When you have a 9 volt battery and you want to test if a red LED (3 Volts) works, without blowing it, what do you do?

Answer: Make a variable resistor by whittling a pencil.

Supplies

2H Pencil
Knife
4 alligator clip cables
Multimeter

Step 1: Do Some Whittling

Whittle the end of a 2H pencil with a sharp knife untill the lead is exposed. This will allow an alligator clip to be connected.

Connect a multimeter to both ends of the pencil and measure the resistance.

Step 2: Testing Resistant of Different Pencils

Pencils use graphite as lead. They come in different hardness from 6B (almost pure graphite) to 5H (Hard, lead is a mixture of clay and graphite).

Graphite conducts electricity. Pencils which are harder (H's) don't conduct electricity as well as the Blacker, softer ones (B's). The (H's) are more resistant and act as an electrical resistor.

I measured the resistance of different pencils.
5H - 40 ohms
2H - 30 ohms
HB - 16 ohms
6B - 2 ohms
Different pencils of the same type can vary a bit.

Step 3: Whittle Some More

The 2H Pencil seems to be a good one to make a variable resistor from.

Whittle to the lead in the middle of the pencil. Cut away from fingers, towards the centre, rotating the pencil 180 degrees after ever few cuts.

Test the resistance of half the pencil with a multimeter.

Step 4: Maths to Calculate Resistors Required

The website 'learningaboutelectronic.com' had this to say:

'To reduce voltage in half, we simply form a voltage divider circuit between 2 resistors of equal value (for example, 2 10KΩ) resistors.'
It showed the picture above.

It also showed a formula, to select any voltage. It seemed a tad complicated. Picture above.

They even included a calculator to do the maths for you: Caculator

Anyway, back to the pencil.

Step 5: Connect and Test

Connect a 9 volt battery and test the voltage at the end and then half way along. I found the voltage had about halved.

Step 6: More Whittling Then Test

Whittle more wood off the pencil, so that most of the lead is exposed. Leave at least an inch at the pointy end so the pencil can still be used for writing.

Test the voltage output at various places along the pencil. Once you have the voltage required, use it.

Step 7: Testing LED Light

I tested a red LED light. I found it lite quite brightly at 3 volts but also glowed at 2 volts. This is useful to know because I have a project I will use it in, which doesn't generate many volts:

Mini Drone to Mini Electrical Generator

This variable resistor uses up the battery quite quickly so should really only be used when you don't have a better means.