Introduction: Automotive Current Detector

About: I miss the days when magazines like Popular Mechanics had all sorts of DIY projects for making and repairing just about everything. I am enjoying posting things I have learned and done since I got my first to…

I need to replace a current detector probe for 12 volt automotive systems. I decided to use an LED mounted inside a dried up felt tip marker. I will also need to add a current limiting resistance so the LED can safely operate where as much as 14 volts may be present. All of this will need to fit inside the shell of the old marker.
The photo shows my finished test probe indicating current flow between the positive (+) and negative (-) posts of my car’s battery. The LED is red, but the camera saw it as white because it is so bright relative to what is around it in the photo.


  • Marker case
  • Brass screw (8-32 3/4 inch in length)
  • 470 and 100 Ohm resistors
  • LED
  • Alligator clip
  • Plastic covered wire

Step 1: Resistance Value

The LED I am using was an extra in a kit. It came without data. I am assuming 2.2 volts will be a safe forward voltage for it. 20 milliamps is a safe operating current. At those figures, the LED may not light up as brightly as it could, but those figures will also provide an easily seen brightness, and that is all that matters.

If the engine on my car is running, the voltage at the battery could read as high as 14 volts, even though it is classed as a 12 volt battery. I will use 14 volts as the source voltage figure (VCC).
I have an LED calculator app. on my iPhone. When I enter those figures into the app. and press “Calculate,” the app. tells me I need between 550 and 590 Ohms to protect the LED. Parts I have on hand include some 470 and some 100 Ohm resistors. Connecting a 470 and a 100 Ohm resistor in series gave me an actual resistance of 560 Ohms, which is well within the necessary parameters.

Step 2: Open the Marker Case

My used marker is a Sharpie. The case parts are not welded together, but the front section has raised rings around it that make a friction fit when they settle into grooves inside the body. I was able to separate the parts with a slip joint pliers and pushing the barrel to one side as I pulled them apart.
The second image shows all of the parts of the pen. Discard the felt tip and the cellulose ink reservoir. I also removed the little swivel top with the lanyard ring and discarded it.

Step 3: The Tip

An 8-32 screw fits very nicely into the hole for the felt tip. It is just snug enough to screw into the hole and be held firmly in place. I used brass so I could solder one end of a resistor to it. I took advantage of the slot on a flat head screw because it allows the lead of the resistor to pass through it at one end of the slot, but also leaves the slot undisturbed for turning with a screwdriver.
See the second photo. Keep leads for the resistor off to one side and use a small screwdriver to thread the screw into the hole from the felt tip. I had a little difficulty getting the screw aligned to start in the hole. (The screwdriver handle is in a vise so I could hold the camera to take the photo.)

Step 4: Drill the Cap End for the LED

Measure the diameter of the LED and drill into the cap opening and through the top of the cap to make a hole for the LED. I had to use a file for a smoother, better fit.

Step 5: Fitting Resistors Inside the Body

See the first photo. The brass screw has been turned into the front section of the pen as far as it can go. Solder the second resistor to the end of the first. I had to add an “S” curve to the leads so everything will fit as needed inside this smaller pen case. See the second photo. Drill a hole in the end of the pen body for the resistor lead. Push the front section and the body of the pen together so they snap into place.

Step 6: How Will It Fit Together?

There needs to be a hole in the side of the cap for the wire that is the lead on the probe. See the first photo. I checked the fit of the cap and used my thumb roughly to note how far the pen body goes into the cap when the cap is posted on the body. Drill a hole for the lead wire just a little higher. See the second photo.

Step 7: Solder the LED Into Place

The longer lead on the LED connects to the resistors and the tip of the probe. This will connect it to the positive (+) terminal on the car battery or to a “hot” wire on the car. I use a heat sink to protect the LED while soldering. The photo shows soldering the lead to the negative (-) lead of the LED. Insulate the positive lead of the LED with electrical tape.

Step 8: Assemble and Test

Carefully snake the probe lead out the side of the cap while snaking the LED into its hole in the cap and snaking the cap down onto the pen body.
I have an old alligator clip from the back of an analog television and its 300 Ohm antenna connections. I am using it to clip the probe to a ground on the car. I could make the probe lead longer.
If all was done as intended, the LED will glow when placed in the proper polarity on an automobile battery.
This probe can be used to check for power in an automobile circuit, or to check where power is lost. Once there was a period of time when I used a probe like this to static time my car engine, but that was in the days of ignition points and distributors.
The tip of my probe is not pointed, but it is threaded with 8-32 threads. I could fit a sharp point to an 8-32 coupling nut and thread it onto my probe tester when needed for penetrating the outer insulation on a wire. I would probably prefer to stick a thumb tack into the insulation on a wire and touch the brass screw to the thumbtack.
This test probe is designed for a maximum voltage of about 14 volts. I checked and it will glow faintly all of the way down to 1.5 volts, which is a AA battery. Its applications are far broader than automotive circuits.

Step 9: Add Continuity?

At the suggestion of one of the commenters I did more thinking about possible use of this probe as a continuity tester. I did that as an add-on accessory rather than making it an integral part of the primary assembly. The first photo shows touching the continuity lead to the grounding clamp. Notice the LED is lit because there is a circuit powered by a 9 volt battery. The battery uses a common snap on clip for a 9 volt battery. The polarity on the clip cannot be reversed.
See the second photo. I had made a sharp point for the probe. It is from a short piece of 5/16 inch rod center drilled enough for the 8-32 threads on the probe. I tapped the hole to fit the 8-32 threads.
I crimped a short piece of #12 copper wire into a terminal connector with a round end that fits over the 8-32 screw. This also insures proper polarity. When I screw on the sharpened probe, the crimp connector is held securely between the plastic end of the marker pen and the back of the sharpened probe. The other wire from the battery clip is soldered to a piece of #12 plastic covered wire that serves as a probe.

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