Automotive Current Detector

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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.

Supplies

  • 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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    41 Comments

    0
    Build_it_Bob
    Build_it_Bob

    1 year ago

    Hi Phil,
    I seen the 300 ohm antenna clip and had to comment...I have not seen one of those in YEARS! Great way to re-purpose an item that survived years in a drawer somewhere.
    I will check over your posts as well, thanks for sharing!
    Bob D

    0
    Phil B
    Phil B

    Reply 1 year ago

    Thanks, Bob. We have had some experiences with televison things in our house during the last couple of weeks and I encountered that 300 Ohm connector while looking for something else. We have two grandchildren living with their parents less than a mile away. They are doing their schooling on-line and my wife is helping them. I showed the 12 year old girl a Radio Shack experimenter’s kit for electronics and she got excited about a couple of circuits on a breadboard. The next thing I knew she dragged home someone’s 70 inch flat screen television that had been too near a lightning strike and no longer worked. We opened it and I watched a lot of videos. Eventually I ordered a repair kit consisting of the three primary circuit boards. I had her install the new boards while I watched. The TV works sgain. Her father wants local channels over the air, too. I built a basic UHF dipole antenna with 7 inch strips of aluminum sheet. When I went looking for a 300 Ohm to 75 Ohm balun transformer I also found the antenna clip. It saved me from making an alligator clip.

    0
    Build_it_Bob
    Build_it_Bob

    Reply 1 year ago

    It sounds like you may have a future Electronics Engineer! There is nothing like the feeling you get when you build a circuit, or when you change it so if functions in a different way. Following up with the antenna was also a great thing to demonstrate.
    Stay safe, and keep creating Instructables!
    Bob D

    0
    Phil B
    Phil B

    Reply 1 year ago

    In March I will be 75 years old. I have always enjoyed reading a little about electronics and trying a few basic things as needed, but probably will not start any new careers. I spent my working life as a Lutheran pastor. One Sunday morning a couple brought me an electronic fly swatter that did not work, but it had meaning for them. Everyone takes electronics to their pastor for repair, right? It contained one transistor that had been blown to pieces. I guessed that it was probably NPN, that it was a basic switching transistor (like a 2N2222 that I had in my spare parts), and that the pin out could be the same. I was right on all three counts and their fancy fly swatter worked again.

    0
    Ag800Hans
    Ag800Hans

    Reply 1 year ago

    If not you, then maybe your pre-teen granddaughter will be an electronics engineer. That must’ve been some cool connecting time that you had with her.

    0
    Phil B
    Phil B

    Reply 1 year ago

    Thanks. Once the new boards arrive for a flat screen TV it is only a matter of unsnapping ribbon connectors, removing screws, and then putting screws back without cross threading them and carefully attaching the ribbon cables again. Nothing is assured until the TV powers up and you see the maker’s logo on the screen followed by the setup menu. Even then you hope there will not be a mass of pixels somewhere that remain dark. She probably took it all for granted a little. Still, it would be fun for her to see her friends again for a discussion of, “What did you do during Covid restrictions?” and she can say, “I rebuilt a big screen TV that had been struck by lightning.” She talks about computer graphics as a career interest. A week ago she mentioned she had read about Boolean algebra logic. I mentioned to her that those propositions are all electronic gates used in computers. Then the subject changed.

    0
    Build_it_Bob
    Build_it_Bob

    Reply 1 year ago

    That is an interesting story, It sounds like your reasoning skills are spot on.
    I now know where to bring electronics that I can't fix...
    Bob D

    0
    Phil B
    Phil B

    Reply 1 year ago

    Thank you for the endorsement. I have “let the smoke out” a few times, too. By the way, there is a free circuit simulator on-line at falstad (dot) com. You are probably already aware of it.
    In case you are curious, the transistor suffered catastrophic failure because a wire from the high voltage, fly killing side was too close to a wire from the low voltage side that boosts the voltage before sending it to a voltage multiplier that charges the wire grid. I moved the wires apart as much as I could to keep a charge from jumping over to the low voltage side sgain.

