Designing the Ultimate Continuity Tester

Discussion in 'General Electronics Chat' started by Sensacell, Jun 10, 2017.

  1. #12

    Expert

    Nov 30, 2010
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    It only took me a short nap to realize the driving impedance is going to change if the voltage target is 3.75 mv.
    Now you're going to need 7.5 ma through 0.5 ohms to get to the measurement threshold and you can use up 0.49625 volts to get there.
    that's 66.1666 ohms. Hmmm...not much improvement in safety from external voltages.

    Good start in post #20. You can get 3V chimes but they max out at 80 db IIRC.
    I was looking at a 9V chime for 95 db but you chose a chip that limits at 7 volts Vcc. Not a problem. Add a few volts of zener.
    Typo: You need 5mv for the threshold with that chip, and you did the math right on the resistors.
     
  2. wayneh

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    Sep 9, 2010
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    My 2¢: You need a microprocessor to run a test routine for an ohmmeter, connected to a noisemaker. The µP would be off most of the time but would be awakened by sensing a small current between the probes. It would then proceed to execute a sweep of increasingly 'harsh' tests to determine the ohms between the leads. The tests would be designed to protect the device, and would only advance when the safe conditions were confirmed. This could all happen in the blink of an eye.
     
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  3. ian field

    AAC Fanatic!

    Oct 27, 2012
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    Horses for courses - I have various continuity testers, but often have to pick the right one for the job.

    When I serviced monitors for a living, there were no shortage of fatigued VGA cables. Most continuity testers would read OK but I end up with a call back. My solution was a H4 headlamp bulb and a motorcycle battery. It only takes 1 intact strand to give a false OK - the current I put through it made sure. It also works for setting the timing on energy transfer ignition - the points short the coil so a regular timing light won't tell you anything. The DC resistance of the coil is enough to dim the headlamp bulb when the points open.

    You do have to watch it though - the lighter cables warm up a bit with headlamp current.
     
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  4. wayneh

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    That's exactly what I was thinking of when I offered my strategy above. If you don't test with a significant load current, where justified, you can be fooled. But you can't just set your meter to drive 1A through everything you test.
     
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  5. #12

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    Today, I like a uP on this job. Low currents being measured by high impedances is inviting mistakes.:(
     
  6. RichardO

    Well-Known Member

    May 4, 2013
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    I have posted this before. I does not quit meet all of your spec's but I think it is close enough. The threshold is more like 100 ohms and it might not withstand 15 volts. If surface mount parts are used it can all sit on the end of a 9-volt battery.

    Surprisingly, the 100 ohms seems about right. A lower threshold is a problem when there is oxidation on solder joints. I have been using variations of this circuit for decades and I've seldom permanently damaged one.

    The biggest problem I have been having lately is that it is not loud enough for my failing hearing.

    upload_2017-6-10_10-26-57.png
     
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  7. Sensacell

    Thread Starter Senior Member

    Jun 19, 2012
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    A final follow-up on my project:

    After a few obsessive-compulsive hours of soldering and fiddling, the project is completed.

    The main change I made to make it bullet-proof was adding the Bournes TBU to protect the input from negative over voltage.
    This part is perfect for those situations when you cannot figure out how to protect an input from damage, when all conventional ideas fail.

    The TBU acts as an ultra- fast (1 us) mini circuit breaker, when the TBU sees 50-75 mA, it snaps open and stays open until the voltage drops to a low level, then it resets to look like a low value resistor again.
    Since it's a chip, not a thermistor, it's really fast, both breaking and resetting.
    It's rather expensive, I might add.

    With this part in the circuit, I believe I could plug the tester into 220 mains and it would not die.

    I added an LED to the switch output, this turns on at a few mA, giving you a visual that some current is flowing, but it won't beep until it sees 3 ohms or less on the test leads. This allows me to poke around and even test diode and transistor junctions with it, the LED lights, but the beeper stays quiet.

    Very happy with the results.

    IMG_6064.JPG
    Coninuity_tester.png
     
    Last edited: Jun 19, 2017
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  8. RichardO

    Well-Known Member

    May 4, 2013
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    Excellent. Can you post the .asc file for me to play with?
     
  9. #12

    Expert

    Nov 30, 2010
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    All that problematic input protection in a dedicated IC?
    Nobody can beat that with discrete components!
    One of the good reasons to be here. New stuff happens, and it's almost impossible to keep up with everything that becomes available.
     
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