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OT Printed Ckt Board Question

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  • OT Printed Ckt Board Question

    I am attempting to replace a membrane touch pad used on a 70s sewing machine. The original used a mylar film base with the circuit traces printed on it. The board formed the push buttons by folding the mylar with a sticky filler between them to hold the "push button" contacts apart. I want to replace the mylar board.

    What I want to do is use small "Klixon" type surface mount switches for the push buttons. This will allow me to use the original faceplate with all the functions showing. There is 20 switches all running on low voltages (5V or so ). The internal controls/mechanism function, based on testing the functions that did work.

    After all that B/S, in the past copper stick on solder pads where available so you could construct a PCB using the stickies, a soldering iron, tweezers, and a loupe and patience.

    Can't remember what they were called. Does anyone have names, sources, suggestions, etc? I know I could etch a board, don't want to do it if I can avoid it.

    Old as Dirt

  • #2
    i KNOW how this started: "Honey, can you fix this?" And dumb you said: "Sure, Sweetie Pie, I can fix anything." Now you're committed.

    I been bit by that snake too many times.

    You can't use a prototype board?
    Last edited by Forrest Addy; 11-06-2015, 12:31 PM.


    • #3
      The available space, depth is limited. As I stated I'm trying to reuse the original faceplate that has all the functions and lights so you can tell quickly what stitch width, length, etc you're set on. Actually, I could just put switches on a metal/plastic panel, but it would look like ****, plus a card to tell where and what the needle is doing.

      The sewing machine is decent. Before it went to "attic heaven" it sewed well. I used it to sew some auto related plastic. Certainly better that the last POS she bought.



      • #4
        I am not aware of the "stick on" copper pads you refer to, but there is, or used to be, dry transfer pads and tracks and curves, etc.

        You would rub them on clean bare copper board, as required, to construct your component footprints and tracks, then etch the board with ferric choride or permonium persulfate.

        The exposed copper would etch away, leaving the pads and tracks under the dry transfer un-affected. Then you would use scotch-brite to clean the dry transfer off, then drill and stuff the parts. A little labor intensive, but cheap.

        Radio Shack used to sell the dry transfer stuff up until recently.


        • #5
          I have not seen the adhesive backed copper pads for sale in years. If you insist on avoiding etching, you can make your own pads from the foil used in Tiffany style stained glass. Cut out the traces, peel off the protective backing and stick in place. I've never done a full board, but have certainly have rerouted traces with this technique. Caution, the foil adhesive is not nearly as strong and as heat resistant as a clad board.


          • #6
            3M sells copper foil tape..... Maybe you can cut what you need with an Xacto knife.... I have tried doing what you are attempting, may the gods be with you!



            • #7
              Do you mean stuff like this?


              Look here:


              Digi-Key/3M does not seem to have any PCB pads in the stuff but you could cut it into pads for surface mount components. I would keep the trace wide and only narrow it down to the size needed for the pad at the last fraction of an inch.

              Digi-Key does not seem to stock this produce so you would have to wait and they may have a minimum order ($$$). Perhaps another electronic supplier would have them available from stock.

              Personally, I would get a piece of perf-board with a "pad per hole" and try to use that. If the 0.1" grid does not work, perhaps turning it at a 45 degree angle and using the 1.144" diagonal spacing would. Another choice would be the strip board made by Vector and others. It has foil strips along every row of holes and you cut it at a hole to isolate horizontal runs of foil. Jumpers are then added for vertical conductors to form the full circuit pattern. Gaps in the foil could be tailored to the spacing of the switch contacts. It is a bit wasteful of board space, but a keypad does not have to be super compact.

              Warning: I could not find the switches you referred to. The word "Klixon" returns far too many references. If they are the type that use the circuit board traces for contacts, then bare copper will not work for long. It will corrode and the resistance will go way up. That type of switch requires that the copper be plated with a more corrosion resistant alloy. If the copper is just to solder them to, then you will be OK.
              Paul A.

              Make it fit.
              You can't win and there is a penalty for trying!


              • #8
                You can get pre-printed membrane keypads for cheap, and they would be ideal if the size is right:


                They have other types as well.

                If you want to go with the conductive adhesive pads and tracks, there are kits available here:

                This "frame" with bright tin plated pieces is $39:

                You may need other items as well, so the cost may become significant. Their complete kits are $680.

                I found this by a simple search, which also came up with other options:

                P S Technology, Inc.
                and Muttley


                • #9
                  Perfboard or protoboard is what is is called.

                  Or you can design a board in circuit maker and have it sent out to OSHPark for manufacture and get it in a couple weeks. It is really simple to do.