    0
    Build_it_Bob
    Build_it_Bob

    Reply 1 year ago

    I had not heard of the simulator...looks interesting. Nice info on the swatter issue as well, it is best when you can figure out the cause of the problem as well as fix it.
    Bob D

    0
    Phil B
    Phil B

    Reply 1 year ago

    The Falstad simulator seems to be adding components over time since I became aware of it. I get along better with it on a Windows machine with a mouse than I do with an iPad. There is a little learning curve. You can also save a circuit and come back to it later to add to it or make changes.
    The fly swatter uses a transformer to boost voltage, but a transformer requires an alternating, or at least a pulsating current, to work. The primary has a feedback loop that saturates the transistor and causes it to shut off. When that happens the transistor is no longer saturated and current flows again until the feedback saturates the transistor again. The transformer output goes to the voltage doubler. When a fly shorts the grid wires, the voltage stored in the capacitors of the doubler dump on the fly. There is also a bleeder resistor across the voltage doubler. I apologize if this is redundant.

    0
    Build_it_Bob
    Build_it_Bob

    Reply 1 year ago

    I appreciate the circuit theory as well. It always amazes me at what can be made with a small handful of components and some engineering.
    Bob D

    0
    amariller
    amariller

    1 year ago

    Handy voltage detector but limited in scope. All that tells you is there is “some” level
    of voltage present but not how much. The intensity of the LED offers a little more info as a relative indication. It occurred to me that a three-color LED and a little voltage divider circuitry would give you (by the color and degree of illumination) much better picture of the actually voltage of the battery.

    0
    Phil B
    Phil B

    Reply 1 year ago

    The unit I had but cannot find now only indicated the presence of a voltage, but that is a handy tool for anyone tracing an automotive circuit to determine where a break in the circuit is. A man I know used one a few days ago to determine if a neutral safety switch is good or not.

    0
    amariller
    amariller

    Reply 1 year ago

    Right. Assuming there is supposed to be a voltage present in the neutral switch circuit. Same principal. You are simply illuminating an LED w/ a voltage (unless I’ve missed the larger context).
    The resister serves only to limit current through the LED so you don’t smoke it. Now if you are wanting to find an open circuit that doesn’t have an inherent voltage you’d need to add a battery to your circuit but that would require extra cause as even a small reverse voltage on some circuits such as a mass air flow sensor can be damaging.

    0
    Phil B
    Phil B

    Reply 1 year ago

    I have thought I could add a 9 volt battery, but with connectors that allow only the correct polarity. Then the unit could be used as a continuity tester.
    Because it is not made to indicate a voltage level, I can use it across a variety of voltage levels to know if power is present. I did want to fit the device into a used marker pen shell, and that really limits space available for components.

    0
    amariller
    amariller

    Reply 1 year ago

    If you find a larger pen like maybe a sharpie that might work if you use a AAA battery. That will work.
    The resistor value would be less but 1.5v is plenty to illuminate the LED.

    0
    Phil B
    Phil B

    Reply 1 year ago

    I added a step to the Instructable. It shows the 9 volt battery and probe I added as a continuity tester.

    0
    amariller
    amariller

    Reply 1 year ago

    Another thought. If you can scrounge a tiny SPDT switch from an old PCB or whatever you could test both voltage and continuity.

    image.jpg
    0
    amariller
    amariller

    Reply 1 year ago

    Phil,
    I guess what I was getting at with the voltage level idea is that it serves as a broader trouble shooting tool. If you are looking for a broken wire and it’s a go-no/go situation then that’s perfect. I’ve periodically seen circuits that are a bit more hectic. If the circuit goes through a set of relay contacts that are messed-up or going bad (poses a higher resistance than normal) a pure continuity test might not be enough to find it. Having an indication of level can help identify a contact going bad and that may be intermittent. A dirty or corroded plug contact can produce the same symptoms. The LED illumination will be slightly or even notably dimmer in such a case. Hope that helps clarify or maybe just some food for thought. Thanks.

    0
    Phil B
    Phil B

    Reply 1 year ago

    Thank you for all of your suggestions. I do not have any unused LEDs in my “junk box” right now, nor any zener diodes. But, I have thought about running a simulation at falstad (dot) com. Originally, I wanted to use the shell of a larger felt tip marker, but the smaller one I did use was the only marker in our drawer drying out at the time, and its size did impose limits. A multiple use tool is always an attractive idea. But, I now have an abundance of digital multimeters, even if several of them are the free import variety I got from Harbor Freight Tools with a coupon, and all of them have a continuity Indication capability. For the most part, I simply wanted to replace my missing incandescent power probe for automotive circuits and thought a simple LED circuit powered by the car battery would be interesting. If you would like to build and post a tool with more capabilities, I have no objection and would very much like to see what you did.