I was tasked with fabricating a replacement housing for Merlin M842 remote control for a garage door. Given that the device is most commonly carried on a key chain or thrashed around in a car glovebox, the housing ends up taking a beating with age and a replacement can cost as much as $60 to $90. My goal was to create a competent replacement that would:
The circuit board of the remote is a circle, the battery is centered on the back and the buttons are simple printed contacts that get shorted by the rubber buttons. One option for the buttons I had was to reuse the original rubber housing, but it was also damaged (the big button was ripped) so it wasn’t an option. I ended up salvaging buttons from a calculator with the same style rubber buttons that has a conductive pad on the bottom.
My design essentially boiled down to two components – the “housing” that’s the main body holding all the components and has the keychain loop, and the “cap” that holds the battery in place.
The design went through 6 iterations on the main body and 3 iterations on the cap as I was printing prototypes in order to test the assembly and how everything fits together. To further “slim down” the profile I made the cap have rounded edges to give a thinner feel to the whole assembly. Apple gets away with it and so will I.
Getting the circuit to sit right in the housing was easy, the circuit is pretty symmetrical and key components sit at neat 45 degree angles to everything. The main challenge was to get the tolerances right on the cap for the battery to be centered on the power connector. The led indicator is simply hot glue melted through the housing led window.
I opted to use the holes in the circuit board to thread screws through to hold the two parts together. Every print took 30 mins before I could test out the fit and I didn’t want to spend too long, the screws were the easy way. Ideally I would design a bayonet type assembly where no hardware would be required to put the two pieces together. This way I’d do away without screws on the button surface (I didn’t have any nice countersunk ones, so I used what I had) as well as be a more durable assembly. There’s only so many times that you can screw and unscrew them before the plastic thread gives out.
While the calculator buttons look funky and unique, I would ultimately prefer for the buttons to be hidden under the housing. They’re too easy to press accidentally or pull the rubber out of the housing. The best alternative would be to solder in some low profile hardware buttons (like these) and use the flexibility of the housing to press them through the plastic. This would also offer more dust resistance.
If you own one of these fobs and yours is falling apart, feel free to download the files and print them on your own. There’s no guarantee that the buttons that you will use be of the same dimensions, so I provided a version with the center point of the buttons for you to cut away (physically or digitally) as much material as you need for the buttons you’ll end up using.