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If I connect the positive and negative side of an LED to positive will anything break? I ask this since I want to use an Arduino pin as a transistor. Low would be ground and high would have the same voltage so nothing would happen.

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  • \$\begingroup\$ Nothing will break. It is like shorting the pins of the LED together. In that case, there is no voltage across it, so it can not be damages. But, unless you add a resistor that is inline with the LED when it is ON, you will draw too much current from your Arduino pin. That could damage both the Arduino and the LED. \$\endgroup\$
    – crj11
    Feb 19 at 2:08
  • \$\begingroup\$ As far as the led is concerned there is no current, so not a problem. \$\endgroup\$
    – Kartman
    Feb 19 at 2:09
  • \$\begingroup\$ @crj11 I have a 4 digit 7 segment display and I need to turn the digits on and off. It requires that it has a 220 ohm resistor between it and ground. If you think of it as an led it would be + to the cathode and the anode to a 220 ohm resistor then to the +. If I instead change the ground to a arduino digital pin. Then set it to output. Writing the pin high(5v) would not be an issue right? Writing it low would also just connect it to ground which would complete the circuit. \$\endgroup\$
    – jujumumu
    Feb 19 at 2:23
  • \$\begingroup\$ Correct. 5V to 5V means potential difference across the circuit, which means no current flow, which means no damage. \$\endgroup\$
    – crj11
    Feb 19 at 2:33
  • \$\begingroup\$ It would help if you drew a schematic. But it sounds like all your thinking is correct. Just take care that you don't try to share a single resistor with all the segments of the display. Each segment should have its own resistor. \$\endgroup\$
    – mkeith
    Feb 19 at 3:04
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Your assumption is correct. If you connect both the anode and cathode of an LED to + nothing will happen. Since there is 0V potential no current will flow.

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    \$\begingroup\$ What if the voltages are different? If I have the + to the cathode and the anode to a resistor then to the +. Will this cause any issues having a resistor in between? \$\endgroup\$
    – jujumumu
    Feb 19 at 2:18
  • \$\begingroup\$ If cathode to + and anode to -, as long as the voltage difference is smaller than the reverse breakdown voltage Vbr of the LED, nothing will happen. If the voltage is higher than Vbr, then the resistor will limit the current once the LED conducts reversely. \$\endgroup\$ Feb 19 at 3:02
  • \$\begingroup\$ @jujumumu The voltages won't be different in that case. \$\endgroup\$
    – Hearth
    Feb 19 at 5:00

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