage this, but another problem now showed itself. The armatures are not held in the solenoid body, so its only the ties that prevent them from "escaping". Ping! More than once I had to search the garage floor to find the armature and the spring. I resorted to putting a blob of hot melt glue on the tie and armature to hold them together.

Ties glued in place
Armatures glued to ties

 So, putting it all together I now went and tried the solenoids with some power. They are labelled as 12V but on some previous tests I found they didn't work too well until about 15V was applied. I tried this, and although there was a faint movement, it was nowhere near enough to make them strike the xylophone bars. Examination of the ties and solenoids convinced me that the ties are just too stiff.
Cut tie now held by some 'foamy' double sided adhesive tape

Drastic action is needed. I tried cutting the tie near the end where the hot glue was holding it, and ping, another scrabble on the floor. Then I put the bit of double sided tape to hold the tie in place, and this gives much more movement to the tie and is a much softer 'hinge'.

Another problem:- I dropped the frame while manhandling it, and two of the solenoids fell off! It seems the hot melt glue does not adhere well to the aluminium surface and a sharp jolt is enough to break the adhesion. This is not good!
More double sided tape!

More tape! More tape! Yes the double sided tape seems to do the job, so my plan is to undo all the solenoids and re-attach them using the tape, and also to cut the ties as above, and fix them with tape too. Thats where I'm at. Next time I'll let you know how that has worked out, and some details of interfacing the solenoids to the Pi.


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