If you want to test your electrical circuits, here's a way to make a great test probe for your car. I made mine in about an hour. It is great for probing the rear of your lamps / fuses to see if power is available.
The advantage of making this probe is that unlike those in shops, this one uses a LED to indicate. LEDs indicate with FULL BRIGHTNESS @ just 15-30mA of current. That 15 to 30 thousandths of an amp. You can buy High brightness leds of your colour choice at electronics shops.
LED's are much more robust than globes and can take the rough testing environment.
Items you'll need
1 Plastic Bic pen. Straight as possible with no clip, make sure it is all plastic in construction.
1 Black test lead with insulated alligator clip, thin wire light duty only, required.
1 LED hi Brightness. Colour, your choice, I use red.
1 1/2 watt 1000 ohm resistor.
1 Nail
1 Drill
Soldering iron & rosin cored electronics solder.
150mm length of small multi strand wire i.e. speaker wire etc.
Heat shrink tubing to go over resistor & Led leads.
Procedure
Take the pen & remove the ballpoint & ink etc.
Remove the little indicator cap at the end.
Drill 2 holes in the indicator cap and glue/ silicone the LED to the cap.
Take your nail, test that is is a snug fit into the writing end of the pen.
If you wish, shape the pointy end of the nail with a grinder or file into your preferred tip, I made mine like a screwdriver, so it would fit inside electrical plugs better.
Lay some solder 1 cm down the HEAD end of the nail.
Bare the speaker wire & pre solder the bare wire.
Wrap the bare wire over the solder on the nail & apply heat to fuse the two together.
Trim the other end of the speaker wire about 2 cm shorter than the length of the pen.
Bare 1 cm of this wire & solder 1 of the resistor wires to this wire.
Solder the other resistor wire to the
LONG LEAD OF THE LED. This is the anode and if you get it wrong, the led won't work.
Insulate all exposed soldering with tape or heat shrink tubing.
Take the black wire 400mm long if you can get it. Clip off 1 of the alligator clips.
If necessary, drill the little breather hole on the side larger to accept the wire.
Feed the wire out the large end of the pen, not the writing end.
Solder this wire to the short end of the led using the same method as described above.
MAKE SURE THE TWO LED LEADS ARE INSULATED FROM EACH OTHER ADEQUATELY. I used silicone which took overnight to cure.
Place the tip on 12v+ and the alligator clip on 12-, the led should light.
If so, gently move the assembly down into the pen casing and glue / silicone the nail into position.
Push the indicator cap with LED into the pen housing. You're done.
Remember, the Light will only work if the tip is touched to +. If you connect it backwards, the led will not light. Diodes are like valves, they only work in 1 direction. The alligator clip has to be attached to an acceptable earth as well for the circuit to work. There is a lot of plastic in modern cars, so look for strong bolts / nuts or chassis components to attach to.
Maximum test voltage is 30V DC
Warning
Do not use this device on household voltage. The Light will explode & electrocution could result.
