6/12/24v circuit tester

Discussion in 'The Projects Forum' started by 3rror, Nov 10, 2011.

  1. 3rror

    Thread Starter New Member

    Oct 29, 2010
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    Hey guys, I’m looking for a wiring schematic I can use to attempt to construct this. I know I can buy a nice Mac/Snap-on/Craftsman one for around 100 bucks, but I’m really just interested in building one myself. I enjoy this type of tinkering.

    What I’m looking for is a 2 lead (+/-) hookup that will connect to the battery and a ground. When connected, a red led will light up indicating power. Once u touch 6/12/24v with the tip, another red led will light up. The second led will be a bi-color red/green so when u touch a ground, it will light up green.

    I have a test light body to put a small circuit board in to make it all work, just need the info on how this can be done.
    If anyone has any ideas, please let me know.
    Thanks in advance.
    Eric
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    These are all DC voltages, I assume, and this tool is for working on cars and such?

    Do you need it to work for 6-24V without switching from one to another? (It'd be easier to design for a single voltage at a time.)

    It'd be good for someone that knows these tools to chime in. I'm wondering if it's OK to power the LEDs directly from the test or whether the LED power is isolated from the test. The former is obviously much simpler.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    3,236
    If you want one led to light to indicate power then you will obviously need three wires, power, ground, and test.

    You could have the red/green LED connected with the red between the test lead and ground (with a suitable series resistor) and the green connected between the power and the test lead (also with a suitable series resistor). That way the red LED lights when you touch power and the green LED lights when you touch ground.

    The power LED would be connected between the power lead and ground, again with a suitable series resistor.

    The series resistor value is such as to limit the LED current to the maximum desired current with the maximum voltage.

    With this design the LED brightness will vary with the test voltage but that should not be a problem. The minimum test voltage should still give adequate brightness to readily see the LEDs light.
     
  4. 3rror

    Thread Starter New Member

    Oct 29, 2010
    9
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    Thanks guys for all your help. Here's something i threw together with your suggestion ...will this work? It really don't have to be too complicated wtih 3 different voltages, just a though.

    Eric
     
  5. wayneh

    Expert

    Sep 9, 2010
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    Here's a simple version. The two LEDs will be lit at half current when the probe is not touching anything, but maybe that's OK. The appropriate LED will get brighter and the "wrong" LED will go dark when you touch the probe to anyting that completes the circuit.

    The other problem with this simple design is that it sources or sinks ~20mA into the tested point. If that's OK, it'll work fine.

    You'll want to adjust the resistor values for your specific LEDs, so that they receive maybe 2/3 to 3/4 of their rated current at the highest voltage. For instance 20mA for 24V using a standard LED.
    [​IMG]
     
  6. 3rror

    Thread Starter New Member

    Oct 29, 2010
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    That will work fine as i won't be probing air bags or computer circuits. If one wanted to make it circuit safe, would that be big ordeal? Thanks for all the help!

    Eric
     
  7. bertus

    Administrator

    Apr 5, 2008
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  8. 3rror

    Thread Starter New Member

    Oct 29, 2010
    9
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    Thanks guys for all your help. I think between all the info i've gotten here, i can put something together. Thanks again!

    Eric
     
  9. Ehsjr

    New Member

    May 14, 2011
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    Wayneh's nice circuit does it for you. I would suggest a slight change to eliminate the issue of both probe red & green probe LEDs glowing at the same time. Put the LEDs back to back, or use a bicolor LED. See attachment.

    Ed
     
  10. 3rror

    Thread Starter New Member

    Oct 29, 2010
    9
    0
    Ehsjr, thank you for your time with this. I really appreciate all of you guys helping me. This will work nicely for what i want to use it for.

    Eric
     
  11. wayneh

    Expert

    Sep 9, 2010
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    I just KNEW there had to be a simple way to do that, and indeed there is. Ehsjr's solution allows using a bicolor LED with a common connection to the probe.
     
  12. 3rror

    Thread Starter New Member

    Oct 29, 2010
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    0
    I seem to be having a problem. I wired it up on a bread board and the only way i can get it to work is if i reverse the red led on the prob side, but then the red/green stay lit untill u probe a hot then the green goes off, ground and red goes off. If i put the red/green back to back, neither work. What am i doing wrong? Thank you.

    Eric
     
  13. wayneh

    Expert

    Sep 9, 2010
    12,145
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    A picture would help, but one obvious possibility is that they are both backwards. You could try turning both of them around.
     
  14. 3rror

    Thread Starter New Member

    Oct 29, 2010
    9
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    Both Leds light up this way. When i touch a ground, the red goes out, and positive, the green goes out. Otherwise they are both lit. I've tried turning them around in all kinds of diff configurations, but no results. If i put the cathodes back to back, they are both off untill i touch ground witch will lite the green led, but touching a positive does nothing for the red.
    Thanks guys.

    Eric
     
  15. wayneh

    Expert

    Sep 9, 2010
    12,145
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    Wait a minute, at first look I thought that circuit would work but looking at it again, I see it cannot possibly work. There's no way to ever light the red LED (except by probing a negative voltage), as of course you've discovered. Revert to the circuit I first offered.
     
  16. 3rror

    Thread Starter New Member

    Oct 29, 2010
    9
    0
    Ok will do. Thanks for all your help, i really appreciate it!

    Eric
     
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