Can I enlarge mounting holes in an Uno?

I have quarter inch screws ...


Just kidding! ;) The smallest nylon screws I found at my local hardware store were #6x32 which requires a 0.146 inch (3.71mm) hole and it appears that the holes are sized for M3 screws which are 2.98mm (0.117 inch.) I would need to enlarge by about 1.2mm diameter (though I might get by with a trifle less since the nylon screw threads might deform a bit to get through a smaller hole.)

I did a google image search for "arduino uno pcb layout" and from the images I found, it appears that there are no traces so close to the holes that they are likely to be damaged if I proceed cautiously. One concern I have is the drill bit grabbing the edge of the hole and doing damage beyond the diameter of the hole.

I searched the forum and found nothing on this so I presume it is either an unspeakable travesty or those who have done so speak not...

Yes you can do it but the holes are very close to the edge of the board and that might pose a problem.

Also you will cut into the ground plane so your pillars could become connected to ground.

Something I’ve found useful is foam rubber carpet underlay. If you then put double sided carpet tape on each side of it, you can stick your board practically anywhere you want.

I presume @Grumpy_Mike knows his stuff - he usually does.

But on my Uno and Mega some of the circuit traces seem to run very close to the holes. I wouldn't try it.


Thanks all for the help. The deed is done. I worked carefully using a (rechargeable) hand drill with the clutch set to the lightest setting and going at the slowest speed. Drilling went well. I won't know about damage until I hook everything up. I specifically used nylon screws to sidestep electrical issues. I considered double-stick tape (and plan to use that for another circuit board) but I needed a solid mount for this because the USB connector will be exposed and users will be pushing the USB connector on it in order to power it. I wasn't sure that double stick tape would provide a solid enough

Today is mechanical day. If work goes well I might get to electrical, but I have a fair amount of mechanical to do. I'll be sure to report back when I know the final result.

Incidentally, not to be too obtuse about the project, this is going to be a toy for my 3+ year old grandson. He likes music. I think he'll like flashing LEDs and buttons to push. I have a clear acrylic box about 5" x 4" x 2". The Arduino Uno and a rechargeable USB battery (sold for recharging cell phones) and the necessary circuitry will reside inside. On the lid there will be four push-buttons arranged in a square. Just inboard the buttons will be four LEDs (and probably some additional LEDs between them). In the middle is a speaker.

He will need to press various sequences of the buttons to get the box to do what he wants. Hopefully what he wants will be to play various songs and play a game or two.

There is a now a secondary goal. A week or so ago 'the kids' were over and I showed them progress so far (with no hint to the toddler that this was going to be his.) It was just a pile of wires in a wood frame to hold the buttons with the LEDs in the prototyping board. My son took an instant liking to the setup and in a moment was composing his own music for it to play. (We both write S/W for a living so it was duck soup for him.) He mentioned how nice it would be to have one of his own to which his wife said no... Later on I realized that when the grandson wasn't playing with his toy, his father could play with it himself. I just need to give him a copy of the sketch to reload when he's through with his efforts. That's another reason to keep the USB connector exposed.

HankB: That's another reason to keep the USB connector exposed.

You know that's going to be used as a pencil and crayon sharpener don't you :)

I have a clear acrylic box about 5" x 4" x 2"

Have you considered this case?

It won't fit a Uno in but a Trinket or a Micro would fit.

You're right Ken, I should never underestimate the ingenuity of a toddler. ;)

Grumpy Mike, the bear case looks interesting. I could use the nose and eyes for three of the four LEDs. Where to put the fourth? :D :smiling_imp:

HankB: You're right Ken, I should never underestimate the ingenuity of a toddler. ;)

Grumpy Mike, the bear case looks interesting. I could use the nose and eyes for three of the four LEDs. Where to put the fourth? :D :smiling_imp:

A navel display?

Henry_Best: A navel display?

She was only the Admirals daughter but... ;)

Grumpy Mike, the bear case looks interesting. I could use the nose and eyes for three of the four LEDs. Where to put the fourth?

I know it was a joke but:- The case actually has provision for 8 LEDs, three down each side below the arms and one on each side of the bow tie. These are places where the case is made thin, to allow the light to get out, and is surrounded by a thick region of plastic to stop the light spreading. Also the back of the head has holes for the sound to get out.

But on the subject of jokes:-

She was only the Admirals daughter but.

She was only a logicians daughter but she wasn't an ^

I have had some setbacks. :( I used solid wire for connecting the LEDs and movement around them resulted in breakage of the leads off a couple LEDs. The LEDs are glued pretty solidly into the acrylic box. Back to the drawing board! (And a replacement box ordered.)

I'm rethinking the wiring to the LEDs. I need to use stranded wire and fasten the wire near the LEDs to reduce movement. Second, I plan to use headers on the interconnect board and socketed leads on the LEDs rather than solder everything. That way I can get the interconnect board in place and then easily make all of the connections. (The interconnect board holds the series resistors for the LEDs, the grounding resistors for the push buttons and a small amp for the speaker.) As I pondered that, I realized that I could put the processor on the interconnect board as well, eliminating all of the connections between the Arduino I/O sockets and the interconnect board. Except for the lack of an Arduino for my son to play with, this will make a much cleaner project. But it also opens up a couple more issues I need to investigate.

Can I apply S/W updates in circuit using another Arduino (Quick research - yes:

Does power via a USB connector require conditioning or regulation? I looked at the circuit diagram and could not answer that. I see the 5V pin from USB going through a pot and I don't see where it goes from there. At a minimum it seems like a bypass cap would be wise.

I plan to socket the ATMega chip. It might make it easier to reprogram (though the connections in the tutorial I cited above look pretty straight forward.) It also makes it easy to test out the peripherals by connecting I/O pins from an Arduino Uno (or breadboarded chip) to the corresponding pins in the socket.

But back to the LEDs. I also plan to put a dab of silicone sealer at the base of the LEDs to act as a bit of a strain relief (along with a bit of shrink wrap over the solder connection. And finally... A dab of silicone sealer to hold the LEDs in their holes rather than super glue. If I need to remove them for some reason,it should be possible.

If you connect the buttons between input and ground there is no need for an external resistor as you just enable the internal pull up resistor.

Watch out for the sealer, unless it is electrical grade it can release an acid during the cure phase and if it is in contact with metal or wires it can cause corrosion in th long term. Hot melt glue is good for semi permanent fixes like this.

You can put the resistor in line, that is in effect as partof the wire with heat sharing sleaving over to make it insulated, or hot melt glue again but that can look messy.

Thanks for the tip on hot melt glue vs. silicone sealer, Grumpy_Mike.

I’m trying to figure out why I thought I needed series resistors for the push buttons. I looked up internal pullup (Which I’m using anyway) and found form 20K to 50K ohms. That will save additional space and connectors on the board.

I’ll give some thought to i-line resistors for the LEDs. My first reaction is to prefer soldering plated through holes to wire to wire, but I try to keep an open mind.


HankB: I used solid wire for connecting the LEDs and movement around them resulted in breakage of the leads

It is not very obvious at the human scale because the wire is so small but copper work-hardens with just a few bends. Try a piece of the soft small diameter (8mm ?) copper tube that is used for gas installation. When new it bends like putty - but only for a very few times.

Always use stranded wire unless there will be no movement.


Yes that is good advice on stranded wire. However if there is a lot of movement this will crack at a soldered joint. Therefore either use some sort of strain reliefs to stop the flexing of the joint, or use crimped connections.