Post by Roger on Jul 14, 2022 15:10:13 GMT
I've been sent this job in which three units were supplied, with none of them working and two having been robbed for spares. They want them all repaired if possible.
After Reverse Engineering this, it turns out to be a decade counter which latches a relay which starts something in a machine. There's some kind of optical sensor that sends pulses back which counts down the preset count until it reaches zero, at which point it releases the relay which presumably stops the machine.
The circuit is simple enough, just old school integrated circuits, all of which work ok. The problem is in the Binary Coded Decimal rotary switches which are worn out. The red ones on the right are the originals, and the LH one is what I've replaced it with.
PXL_20220714_140929335 by Timothy Froud, on Flickr
So here's the original one on the left and the new one on the right. Obviously they are physically different, and the pins come out in different places. There's a Common pin and then one for the binary digits 1,2,4 & 8.
Unfortunately, search as I might, there are no drop in replacements and no old stock that I can find and that's a real problem. The switches need a bit of force to rotate them, so they need to be held firmly, but there's no locknut on the panel to support them. The centre of the new switch needs to be on the centre line of the old one, so the pins foul each other.
Fortunately, there's 15mm between the PCB and the underside of the plate the shaft passes through. I originally thought I might use two pieces of Veroboard and stout wire, possibly potting it in Epoxy resin to make it rigid enough.
PXL_20220714_141020427 by Timothy Froud, on Flickr
Eventually, it dawned on me that this might be an ideal job for 3D printing. So below is a see the solution that I came up with. The body is a taller version of the old switch with a pocket for the new slim version.
The pattern of holes in the top and bottom match the new and old ones respectively. Then it was a matter of creating three more intermediate planes that steer where the holes meander through the part to join them up without touching each other. That was harder than it seemed at first, and it was easier to export and slice it so that it could be viewed slice by slice to check the clearances at every level.
BCD adaptor by Timothy Froud, on Flickr
Anyway, as you can see from the first picture, it goes together nicely. The bare solid core wires were threaded through first and the new switch soldered to them. The wires were then gingerly pulled down in sequence to get it flush with the top, and the bottom ones cut off.
Thankfully there were no cockups and the coding is correct so this is definitely going to work. I love this sort of work. It never pays back what it ought to in terms of cash, but it's really satisfying.
After Reverse Engineering this, it turns out to be a decade counter which latches a relay which starts something in a machine. There's some kind of optical sensor that sends pulses back which counts down the preset count until it reaches zero, at which point it releases the relay which presumably stops the machine.
The circuit is simple enough, just old school integrated circuits, all of which work ok. The problem is in the Binary Coded Decimal rotary switches which are worn out. The red ones on the right are the originals, and the LH one is what I've replaced it with.
PXL_20220714_140929335 by Timothy Froud, on FlickrSo here's the original one on the left and the new one on the right. Obviously they are physically different, and the pins come out in different places. There's a Common pin and then one for the binary digits 1,2,4 & 8.
Unfortunately, search as I might, there are no drop in replacements and no old stock that I can find and that's a real problem. The switches need a bit of force to rotate them, so they need to be held firmly, but there's no locknut on the panel to support them. The centre of the new switch needs to be on the centre line of the old one, so the pins foul each other.
Fortunately, there's 15mm between the PCB and the underside of the plate the shaft passes through. I originally thought I might use two pieces of Veroboard and stout wire, possibly potting it in Epoxy resin to make it rigid enough.
PXL_20220714_141020427 by Timothy Froud, on FlickrEventually, it dawned on me that this might be an ideal job for 3D printing. So below is a see the solution that I came up with. The body is a taller version of the old switch with a pocket for the new slim version.
The pattern of holes in the top and bottom match the new and old ones respectively. Then it was a matter of creating three more intermediate planes that steer where the holes meander through the part to join them up without touching each other. That was harder than it seemed at first, and it was easier to export and slice it so that it could be viewed slice by slice to check the clearances at every level.
BCD adaptor by Timothy Froud, on FlickrAnyway, as you can see from the first picture, it goes together nicely. The bare solid core wires were threaded through first and the new switch soldered to them. The wires were then gingerly pulled down in sequence to get it flush with the top, and the bottom ones cut off.
Thankfully there were no cockups and the coding is correct so this is definitely going to work. I love this sort of work. It never pays back what it ought to in terms of cash, but it's really satisfying.
