DIY LED Bulb Lacks Correct Lugs, So M3 Bolts To The Rescue

[Damo] has an interesting LED bulb project with a neat twist: he converted some outdoor lighting to 12 V LED lighting with some self-designed bulb replacements and decided to reuse the existing GU10 sockets and wiring. That meant putting GU10 lugs onto his custom PCBs, but he ran into a snag.

These GU10 bulb lugs are tough to buy in small quantities. Luckily, M3 socket head bolts are a near-perfect match.

Those distinctive lugs that twist into GU10 sockets? [Damo] simply couldn’t find anywhere offering to sell them in small quantities. So he did what any enterprising hacker would do and found a substitute that was both accessible and economical: M3 bolts. Apparently, socket-headed M3 bolts are pretty much identical in size to GU10 lugs. Who’d have thought?

[Damo]’s retrofit worked great, and thanks to M3 bolts he was able to re-use the existing weatherproof wiring and sockets in his yard. His design files are here on GitHub.

We do love using things for other than their intended purpose, but as [Damo] points out, GU10 sockets are normally connected to mains power. So if you decide to use his design (or use GU10 sockets for your own purposes), be aware that you’ll have hardware that looks interchangeable with other (mains-connected) sockets, but isn’t. Be mindful of that, and take appropriate precautions. Avoiding electrical oopsies is always worth putting effort into, after all.

30 thoughts on “DIY LED Bulb Lacks Correct Lugs, So M3 Bolts To The Rescue

  1. For a moment I thought this runs on mains, then I saw the 12V print. I only have LED bulbs running on mains at home. At least this is not another self-electrocution HaD :)

    1. We had Schuko sockets on our ship that gave out 230V (only when connected to shore or the inverter was on) and Schuko sockets on our ship that gave out 24V. To differentiate them, the former were painted red with a sharpie, which weathers away quickly in open weather and off course is very visible at night, when you need a socket to plug a light in.

      I am very surpised no lamps burned and no electrocutions happened, ever.

      1. The TUV does not approve. Seriously, WTF? Using the same connector for 230V and 24V?!?! Whoever did this is not sane in mind. This is horribly dangerous! There are so many connectors, shouldnt be too hard to find something else for 24V power.

        1. Plugging 230v devices on the ship’s 24v outlet won’t do anything as there isn’t enough voltage to make a 230v light bulb light up or anything. No harm.

          Buuuut… if someone were to plug the 24v device from the ship in a 230v outlet, something will blow.

          1. Well, if it’s 24V DC and there is a transformer in the 230V device some bad things will happen… But yeah, “classic” transformers are not that common now (maybe on ships?). And yes, if a 24V device goes into the 230V outlet it will go BUMM. That’s why the 24V stuff should have some other sockets *and* connectors.

      2. Being currently in the market for a new boat, and looking at a lot of them, I also encountered a few repurposed schuko outlets (admittedly not modern schuko, but an old fashioned non-recessed two pin outlet)

        Only identifying information was a permanent marker scribble saying ’12V’ on the socket face.

        Didn’t see what item was supposed to plug into it, but I didn’t like it. Easy to replace of course.

        Real problem is a lack of ubiquitous 12v outlets that aren’t a shitty lighter socket. I don’t know what I’d use if I wanted decent 12v sockets. Perhaps a UK socket and plug, at least then the chances of confusion with a schuko plug or socket are less likely.

  2. Neat hack but I have to say misappropriating mains connections for other purposes is bound to end in trouble. I know because Ive been there it works great while your the only one that touches it but one day some one will give you a hand and do it wrong. Thankfully in my case it just resulted in a burnt out motor but could easily have been much worse.

      1. Yeah, if you allow something by design, it will eventually end up in a catastrophe.

        Also the bolts will probably get loose unless long list of requirements are filled during installation. Just watch any Sandy Munro clip on YouTube and there are opinions of a person who dealt with bolts in large scale assembly operations.

    1. I don’t know about GU10, but 12v sockets are pretty commonplace in my experience? Bi-pin lamps come in either 12v or mains, and there’s nothing in the socket to tell you that they’re either kind other than the lamp that was connected to it. I have a few toroid transformers from ceiling fans that I converted to 220v led

    2. As long as you keep it this way and the wiring in the walls switches etc is all clearly not mains cable anybody coming upon it should go ‘huh’ and figure out its low voltage using higher rated connectors. Really its the best way round to do this as anything ‘standard’ for that connector will at best work really poorly at worst just not work at all.

