ajfisher's doing space

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Posted by ajfisher at 9:39 pm

Creating a glowing globe is pretty easy – my son showed me a really quite poor video on YouTube where a glowing globe was created with just an LED, some wire and a battery frozen in a water balloon. We decided to do it properly (you know with like current-limiting resistors and all that) so he could take it for show and tell at school.

It’s still pretty simple, but in typical 6 year old style he wanted multiple LEDs, of different colours and the largest balloon he could find.

An early prototype using a ice block maker

So we decided that a 9v battery meant you could have a string of 3 LEDs and we could run at least a couple in parallel so he could have a red set and a yellow set that could make a nice orangey glow inside the ice.

Calculating the resistor size

The process was pretty simple, 3 of the same coloured LEDs in series with a current limiting resistor – assuming 20mA from the battery we can calculate the resistor as:

R = \frac{\left ( V_(supply) - \sum V_{f(LED)} \right )}{I}


R = \frac{V}{I}

In our case the red LEDs are 2.1Vf and the yellow ones are 2.4Vf so:

Resistor (Red) = 9 – (2.1 + 2.1 + 2.1)/ 0.020 = 135Ω

Or next closest is 150Ω so we’ll go with that.

Resistor (Yellow) = 9 – (2.4 + 2.4 + 2.4) / 0.020 = 90Ω

Or next closest is 91Ω so we’ll use that one.


For each LED chain you’ll need:

3x LEDs
1x Resistor as calculated above
2 lengths of hookup wire at least 40cm long

Additionally you’ll need:

Balloon that can hold water
Some cable ties
A 9V battery

Wiring it all up

Chain the LEDs of the same colour together in series – ie + side of one LED (the long leg) goes to the – side of the next (the short leg) so you have the three linked up. You can add the resistor either side, doesn’t really matter because everything is in series. Last of all join your set of LEDs and resistor to your battery using the hook up wire. You want to give yourself a decent length of wire or else you’ll run into problems.

Do the same thing to your other set of LEDs, again linking it back to the battery.

Extra points awarded if you choose to solder all your legs together and clip them so you don’t short anything when you put it into the balloon. I also heat shrunk all the connections to stop any rogue bits of wire piercing the balloon.

Test the circuit by connecting the battery and you should get some nicely lit LEDs if everything has gone correctly.

Insert LEDs into the balloon

There’s two ways to try and do this:

The first method is to insert your LEDs into the balloon with as much wire as possible leaving the last little length hanging out. From there if you can put over the end of a tap and fill with enough pressure the balloon will fill. Caution – you are likely to end up quite wet doing it this way but if you can stand water spraying all over the place then this is the best way to do it.

The second method takes a lot more patience. Blow some air into the balloon then fill with water to well over the desired size. From there grip the neck of the balloon and then feed your LEDs into it. The balloon will leak but if you put air in you should get a way in before the balloon leaks too much.

From there cable tie the neck then attempt to tie a knot in it if you can. In the end we did a loose know and used about 3 cable ties.

From there into the freezer. The balloon will likely leak so the best thing to do is put it in something for a little while like an ice cream bucket. After 30 mins or so the neck will freeze creating a dam. From there you can remove the spilt water and you should be right to complete the project.

Let it freeze solid for a day or so.


Once the water is solid, remove from the freezer and remove the balloon. We ran it under a little boiling water to melt it slightly, slit it with a knife and it came off nice and easily. Watch your wire if you use a knife near the neck of the balloon.

Once you have it out, connect it to the battery and you should have a nice glowing orb of ice. You can now do all sorts of things like spin it whilst taking long exposure photos to make it look like a planet or something. You’re only limited by your imagination – you could try sculpting if you were brave too…

You can check out the photos in the project here