12 Volt Smart Test Light

Discussion in 'The Projects Forum' started by dgajp, Jan 10, 2014.

  1. dgajp

    Thread Starter Member

    Mar 16, 2009
    12
    1
    I have built a test light using relays.I want to distinguish positive voltage (Red Light) from negative voltage (Green Light) depending on input from probe.Can I have some suggestions ,on diode placement,feedback, design ,etc. please? . I have a very basic knowledge of electronics .
    Thank you
    David

    :)
     
  2. Athineos

    New Member

    Dec 18, 2013
    9
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    Connect at the edge of our probe 2 diodes (1n4000).One with her anode to the edge and the other with her cathode.So at each oposite side of the diodes you will have a voltage polarity accordenly to the inpout.Of cource we are talking for DC current.
     
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  3. Athineos

    New Member

    Dec 18, 2013
    9
    1
    Sorry just now I relise that you include a sircuit pdf.Tell me are you using a separate power supply to operate your probe?Because if you want to test different polarities of voltage up to 12v ther are simplest sircuits.My above proposal needs an aditional wire for ground.
     
  4. dgajp

    Thread Starter Member

    Mar 16, 2009
    12
    1
    I only have one probe therefore I I need the positive and negative voltage .Also for expansion of circuit.Extra function using positive.
     
  5. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    If you would post diagram showing the contacts rather than pictorial.

    It will be easier for others to follow.
     
  6. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hallo,

    Here I have a simple polarity tester with leds.
    One of the leds leds will start glowing at about 10 Volts
    and have about 20 mA through them at 14 Volts (depending on the polarity).

    [​IMG]

    Bertus
     
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  7. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    Op wants to test polarity without reversing leads, I believe.

    Iow connect ground probe to chassis, Tnen an indicator would show if probe was + or ground.

    I have the perfect tester for that. I'll see if it's still made.
    From Ireland.
    UNITEST440

    If it's no longer made, it would be a perfect project for someone to get pcb's for.

    Uses one AA battery.
    Led for continuity. (Ground)
    Led bar for 6-440 ac-dc. Indicator for + or ac.
     
  8. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
  9. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
    1,190
    156
    Check out Power probe
     
  10. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    If a line diagram is include you may get more help.
    It is confusing to convert the pin-outs of relays to function. IMO

    Meanwhile, how about this for a low impedance logic probe?

    Gut a standard 12 volt test light.

    Try to stuff two small 12 volt indicator bulbs inside.
    One red and one amber.

    Connect the two in series, with a red and black clip lead to battery and ground.

    Tie center point of the two bulbs to test probe.

    Dim light = power available. Or probe = nc.
    Red bright / amber off = probe plus.
    Amber bright / red off = probe ground
     
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  11. dgajp

    Thread Starter Member

    Mar 16, 2009
    12
    1
    More information.
    5 volts is measured constantly measured at probe tip.I don't know where to add diodes to stop feedback.

    :confused:
     
  12. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    I've been too lazy to print and draw in the contacts.
    I was hoping that you would re-draw it.

    I'll print it and see.

    It would seem that you have to have voltage at the probe.:confused:

    What else are you trying to do? Other than sense +12 or ground?

    To sense "ground will take voltage at the probe.
     
  13. dgajp

    Thread Starter Member

    Mar 16, 2009
    12
    1
    Thank you for reply.I want to use to establish positive or negative voltage.I accidentally sat probe on test light on bench.The test light glowed.Thats I used multimeter to discover 5 volts.Not being as smart as I look.I posted here for help
     
  14. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    I printed and examined the diagram.
    Can't find the logic in the circuit.

    When the probe is at +12:
    Red led comes on.
    Left relay pulls in.
    30 connects to 87A.
    Which connects thru nc. in middle relay to ground.
    Short circuit.
    How would you like this to work?
    What are the relays used for?

    Do you want a relay to pull in for +12, and another for ground?
     
  15. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    You don't need all the circuitry to do that.

    For probe to indicate +12 and ground, powered from a 12vdc volt supply, there must be voltage at probe.

    All you need is two bulbs connected in series across the 12vdc.
    Connect your probe to the middle.

    The same will work with leds.
    It will not work with relays.
    If you can't tolerate the indicators being dim on standby, it will get a little more complicated.

    However I thing that's a good thing. It verifies that probe is powered and ready to test.
     
  16. dgajp

    Thread Starter Member

    Mar 16, 2009
    12
    1
    This was an experiment from conception to inception.(using relays)

    Inwo: I would like the relays to work as you you asked

    a relay to pull in for +12, and another for ground?

    David
     
  17. dgajp

    Thread Starter Member

    Mar 16, 2009
    12
    1
    Thanks to all for replies
    David
     
  18. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    I'll try to put something together tomorrow.
    Some with a simulator could check it in an hour.

    It will need some gain as your relays pull-in and drop-out are ≈8 and 4 volts.
    At least I haven't found a simple way.

    Are you comfortable connecting two transistors and some diodes to the relays?
     
  19. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    Here's one idea.
    If someone would simulate it for you, I wouldn't have to test it.:)
    Relays are 12V @ 100MA

    Another way doesn't use transistors, but uses 6V relays (or lamps), 7806, and a few diodes.

    To OP,

    What will the relays be used for?
    It would be a so much easier to use indicator lights.

    Oops, I left some garbage on top of diagram.
    Just ignore D3 & D4.
    Also see that R1 might be better split between bases. In case of over voltage.

    Supply through a bridge would make input non-polarized.
     
    Last edited: Jan 12, 2014
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  20. dgajp

    Thread Starter Member

    Mar 16, 2009
    12
    1
    INWO.Thank you for suggestions. I like the bulb idea.i will be in my shed tomorrow modifying old test light.
    David
     
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