Introduction: DIY Circuit Flow Direction LED Test Pen

Ever wondered in what direction the electricity travels? Build your own circuit flow direction LED test pen to find out!

It is quite a long and fancy name for something that really is a LED on a stick. The build time was about an hour while taking photos and the total cost is somewhere around a dollar.

You might already have a tool that does this, but it is much funnier to use tools that you made yourself.

In theory:
Electricity travels from the positive end to the negative end. By using a conventional test pen we can see if there is two cabels connected, but using the LED pen we are building we can also see in what direction it travels. This can be useful when you have LEDs in your circuit or other components that only lets electricity travel in one direction.

LEDs will only let current pass from the anode (long leg) to the cathode (short leg) and not the other way around. We will therefore build a tool that will let us connect two points and see if our LED lights up. If it does, the tip is touching the positive point. If it doesn't, we will switch points. If the LED lights up we now have found the positive end and if it doesn't, we know there is something wrong in our circuit.

Step 1: Parts and Tools

In order to build this pen you need the following parts;

- 1 Pen
- 1 LED
- 1 Resistor
- 1 Soldering tip
- 1 Alligator clip
- 2 Wires
- A piece of heat-shrink tubing


- You can replace the pen with a straw, pipe or something similiar

- Choose a LED that matches the diameter of your pen

- I used a 560 Ohm resistor along with a red LED. This makes it possible for my pen to test circuits up to 12V DC. Anything higher will ruin the LED. Use the following calculator to find out which resistor to use in your pen;

http://led.linear1.org/1led.wiz

- The soldering tip can be replaced with anything pointy. I used when because it was lying around and had proper dimensions.

- One of the wires should be slightly longer than the pen. The other should be at least twice the length of the pen.

Tools I used:

- Screwdriver
- Wire stripper
- Helping hand
- Soldering stick
- 9V Battery + clip for testing
- Wire cutter
- Super glue
- A piece of tape

You could pull this off using only tape and glue. The other tools just makes it easier.

Step 2: Take the Pen Apart

Screw of the top and bottom and throw them away. Keep the body of the pen.

Step 3: Start Soldering

- Solder the resistor to the anode (long leg) of the LED. The reason we do this is so we can hide it inside the pen. The pictures illustrates why it is wise to invest 10$ and get a helping hand if you plan on doing any soldering.

- Stripe one end of the shorter wire. Attach it to the tip of your choice. I decided to use tape for this. That way I could make it easier to fit the tip inside the pen.

Step 4: Add the Wires

- Attach the tip of your choice to the pen. Pull the wire through.

- Drill a small hole in the pen. This is for attaching the ground cable. Make the hole about as far down on the pen as the LED cathode will go.

- Cut the wire that is attached to the tip so it is just slightly longer than the pen. Enough for you to do some soldering. We will stuff all that wire inside the pen later on along with a resistor.

- Strip both wires.

Step 5: Connect Everything

- Solder the positive wire (going to the tip) to the resistor on your LED

- Pull out some of the wire that leads to ground. We need to use some heat shrink tube to prevent any short-circuits. Make it slightly longer than the cathode and put attach it on the wire. This will also help us keep the ground wire in place.

- Solder the ground wire to the cathode. Pull the tube over it and shrink it.

Step 6: Put It Together

- Gently push everything inside the pen while pulling the wires out. Take off the tip if necessary in order to be able to pull the + wire out and then put it back.

- Attach the alligator clip to ground.

Step 7: Test It

Try it out on your battery!

If everything is done correctly the LED will light up when you have the tip on + and the clamp on -. Nothing should happen when reversed. If it doesn't work, make sure you didn't break anything by using too much force when you put the pen together.

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