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Step 6Secure Mounting Nuts, Drill holes in tin
At this point, you need to drill the holes for your jacks, LED, and switches. You may have marked the holes for the jacks in the last step, or you may have waited until the jacks were mounted in the board so you could just trace around them, you crafty devil, you. I'd prefer to mark them before so I can get a more accurate centerline, then check for fit once the jacks are in place, but whatever works for you. Either way, double check the placement of your holes, and break out the drill.
I'd really recommend using a drill press to drill the holes if you have one. The bit likes to wander a bit if you don't. Regardless, use a spring-loaded center punch on the center of the holes. Chuck up a small drill bit. If you just jump to the bit you want for the final size, you're likely to have it catch in the thin metal of the tin and either damage it or worse, damage you. You'll want to start small, maybe 1/8", then work up to a 1/4" bit. "But wait," I hear you say, "the jack's more than 1/4" in diameter, and you said we'd need to make the hole oversize!" That's what the needle files are for. Once you've reached 1/4", break out the needle files and enlarge the hole slightly. How slightly?
The reason for the oversized holes is likely obvious to you by now, but in case, like me, you're a little slow on the uptake, allow me to explain my stupidity in detail. The metal barrel on the jack into which we place our plug is the ground connection for the jack. On the line input and headphone jacks, it doesn't matter if the ground contacts the metal tin, because both jacks are tied to a common ground and we went to great pains to ensure that none of the other components contacts the tin. The audiostrobe output, on the other hand, must be isolated from ground. The ground from the audiostrobe line goes to the tone decoder; the circuit remains open until the audiostrobe signal is detected, when the decoder ties the line to ground. Current flows through the LEDs and they light up until the signal stops, at which point the decoder breaks the connection. Short the audiostrobe's ground connection to ground, and the LEDs come on and stay on, regardless of signal.
I hadn't realized until I had soldered the jacks in and was poking around with the multimeter that the external metal barrel was the ground connection. This really made placing the board into a metal tin sound like a pretty crappy idea. Nevertheless, it was one I was loath to give up, especially after I went to the trouble of sanding pretty rounded corners on my PCB. So, after an exhaustive search for the best solution, during which I must have looked through at least two (2!!) drawers, I settled on paint. White, oil based enamel paint, to be exact.
Flash forward. Paint sucks. Scrapes right off smooth metal. This, by the way, is going to be a recurring theme in this little saga. Tried a thin layer of epoxy. I'll spare you the details. Heat shrink tubing would probably work great, but I didn't feel like performance testing the heat resistance of my thin, cheap plastic stereo jacks, so I opted for my favorite, electrical tape. I cut a piece of electrical tape about 1" long, then cut it into 3 strips along the length. Started the wrap just on one side of the bottom, wrapped 360 degrees to overlap the start, then cut the tape about midway up the side. The idea is to get just one layer thick along most of it, but two layers along the bottom. You'll be levering the jacks along that bottom edge, so you'll want the extra layer. Don't trim the tape yet! Pinch it in neatly in several places and try to turn it into the barrel of the jack. If you trim the tape now, it will catch on the edge of the hole when you're inserting the board and get skinned back. If you've gone up from 1/4" to the next size drill bit (my next size is 5/16"), this probably won't be an issue. If, on the other hand, you've used needle files to enlarge the holes for a cleaner look, you may be in for a tight squeeze.
Continue with the drilling. Mark out the locations of your switches and any mounting hardware, if necessary, and drill. Don't forget the hole for the LED. You're on your own as far as placement goes, as your switches won't be the same as mine. Plus mine look like crap, so you don't want to copy me. Once you're done drilling, be absolutely certain that you've removed all the metal dust, chips, and scrapings from the interior before you continue.
Hopefully in the last step you scuffed and cleaned the inside bottom of the Altoids tin. I didn't, so listen carefully when I tell you that you need to scuff and clean the metal like an obsessive compulsive. I can assure you that, should you go to the trouble of levering the PCB into position, giving yourself a blood blister and nearly killing the dog in the process, only to find, as you triumphantly screw your mounting screws down, that the nuts you so carefully epoxied into place are prone to popping loose with reckless abandon, you will not be a happy camper. So I once again strongly encourage you to scuff and clean the metal before you epoxy the nuts.
Should you be one of those carefree souls who blithely forges ahead without fear or caution, regardless of the warnings of others: have hope. I have discovered, to my surprise and delight, that humble super glue will serve to rebond the nuts to the metal. I don't know how well it would work to bond bare nuts to the tin, but it bonds nuts with a fillet of hardened epoxy around them to the tin exceedingly well. The problem being, of course, that you have one shot, and one shot only, to get the nut in the right place. Once it's down, it's down for the count.
At any rate, mix up your epoxy, spread a thin layer around the holes you traced, and press the nuts down, making sure they're aligned with the holes. Then either lay a fillet using a syringe or just use a thin stick to place epoxy. You want the epoxy fillet to be even with the top of the nut, and the diameter of the fillet should be at least twice the diameter of the nut. Be very careful not to get epoxy in the nut! This is very easy if you're not using a syringe. If you do get some in there, just chuck the nut and start over fresh. You'll likely need to remove all of the epoxy from the first nut before laying its replacement, lest the epoxy ooze up into the center of the nut.