      Just don’t try and be ‘smart’ and have both voltages potentially available with the same connector, even if its a carvan/boat system which might reasonably be dual voltage as that will get confusing even to you the maker I expect in a few years time – either everything with that connector is 12-36v DC and you have a transformer, or its all 240V AC with the inverter, or you have entirely separate wiring and connectors for shore power and onboard battery power. The latter being the simplest but most inconvenient IMO as you don’t want to have two of every device and battery charger for the dual voltages but a large enough inverter or transformer to cover your entire draw isn’t trivial to find or cheap.

      1. The trouble is that line voltage AC connectors, wiring, and switches are not low voltage DC rated for short circuit current protection and things like arc suppression / contact welding etc.

        Re-purposing the stuff for what it’s not meant for is a recipe for a house fire.

      2. In case you didn’t know, connectors and switches have lower rating for DC than AC because there’s no zero crossing to kill off an arc or stop contacts welding together, and lower voltages need thicker cables to make the circuit breakers work properly. Thinking that a system is “higher rated” because it was built for higher voltages is entirely misleading.

        1. Yes I am aware, but at the low voltage and rather low current DC end used in caravans and the like you well within the capabilities of any mains AC gear I’ve ever seen (some even have DC ratings stamped on them – usually rather high DC rating – well high in terms of what you usually see in caravan/boat/battery power tools etc).

          You are entirely correct, but in the low end DC we are talking about here its pretty much safe to use pathetically small sized bare conductors and a clothes peg to hold them together as a switch. Maybe there is enough energy with a short to melt that wire and make a stink but actually igniting anything else is rather unlikely. NOT that I am at all suggest anybody should do such a thing, when its so easy to actually be in the specs or well over engineered with cheap off the shelf parts.

          1. The number one reason that cars burn down is because of electrical faults. RVs are no different.

            The problem is that the low voltage system has to pass enormous amounts of current to drive any meaningful load, so any fault current is easily the same as normal operation. A frayed wire touching another in a short circuit can pass 10 Amps but not more, and heat up with 100 Watts of power at the contact point which is not enough to melt the wires and break the contact, but easily enough to make everything else smoke and catch flame.

            The flimsier the wiring, the more dangerous it becomes because normal circuit protectors just won’t work.

          2. >The flimsier the wiring…
            Yes and no – as what is a fuse but deliberately flimsy wire that will fail rapidly to make everything safe.

            And even if you assume the wire won’t get hot enough to actually fail its going to stink like buggery rather early on in the process set of fire alarms etc all long before it should have been able to ignite anything else – as you don’t tend to surround your electrics with highly volatile materials..

            Cars may start to burn for an electrical fault, but generally they really burn because they are full of stuff that really likes to burn and that stuff was given that kick by the electrical fault, take the fuel and oils away and usually not much happens…

            BUT NOTE I never said you should use flimsy wiring, or anything of the sort, just that at the low power end of DC what is required for safety isn’t all that vast. Along with that all mains components I’ve ever seen are either actively rated and stamped into the parts with all the other specs to well over the normal Car/RV/Boat/Powertool type end of DC power or at least so overbuilt for that level of DC power that official rating or no it shouldn’t pose any problems in itself. Just obviously use the right type of Fuse etc for the job in hand!

          3. Also worth pointing out I don’t like this idea at all, in case that isn’t clear enough. Anything that can get confusing by misappropriating such a reliable standard isn’t a great idea.

            Just that this is the sane way doing it as low power DC through off the shelf parts able to deal with way way more than that load is by far the better way round than putting mains through the molex, 0.1″ pin headers, or perhaps even smaller less suitable for mains ‘normal’ connectors more common for this sort of low end DC use..

          4. >as you don’t tend to surround your electrics with highly volatile materials..

            Except in cars and RVs where they’re surrounded with everything paper and plastic, including the wire insulator itself. That’s another myth that cars catch fire because of oil and gasoline – it’s the interior lining and seats that go up much quicker.

          5. >I never said you should use flimsy wiring

            You said it was “pretty much safe to use pathetically small sized bare conductors”, which is pretty much the opposite. Using undersized wiring in a low voltage system renders your fail safe mechanisms in the fuse box completely moot because your wiring will limit the current and turn into the load themselves.

