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Topic: Weird led behavior? (Read 333 times) previous topic - next topic

Pancra85

I have made a PCB for the circuit I have attached. Those 12 leds on the left are sharing one resistor because only one of them will turn on at a given time, on those leds I put some I had stored, they don't seem to have a cathode and annode, the orientation work the same and the 2 legs are equally length. What they did is some of the voltage seems that was going trough the leds into GND.
I really don't know a lot of ellectronics so really can't explain it very well, but when connecting the PIN13 of arduino to the GND terminal of the leds, if one of them was turned on, I could get a "1", and so some of the other leds got turned on as well. After replacing those leds with normal ones it did work as intended.

Could someone explain why I got a "1" on the GND terminal after the leds? Of course if I connected PIN13 after that shared resistance I got a "0".
Thanks!

funkyguy4000

Wait wait, did you make your circuit exactly like that in the picture?  If so, no matter what, if any shift register output pin is high, all of them will turn on.

In regards to your question,  maybe you had the leds in backwards and there was discharged current.  Leds can also act as photodiodes if you put them in the wrong way.  As the power went through your led, the voltage drop was enough to power them, but not enough to direct the left over current through the resistor, so it would go through the leds that were inserted backwards possibly? 

Its generally a good practice to calculate the resistance needed and put the resistors in before the led itself because then you don't get reverse current flow like I stated.
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Pancra85

Thanks for answering!
yes I did it like the picture, now I see what you mean, as the leds were not polarized (I mean they turned on on whatever orientation) once one shift register output becomes high it will get trough the led and as the other outputs of the SR are on low it will make the other leds turn on (on inversed polarity)... so that is what happened.

Now that I changed the leds to polarized ones it works perfect!

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