Attaching a PCB to a curved, vibrating surface....?

[kriista]

So I'm working on a project that will now required me to attach a 4" x 4" PCB to the inside of a curved surface, that vibrates.
A drum.

More specifically, a floor tom.

I'm building an updated version of this basically:


http://rodrigoconstanzo.com/Sidrassi-Tom.html

I can't really bolt it down as the angles would put tension on the board, not to mention I worry that it would eventually vibrate/break things.
I had originally planned to attach it via some kind of elastic suspension, so something on each corner floating it in space, but now that it's come time to doing it, it seems way impractical, not to mention it would likely bounce/hit the wall of the tom.
Next I thought about using PCB standoffs stuck to the inside, which seems like it could work, but doublesided tape would eventually come free, again from the vibration, and constant moving of the drum (to and from gigs).

So, has anyone mounted a PCB inside something similar, and/or have any ideas on what could be done here?

[ThunderShowers]

Just a 4x4?

Uuuuuh, float it in the middle of the drum with elastics; Four from the top, four from the bottom, so it'd float dead center in the drum and stay there? I really don't see how that'd be impractical. 

[artifus]

float it in the middle of the drum with elastics

or springs?

rubber gromits?

[R.G.]

You could cast a holder out of silicon rubber, epoxy or polyester casting resin with a flat side to match the PCB and a curved side to match the drum. The curved side would give you enough surface area to have double sided foam tape hold even under vibration, and the foam tape is renewable if it ever fails. The hold of foam tape goes up linearly with surface area. Sixteen square inches is a really, really firm attachment.

The casting could also have imbedded threaded inserts to take the PCB (with any of the recommended materials) or could be drilled and tapped (epoxy/polyester).

You could even make a block of wood match the inside curve of the drum by some carving, then placing sandpaper inside the drum, abrasive up, and sanding the wood on the sandpaper to match. But that sounds like a lot of work to me. 

[derevaun]

Seems like the two best choices are brute force nailing it down bombproofly, or suspending it with some elastic. I suspect that the force of air vibrations inside a drum could vibrate a suspended board pretty well. I wonder if there are illustrations of typical vibration nodes inside a drum, somewhere on the interwebs?

In any case, the most solid, least violent places are probably going to be where the legs attach. You'd also get the benefit of a place to drill/bolt/etc. Maybe attach one edge; two points of attachment on the board? There'd be less danger of flexing the board via the attachment points.

Any way you do it, the board's going to vibrate. But I suspect only socketed components would be in danger of failing. Hm, where's my copy of "PCB Attachment in Aerospace Cowboy Applications: First Principles" when I need it?

[kriista]

The thing for floating it is that it would have to be near or up against one of the sides. I would imagine a flat square between the two heads would dampen the drum a bit (seems logical).

Attaching it physically would break the PCB I'm sure as it's a small drum, hence a small radius, so there's no way the screws would go in anywhere near straight.

I like the casting idea as it would hold it nice and snug and be non-invasive (as non-invasive as putting a PCB in a tom can be), but I don't have any of that kind of stuff, and 4x4 is a lot of goo.

I just had a thought about using metal brackets coming off the tuning lug screws and attaching to the PCB via bolt/gromits. That would give it a decent stability and a bit of give with the brackets and gromits.

[kriista]

Speaking of which, what is that kind of bracket called?

It's about 3/4" or so, is kind of thin/flexible and has about 1/4" holes across the whole thing.

It might be some kind of strip or stripping?

[waltk]

If you are etching your own PCB, you could use a very thin and flexible PCB material.  I have some that would easily fit the curvature of a drum - just never had a reason to use it for anything.

[kriista]

It's funny as I specifically did the opposite.

The circuit is an Old Mr Grassi.
http://ciat-lonbarde.net/paper/grassi.gif

It's a "paper circuit", meaning you print it out, glue it to a bit of cardboard and build it that way. I wanted a sturdier version to live in my floor tom as the other paper one I built is a bit temperamental.

Here are a couple pics of the tom/PCB for reference:






And a video of my other Old Mr.Grassi in action:

[greaser_au]

I'd say those screws holding the lugs look like suitable candidates for holding a bracket. Take a strip of 1/2" wide steel, say 1.5" longer than the straight distance between the lug screws, bend it down at 90 degrees 1" in from each end. bend last 1/2" outwards a little more than 90 degrees & drill holes 1/4" from the ends to suit the lug screws...  Drill the bracket & board to match each other & bolt on with some minimal amount of standoff (more standoff = more torque in the vibration).  Rinse & repeat at the other end of the board. I imagine it should have fairly minimal effect on  the sound of the instrument, & no permanent changes.

david

[kriista]

I found some nice PCB standoffs that are made of rubber and go through the board (to absorb/isolate a PC motherboard I believe?).

I mounted those on the PCB (a bit further into the board so it's not at the very edge) and used some thin metal (similar to what you described) from the screws at a 45degree angle to support it. I used altoid tin type aluminum as its thin, easy to cut, flexible enough to bend by hand and to absorb some additional shock.

It's looking good so far. I've not fully mounted it yet as I'm going to do all the additional wiring in the tom before sticking a big PCB in there that I have to work around.

[davent]

A block of 1/2"-3/4" Baltic Birch plywood a bit bigger then the PCB.

A rubber/silicon bumper in each corner  of the block.

A screw through the side of the drum shell into the center of the block of wood, the bumpers isolate the block from the side of the shell.

Attach the pcb to the nice rigid, flat surface of the block. You could use wood screws with rubber hole grommets as standoffs to hold the pcb to the plywood block.

dave

[kriista]

I want(ed) to avoid drilling additional holes, not because I'm precious about the drum, but because I have oodles of extra holes already.

Here is the finished board (still plenty more to do though)