I am currently a first time Arduino user. I decided to get one and start prototyping basic types of circuits.

I have built various basic circuits with the Arduino Uno prototype board and it worked completely fine. I am using the Arduino board to source power to my circuit (5V). However, after building the following circuit, everything stopped working:


simulate this circuit – Schematic created using CircuitLab

I don't get output from any of the Arduino pins and I cannot upload programs to the board. The yellow LED still blinks, and the green power LED remains green and active.

I cannot figure out how I managed to burn the board. The current is around 22.72mA, which is pretty far of the 40mA absolute rating.

  • 5
    I don't see an Arduino in that circuit diagram.
    – Nick Gammon
    Jun 4 '19 at 10:24
  • 1
    Did you measure or calculate the 22.72 mA? If not, measure it (maybe the resistor is working) 22.72 mA is just a bit above recommended (20 mA), but I doubt it should give a problem. Jun 4 '19 at 10:56
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    It was calculated. I didn't really get time to measure it since the board got damaged right away. I've ordered a few more boards, so I'll keep that in mind. Jun 4 '19 at 11:57
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    Check out this tutorial for how to connect a 7 segment led to arduino. You will note that it says (about 1/3 of the way down): "Single digit seven segment displays typically have 10 pins. Two pins connect to ground, and the other 8 connect to each of the segments."
    – GMc
    Jun 4 '19 at 22:56
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    I think the whole schematic would help. You are right that, as shown, even with a direct short through the LEDs, that would only draw 22 mA from the power rail which it should easily cope with. I don't see how the Arduino is being used here, and that undisclosed information might be germane to your problem. Are you using manual switches here? Why is the Arduino even involved in this design?
    – Nick Gammon
    Jun 5 '19 at 11:11

You need to have a current limiting resistor for each of the LED segments. Sharing one resistor between them all is a very bad idea, as multiple lit LEDs (in parallel) will be using the same resistor - see this excellent answer to Why exactly can't a single resistor be used for many parallel LEDs?:

The main reason is because you can't safely connect diodes in parallel.

So when we use one resistor, we have a current limit for the whole diode section. After that it's up to each diode to control the current that goes through it.

The problem is that real world diodes don't have same characteristics and therefore there's a danger that one diode will start conducting while others won't.

I would hazard a guess that you've drawn too much current, or, in your case, tried to sink too much through the PWM pin.

Also, it would (probably) be better to use individual data pins, not PWM pins, to turn on each segment, rather than connecting all of the segments to just one pin. It just seems a bit odd to do it that way.


The schematic you have drawn is completely safe to connect to an Arduino PWM pin. It may not be an optimal choice (closing multiple switches will change the brightness of segments), but it's not dangerous. As you said yourself, the current is limited to a safe value even if there's a dead short in the 7-segment display.

Typically, you burn your board when you connect/disconnect stuff while your Arduino is powered (because intermediate circuits may be unsafe even if the final result is OK), or when you built a circuit which is different from what you think you have built.

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