Help me make this LED light up!
September 17, 2021 6:38 AM   Subscribe

I am trying to get this project to work so I can do it next week with kids at my library, but I can't seem to do it right.  The LED doesn't light up. Can you take a look?

Here's what I have tried:
Changing glue from clear to regular white
Using liquid watercolor
Using food coloring
Using different size paper clips
Stripping the wires to make them longer to wrap around the paper clip more securely
Making the path from paper clip to LED shorter
Using tons of salt.
I checked the LED with the battery pack and it lights up fine, so that's not it.
Any ideas?
posted by Biblio to Science & Nature (4 answers total) 1 user marked this as a favorite
Have you tried the LED in both orientations just to be sure?
posted by achrise at 6:52 AM on September 17, 2021 [2 favorites]

Best answer: What colour of LED are you using? If it's a blue or white, it's going to be marginal if fed from two AA batteries unless those are absolutely fresh, and more so with the resistance of the salted glue in the circuit.

Also the only way this is ever going to work is if the glue is still wet. Once it dries, salt is going to do diddly squat to make it conductive.
posted by flabdablet at 6:57 AM on September 17, 2021 [1 favorite]

Response by poster: It was a blue LED. I switched to white and it lit up!
Thank you!
posted by Biblio at 7:08 AM on September 17, 2021 [3 favorites]

One extra AA will help. As Flabdablet mentions, blue and white (which are actually blue internally, with a phosphorescent coating on the chip which emits white) LEDs need a higher voltage than red and yellow LEDs. The resistance introduced by the salty-glue 'wires' will cause a voltage drop so that the voltage at the LED is barely sufficient to light it up if it's a blue or white one, though you may see such a LED still show a faint glimmer if you take this circuit into a darkened room.

You can show that salt makes the water conduct by taking the two AA batteries, the LED, three wires and the two paperclips. Create a circuit so that you have:
[battery +]-wire-[LED, long leg]-[LED, short leg]-wire-paperclip
[battery -]-wire-paperclip
If you touch the paperclips together the LED should light up. Now hang the paperclips in a glass of plain tap water: the LED should not light (unless you have very bad tap water, in which case you should use demineralised water). Gradually add salt, and soon the LED will start glowing faintly. To increase its brightness move the clips closer together or add more salt.
posted by Stoneshop at 11:39 AM on September 17, 2021 [2 favorites]

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