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Quick And Easy Test Light Conversion

By on January 9, 2015
automobile testlights









For many years Automotive Technicians have used a test light with an incandescent globe for testing circuits and electrical fault finding such as to locate a defective fuse. You can check out the video below to see how to use a test light to find a bad fuse:

In recent decades, though, more up to date equipment must be used to protect delicate electronic circuits in modern vehicles.

A workmate of mine has always used the old style test light but realized the advantages of a LED test light and asked me if I could convert one of his old test lights.

auto test lights


After finding a base to locate the LED (I used a spare hose cap for this), I removed the globe and fed the wire through the side rather than the existing hole at the top. It is less inclined to get tangled this way.

I used a Bi-Colour Red/Green LED and a 330Ω resistor to suit. After about ½ hour the successful conversion was complete.

automobile test lightautomobile test light

auto test light

At only 20 cents AUD for components compared to $35 dollars AUD for a LED test light, I’d say that the conversion was well worth the effort and my workmate agreed.

Here is the schematic:

testlightYou can download the LED information from the link below:

This article was prepared for you by Mark Rabone from Australia.


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  1. biruk aweke

    January 9, 2015 at 10:14 am

    Very nice thanks for sharing.

    • joscamil

      January 9, 2015 at 7:23 pm

      Dear Mr. Yong , Mr. Rabone,

      I fully agree that this tester is a very nice, useful project. What is missing, but, is a small circuit diagram with connections.

      With kind regards,


  2. Robert

    January 9, 2015 at 10:35 am

    Thanks Mark. Nice job.

  3. Yogesh Panchal

    January 9, 2015 at 1:05 pm

    Mr.Mark Rabone,

    Good experiment!! keep up.

  4. Ernst

    January 9, 2015 at 5:12 pm

    Well done Mark. this is some thought to remember. I'm going to build one myself. I have these components lying around and never thought to make one myself.

  5. Abdul Haleem

    January 9, 2015 at 7:38 pm

    Hi Mark,
    Could you please send us the circuit diagram of this dual polarity tester? Is it the same like the one once Mr. Robert had published??

    • Robert

      January 10, 2015 at 10:52 pm

      Hi Abdul,
      The one that I made uses 2 cheaper LED's - a red & green soldered together because it's cheaper, although I have a bunch of Bi-color(RG) two lead LED's. I have all kinds of LED's including some expensive purple ones. I don't know how Mark get's them so cheap because they are $.50 a piece at unless you buy 1000 or more, then they are $.35 a piece.
      Maybe Mark can give us the website that sells them so cheap?

      • Mark

        January 17, 2015 at 6:43 am

        Hey Robert,

        Here is a company that I commonly use because of price and availability. Hope it helps
        As you can see the price is 14 cents each in a pack of 5.

        • Robert Calk

          January 18, 2015 at 10:45 pm

          Thanks Mark. I'm sure that will help many people. That is a cheap price on those LED's.

  6. Henrique Ulbrich

    January 9, 2015 at 8:18 pm

    Very good. Just a a practical experiment.

    One thing I could not understand: what is AUD?

    • Robert

      January 10, 2015 at 10:56 pm

      It stands for Australian currency.


    January 9, 2015 at 10:23 pm

    great work Mark.
    Could you add what watt capacity LED used.

  8. Humberto

    January 10, 2015 at 12:33 am

    Good innovation, congratulations. Healthy and Happy New Year for everybody

  9. John

    January 11, 2015 at 8:24 am

    Thanks Mark. AUD is Australian dollar.

  10. Abdul Haleem

    January 11, 2015 at 9:43 am

    Hi Henrique Ulbrich,
    Let me answer your question for on behalf of Mark. AUD stands for Australian Dollar as Mark is an Australian.


  11. camber

    January 11, 2015 at 11:51 am

    I really enjoy your Web site!! I thought I had done something wrong
    when I could not find a schematic for the fuse test light. When I saw other comments asking the same thing I was relieved it was not just me.

    So where can I find the schematic?


    • admin

      January 11, 2015 at 5:20 pm

      HI Camber,

      Mark just gave me the schematic and i have uploaded it.


      • Robert

        January 11, 2015 at 6:39 pm

        In the USA green means go and red means stop. Does red mean go in Australia?

        • Mark

          January 12, 2015 at 10:00 am

          Hey Robert,

          As shown on the schematic, Red shows a positive connection and Green shows a negative connection

  12. G

    January 13, 2015 at 12:13 pm

    Mark,It's a good job and useful for all of us.
    Thank you very much

  13. Taring

    February 12, 2015 at 3:41 pm

    Always on the lookout for the cheapest price. Thank you

  14. Chris

    May 24, 2015 at 11:34 pm

    Hi! Good job and innovative. But, did you know does this cheap tool is not only a test light for newer cars? 🙂
    You can read out whit this tiny LED test light the trouble codes from some older cars and do not have to pay for any diagnostic equipment.
    If you thin I can write some article, about how to test cars for stored errors in their memory module and get out the error number whit some tools like this...

    • Gary Gemmell

      January 5, 2016 at 5:47 pm

      NIce Chris i always look forward to your hacks - you are a true man after my own heart - a true hacker and cracker!
      Keep up the good works!

      Hoots from Scotland!!!


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