Let's Make Robots!

Xmas Lights Tester

So, you accidentally stick a regular clear bulb in your Christmas lights (fairy lights [US] ?) instead of a fuse bulb. Next time there's a bit of a surge, there's no discernable weak point in your circuit and it takes out more than one bulb. Where do you start debugging that?

Sure, you could sit with a multimeter and probe all the bulbs to find the duff ones. Have you ever tried to attach multimeter probes to a fairy light bulb?!?! It's a nightmare. Specially when you only have 8 fingers and two thumbs.

My bulbs have these little skimpy connectors and I think they're enamelled, so you'd be really lucky to get a closed circuit even with a good bulb.


Knickers to that. Chop one of the bulb holders out of the set and attach it onto a pair of 4mm banana plugs. Test all 40 bulbs in about 2 minutes. Patch the bulbholder back in. As it happens, 11 of them were blown (not bad!) and I only had 5 spares. So the whole set went into file #13 (the garbage).


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Sure, you could sit with a multimeter and delving all the bulbs to acquisition the duff ones. Accept you anytime approved to attach multimeter probes to a bogie ablaze bulb?!?! It's a nightmare. Specially if you alone accept 8 fingers and two thumbs.

Which is why I soldered a bulb holder to my multimeter.

I hadn't thought of that! I have a strand I've been dreading to test, now I feel a bit better :P

Nice new icon btw :) 

Fairy lights? Christmas Lights? Oh, you mean Solstice lights...
Are you one of those non-religionists? (That's for a different thread, me thinks.)
I keep trying to think of another use to put a string of Christmas lights to, or the wire. Ready made sockets, and I believe the bulbs pull out of the bases if the wires are straightened. Possibly insert LEDs, and run a group string on a different voltage?

Me too. Trouble is that to be under MCU control it would probably need to be low voltage. These bulbs can run 140mA at 2.5V if you have a 100 light set. If 10 of them are on at the same time, that's 1.4 amps. That's gonna need a heck of a sized PSU.

I have a couple of LED chains running of a 2x AA cells. They weren't terribly expensive, either.

Usually the lights are in series which is why when one goes, they all turn off. With LEDs instead of bulbs, you need to allow between 2.1-2.4V per LED depending on colour so for 10 on a circuit, thats about 23V (go 25V with a 100ohm pot to adjust current/brightness). A digital output driving a NPN transistor to handle the higher voltage will do nicely and the string of leds will only draw 15-20mA unless your using high powered LEDs. Watch polarity, put a LED in the wrong way and you'll get magic blue smoke.

Are you being ironic in telling me that Christmas lights are in series? :-)

That aside, I think robologist was looking for an alternative use for teh lights. There's very little you can do with 10 LEDs in series other than switch them on and off. My commentary on parallel connection of bulbs was aiming to open the potential (!) for sequencing the lights where 10 may have been all on together, sucking big ampage.

In the meantime, I'm going to stick my head in the power supply cupboard and see where I left all my 25V PSUs.
What are fairy lights?