Wait for the epoxy to dry. This would be a great time to skip ahead to step 8 and build your LED glasses. Never fear, you get to play with epoxy there as well. Once the epoxy has hardened, start laying electrical tape. Try to cover every exposed piece of metal along the bottom. If you placed your parts properly when populating the board, the sides of the tin will not be an issue.
I'd really recommend using a drill press to drill the holes if you have one. The bit likes to wander a bit if you don't. Regardless, use a spring-loaded center punch on the center of the holes. Chuck up a small drill bit. If you just jump to the bit you want for the final size, you're likely to have it catch in the thin metal of the tin and either damage it or worse, damage you. You'll want to start small, maybe 1/8", then work up to a 1/4" bit. "But wait," I hear you say, "the jack's more than 1/4" in diameter, and you said we'd need to make the hole oversize!" That's what the needle files are for. Once you've reached 1/4", break out the needle files and enlarge the hole slightly. How slightly?
The reason for the oversized holes is likely obvious to you by now, but in case, like me, you're a little slow on the uptake, allow me to explain my stupidity in detail. The metal barrel on the jack into which we place our plug is the ground connection for the jack. On the line input and headphone jacks, it doesn't matter if the ground contacts the metal tin, because both jacks are tied to a common ground and we went to great pains to ensure that none of the other components contacts the tin. The audiostrobe output, on the other hand, must be isolated from ground. The ground from the audiostrobe line goes to the tone decoder; the circuit remains open until the audiostrobe signal is detected, when the decoder ties the line to ground. Current flows through the LEDs and they light up until the signal stops, at which point the decoder breaks the connection. Short the audiostrobe's ground connection to ground, and the LEDs come on and stay on, regardless of signal.
I hadn't realized until I had soldered the jacks in and was poking around with the multimeter that the external metal barrel was the ground connection. This really made placing the board into a metal tin sound like a pretty crappy idea. Nevertheless, it was one I was loath to give up, especially after I went to the trouble of sanding pretty rounded corners on my PCB. So, after an exhaustive search for the best solution, during which I must have looked through at least two (2!!) drawers, I settled on paint. White, oil based enamel paint, to be exact.
Flash forward. Paint sucks. Scrapes right off smooth metal. This, by the way, is going to be a recurring theme in this little saga. Tried a thin layer of epoxy. I'll spare you the details. Heat shrink tubing would probably work great, but I didn't feel like performance testing the heat resistance of my thin, cheap plastic stereo jacks, so I opted for my favorite, electrical tape. I cut a piece of electrical tape about 1" long, then cut it into 3 strips along the length. Started the wrap just on one side of the bottom, wrapped 360 degrees to overlap the start, then cut the tape about midway up the side. The idea is to get just one layer thick along most of it, but two layers along the bottom. You'll be levering the jacks along that bottom edge, so you'll want the extra layer. Don't trim the tape yet! Pinch it in neatly in several places and try to turn it into the barrel of the jack. If you trim the tape now, it will catch on the edge of the hole when you're inserting the board and get skinned back. If you've gone up from 1/4" to the next size drill bit (my next size is 5/16"), this probably won't be an issue. If, on the other hand, you've used needle files to enlarge the holes for a cleaner look, you may be in for a tight squeeze.
Continue with the drilling. Mark out the locations of your switches and any mounting hardware, if necessary, and drill. Don't forget the hole for the LED. You're on your own as far as placement goes, as your switches won't be the same as mine. Plus mine look like crap, so you don't want to copy me. Once you're done drilling, be absolutely certain that you've removed all the metal dust, chips, and scrapings from the interior before you continue.
Hopefully in the last step you scuffed and cleaned the inside bottom of the Altoids tin. I didn't, so listen carefully when I tell you that you need to scuff and clean the metal like an obsessive compulsive. I can assure you that, should you go to the trouble of levering the PCB into position, giving yourself a blood blister and nearly killing the dog in the process, only to find, as you triumphantly screw your mounting screws down, that the nuts you so carefully epoxied into place are prone to popping loose with reckless abandon, you will not be a happy camper. So I once again strongly encourage you to scuff and clean the metal before you epoxy the nuts.
Should you be one of those carefree souls who blithely forges ahead without fear or caution, regardless of the warnings of others: have hope. I have discovered, to my surprise and delight, that humble super glue will serve to rebond the nuts to the metal. I don't know how well it would work to bond bare nuts to the tin, but it bonds nuts with a fillet of hardened epoxy around them to the tin exceedingly well. The problem being, of course, that you have one shot, and one shot only, to get the nut in the right place. Once it's down, it's down for the count.
At any rate, mix up your epoxy, spread a thin layer around the holes you traced, and press the nuts down, making sure they're aligned with the holes. Then either lay a fillet using a syringe or just use a thin stick to place epoxy. You want the epoxy fillet to be even with the top of the nut, and the diameter of the fillet should be at least twice the diameter of the nut. Be very careful not to get epoxy in the nut! This is very easy if you're not using a syringe. If you do get some in there, just chuck the nut and start over fresh. You'll likely need to remove all of the epoxy from the first nut before laying its replacement, lest the epoxy ooze up into the center of the nut.
Wait for the epoxy to dry. This would be a great time to skip ahead to step 8 and build your LED glasses. Never fear, you get to play with epoxy there as well. Once the epoxy has hardened, start laying electrical tape. Try to cover every exposed piece of metal along the bottom. If you placed your parts properly when populating the board, the sides of the tin will not be an issue.
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