          6. > this is the sane way doing it as low power DC through off the shelf parts able to deal with way way more than that load

            No. It’s not the sane way of doing it. Electrical systems and components designed for household AC are not designed to deal with “way way more than that load” in a low voltage DC situation. It’s the opposite.

            A 230 Volt 6 Amp circuit can handle 1400 Watts but the same circuit with a 12 Volt DC source can only supply 72 Watts, but not safely because the switches and contacts are sized for AC which breaks every half-cycle while DC does not, so you get contact erosion, arcing and welding, localized heating and probably a fire if you try to operate it at the same capacity for too long. A switch that’s rated for 10 Amps AC will typically be rated for 1-2 Amps DC because of that. All the switch gear has to be built up for low voltage DC operation, and then the wiring has to be sized up to account for the fault current situation being close to normal operation.

            It’s reckless and dangerous to suggest that you can “safely” use household AC parts to wire up your mobile home.

          7. We are talking here about really tiny LED loads, the sort of load that is actually rather smaller than the power sent down tiny conductors to your high power laptops and the battery power tools, that have switches that are usually nothing but a tiny bit of brass spring. Its not a massive multi KW load, it is but a relative handful of watts DC – in the case of battery power tools you probably are holding a much much higher load being controlled by vastly thinner wires and switches..

            So they are using connectors and wiring that officially rated or not can take massively more DC than needed – and likely do have a DC rating as most things mains AC seem to.

            And I said it was pretty much safe to use bare conductors at this level, as it is, not that I suggested doing so – this is not high power stuff. So unless you go bathing the area around it with something really easy to ignite or are using a battery that likes to go pop at the tiniest provocation there just isn’t much danger even when things do go wrong. This isn’t much above, if above at all, the sort of power level you let kids just play with in science lessons (or did not all that long ago) with all the bare conductors making shorting inevitable…

            I’m not at all suggesting you should try and pump all the power it can do in AC though it in DC mode, or anything daft like that! But the handful of watt level load we are talking about the standard ‘AC’ stuff (at least in the UK) is massively massively overbuilt for in DC, and often has rating stamped on it for DC that is actually better than most of the DC only switch and connector you can easily buy!!!

    3. Unfortunately there are already a few clueless companies selling 12V GU10 lamps. For example Airam 4713795 and Sunwind 243220.
      They recommend it for boats and caravans but there is always a risk of mixup. The Sunwind lamp even has the uninsulated LEDs exposed for the user to touch, this could get really dangerous if someone replaces the lamp with power on.

    1. Agree. Stainless steel have a durable layer of chrome oxide on the surface, that is not only protect steel fron rusting, but also is a perfect isolator. Witnessed a lot of overheating problems in different fancy, expensive and shiny enterprise equipment where manufacturer used stainless steel bolts and nuts in power connections for “professional” look. Replacing them with brass/zinc plated ones fixed problem.

      Better change M3 bolts & nuts to brass/bronze screws & nuts, or at least regular zinc plated ones. It is also will be easy to just solder brass/bronze/zinc plated bolts to PCB pads making perfect and reliable connection.

      1. whenever I see ugly yellow/brown screws on my terminals I feel comforted that I’m dealing with a proper alloy with enough copper in it to do the job. I find even aluminum screws to be preferable to stainless. but they have their own set of problems. (minimum breakdown voltage for the oxide layer)

        this stuff gets complicated when you are doing a screw terminal strip when you have it exposed to moisture, frequent temperature changes, shipboard conditions, high currents, etc. the wrong material can back the screws out and then you have live wires disconnecting or the ends melting off due to poor contact or arcing.

        The M3 socket heads for this project are a great idea. Using stainless is good for a quick fit test. You could theoretically special order a steel and plate it yourself, or get black oxide screws and grind it off. Aluminum would be a better conductor than even mild steel. Theoretically 12V is enough to make it work reliably, but higher voltage is better without specially designed receiving connector.

        Like all good hacks there are compromises. Between cost, time, effort, and functionality. I don’t think any compromises on safety have been made, so thumbs up from me!

  3. GU5.3 would have resolved the voltage ambiguity, but 12V GU10 bulbs are not unheard of. Still, if the bulb circuit design allows, an extra inductor could probably keep the bulbs safe if accidentally connected to 120V.

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