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Ask an Expert Forum Inline Key Switch or Tamper Resistant control for mains devices?!
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Inline Key Switch or Tamper Resistant control for mains devices?!

shabaz
shabaz over 2 years ago

Hi,

For one reason or another (elderly relative), I'm looking for a way to prevent operation of a mains appliance (about 6A at 230V AC) unless, say, a key of some sort is inserted, to prevent them from using the appliance themselves. It doesn't need to be a high-security solution, it could simply be a plastic key for instance, that could be pushed in and rotated, to enable the appliance.

I don't wish to cut holes in the appliance, and really do want to do this inline with the mains wire. 

Is there any existing product that could do this? If not, is there any suggestion that I could fit into a small plastic enclosure myself? I don't have access to a 3D printer, so it ideally needs to be a solution that I could make using conventional tools (electric drill, saw, etc). 

One way I can think of, is to find some sort of enclosure where a rocker switch could be fitted recessed, and then have a small padlock such that it prevents the switch from operating. But it probably requires a custom box : ( Another way could be to have a Bluetooth controlled relay. I could design this, but I'd rather find an off-the-shelf solution. 

I'm sure I can't be the only one wanting to do such a thing, but I'm having no luck googling for this. 

Anyone come across anything that could help?

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  • shabaz
    shabaz over 2 years ago +6
    It arrived today, I'd say it is a 7/10 product.. it will certainly fit many plugs, but the build quality looks very average. The hinge looks fragile, although perhaps that's deceptive. I don't know. …
  • charlieo21
    charlieo21 over 2 years ago in reply to anniel747 +5 suggested
    Definitely the simplest and cheapest option, we have those in the lab, it works perfect to avoid someone use an equipment without authorization. Another option are those smart plugs that you can control…
  • beacon_dave
    beacon_dave over 2 years ago +5 suggested
    A long shackle padlock combined with a Lego Brick sized block with a hole in it (placed between the pins) could probably fit over a 13A plug top preventing it from being plugged into a socket. I don…
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  • Fred27
    0 Fred27 over 2 years ago

    If you do end up wanting a 3D printed solution then let me know. I'd be happy to print something and send it to you.

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  • Fred27
    0 Fred27 over 2 years ago

    If you do end up wanting a 3D printed solution then let me know. I'd be happy to print something and send it to you.

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  • shabaz
    0 shabaz over 2 years ago in reply to Fred27

    Hi Fred,

    Oh that's awesome, thank you for the support! 

    I've ordered the Martindale product that anniel747  found, and will give it a shot! If it's no good then I think we should design a 3D solution since it could help many people! : )

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  • beacon_dave
    0 beacon_dave over 2 years ago in reply to shabaz

    Thanks to BS1363, at least the pin spacing should be pretty consistent. Not much else will be, so may need a parametric 3D model that can adjust for width/height variations.

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  • beacon_dave
    0 beacon_dave over 2 years ago in reply to beacon_dave

    If you can keep the tolerances pretty tight then something along these lines might work. The trick is not to allow enough slack to be able to wiggle the plug pins upward and out of their retaining holes.

    image

    It has to allow for variations in cable exits / strain reliefs, hence the hole options for the padlock.

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  • shabaz
    0 shabaz over 2 years ago in reply to beacon_dave

    Hi Dave,

    That's very clever! Neat how it would need no special tools to construct, and doesnt matter what shape the rest of the plug is. I think it should be up to the user to drill a hole as close as possible to the plug (since virtually everyone has a drill at home or can use a neighbours one) and then it is tailored to the plug to be as close to the cable entrance as possible.

    It's a fairly perfect solution I reckon. I can't see any flaw. I might prototype up this on Sunday, just as a quick experiment, since all that's required is sheet plastic and superglue.

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  • beacon_dave
    0 beacon_dave over 2 years ago in reply to shabaz

    Yes, I was trying to come up with a design that was able to be be constructed with basic tools/materials if you didn't have a laser cutter / 3D printer / mill but also that could accommodate the large variation in plug designs. The pin spacing is pretty standard gauge unless you are using counterfeit plugs.

    The one doubt I have depends on the padlock used and how much of a gap the shackle leaves when it is unlocked. Some are flush with the top of the body of the padlock which you want to avoid.

    You will need to insert the padlock into the slot and slide it all the way to the non-plug end so that the open end overhangs the end of the plate, so that you can catch the cable into the hoop of the shackle. As you start to turn the shackle into the closed position, this is where the problem may occur. If the padlock doesn't have enough of a gap, or the top plate is too thick, then you may have problems getting the padlock through a hole.

    You may need to increase the width of the slot to allow the padlock to be tilted to slide it back up the slot and into the desired hole before twisting the brass part to line up with the shackle to allow it to lock. Using a thinner top plate will also help here.

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  • beacon_dave
    0 beacon_dave over 2 years ago in reply to shabaz

    One refinement might be to use the plastic cover that is often found on the pins of  readymade 13A/IEC leads. They are usually quite a snug fit which is what we are looking for here.

    So you could drill larger holes and epoxy one of these covers into them. You could probably get away with only two laminated layers here as the cover will now be doing the job of the third layer. 

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  • beacon_dave
    0 beacon_dave over 2 years ago in reply to beacon_dave

    The removable shackle type of padlock would make the construction easier as you just need two holes, no slot cutting required.

    image

    Not as common however.

    Padlocks are difficult to search for - most are shown in the locked position so you can't tell how much clearance they have when open.

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  • anniel747
    anniel747 over 2 years ago in reply to beacon_dave
    This reply was deleted.
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  • beacon_dave
    0 beacon_dave over 2 years ago in reply to anniel747

    Yes, I had thought about one of these and then grinding the ends off as they are easier to identify on-line.

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  • anniel747
    0 anniel747 over 2 years ago in reply to beacon_dave

    They are available in many lengths, probably one just right.

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