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I have here a green 4 digit 7-segment display with a colon and five extra indicators, salvaged from a broken clock.  It's fairly obvious that its eighteen inputs are simply multiplexed, but with so many, I want to have my Arduino just go through all the possibilities and report which pairs conduct.  I want to know: should I use resistors on every input or can I just flicker through them all quickly enough that it won't matter?  I have an Arduino Mega, so there are more than enough digital pins for input and output.

I intend to modify the code I produce here to make a tester for just about any part, too, so tips on the feasibility of that would also be appreciated!  Mainly, can the arduino act as a multimeter (i.e. read current) or should I read voltage instead?
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Dmm's are inexpensive and have 2 prime features... They are built and calibrated, the Arduino is perhaps built but there are no interfaces or calibration. You will have a great deal of work to do if you really want to do that task. a 5V source and a 1K resistor with a couple of cliplead jumpers is all you need to find out how it is wired. The display will be either common cathode or common anode. connect the ground to a lead on the display with one cliplead and with the open end of the 1k resistor connected to the cliplead 'test lead' (the other end goes to the 5V source) start probing the other wires on the display... any one that lights up is an anode. If you then sweep the test lead across the other wires you should find others that light. If it is a common cathode display you will find other wires on the display that light other segments as well, mark them as to digit and segment and move on sooner or later you will figure out whether you have a common anode or common cathode display after that the rest becomes rather easy... Once you find a common point and there might be more than one proceed to find more segments or common points.
There is also the idea of looking up the part number too... Try Googling the numbers on the display. Many displays are mf'd for a common purpose like clock displays and have part #'s, few are unique. I have yet to find a good display that I couldn't read in 10 minutes or less.

Doc
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Oh, I know how to test it that way.  I just want to see if I can get an Arduino to do it.
Besides, I want to have a quick plug-it-in-and-test-it program that can do pretty much anything.
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That would be I think somewhat beyond the abilities of an Arduino just imagine over a hundred different types of 2 terminal devices, resistors diodes of 3 different materials not counting copper oxide or selenium, coils, capacitors leaky? or a complex two terminal network (BTW the same description could be used to define both conditions, which is right?). Armed with the right tools, first your mind and second your mind and how well you use it would save many millions of lines of code that wouldn't fit in an Arduino anyway likely not even the mechanics and electronics just for the probe head... But a Good DMM and your mind will work wonders for you, asking others like you did is in my opinion not a very good way to learn but rather a way of avoiding a little honest work on you behalf for your behalf.
consider the problem you wish to solve well before you attempt it and you might even find the task enjoyable, perhaps. As I do every day.

Doc
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--> WA7EMS <--
“The solution of every problem is another problem.” -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

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