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I am trying to create a simple cable tester, that needs to check continuity of a multiwire cable.

My first thought was to connect all wires at one end to a positive voltage, and connect LEDs at the other end.

If there are 5 wires inside the cable, this would test continuity of all 5 wires at once. 5V to all wires at one end, LEDs and resistors at the other end. If all 5 wires are good, 5 LEDs will light up.

This would work fine - but if any wire was shorted to another wire, it would give a false positve.

Any thoughts about how to create a simple continuity tester that could test multiple wires at one time, but also not give false positives if any of the wires were shorted to each other?

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  • \$\begingroup\$ Clearly if you short them intentionally on one end to 5V as you say, you won't be able to test for shorts between them. To check for shorts you need to test all the combinations of one end and the other separately (well, you might be able to reduce the number of combinations in based on the previous results). \$\endgroup\$
    – Eugene Sh.
    Commented Mar 2, 2021 at 19:40

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Figure 1. A 555 pulse generator driving a CD4017 ring counter will do the job. Image source: Electronics Hub.

You just connect your wires between the 4017 pins and the LEDs. On a good cable each LED will turn on in sequence, one at a time. A missing LED will indicate a break and a double will indicate a short. The 4017 can be daisy chained to expand for any number of cores.

See the linked article or thousands of others on the web. Once you get the idea then read the datasheet to help your understanding further and to learn how to read datasheets. Have fun.

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Eugene Sh. is correct, every wire must be tested separately. There are other considerations as well.

  1. If the cable is very long or in a coil or wrapped on a spool there could be a deadly charge stored in the cable and each wire needs to be shorted to ground via a large value resistor (=>10kOhm) prior to testing.
  2. A wire can test good with a basic continuity test and still fail in use under load. This could be due to a problem with the insulation breaking down. This can be tested with a high voltage megohmmeter also known as a "megger". The unit is dialed to 400,600,1000V etc.. depending on the upper max rating of the wire insulation and usually tested to see if there is greater than 10,000,000 ohms of resistance. Sometimes wires only test bad when they are wet etc..
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    \$\begingroup\$ How does a "deadly charge" accumulate in a coiled or spooled cable? \$\endgroup\$
    – Transistor
    Commented Mar 2, 2021 at 20:46
  • \$\begingroup\$ triboelectricity, previous use, ... \$\endgroup\$ Commented Mar 2, 2021 at 22:52
  • \$\begingroup\$ I should have clarified that the coil would have to be made of wire several thousand feet long. I have seen submarine cables store a charge just sitting in their storage crate. \$\endgroup\$ Commented Mar 3, 2021 at 23:44

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