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I have an LED powered by a serial(Rs-232) port on my PVR. The light is connected to the DTR pin through a 1k\$\Omega\$ resistor. It is set up this way in accordance with the instructions for the software controlling the light. However, I find the light to be too bright and am wondering what the best approach would be to dim the light's brightness. I am not overly experienced in the field of LEDs, resistors, and diodes, but I am assuming a larger resistor (or set of resistors) or diode would drop the current, reducing the brightness. I just do not know which size resistor(s) or diodes to use. I cannot provide specifications for the LED itself, as it is a part from an old set-top box without documentation.

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    \$\begingroup\$ Yes, either replace the resistor with a larger one or else insert another resistor in series (cut a wire somewhere and connect the ends via the added resistor.) Since you don't know much by way of details, there is no way to quantify a value for you. But if you insert another \$1\:\text{k}\Omega\$ resistor in-series, then you will get close to half the current in the LED. That might be a start in working out the better value. \$\endgroup\$ – jonk Apr 25 at 19:41
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    \$\begingroup\$ Possible duplicate of Can I use blue-green LEDs as MCU state indicators on 3.3 V power? \$\endgroup\$ – Ale..chenski Apr 25 at 19:58
  • \$\begingroup\$ If the brightness is really annoying, go with around 5k or as jonk says add 1k for just a little less brightness \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 30 at 22:19
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If you double the resistance to 2k, you will (approximately) halve the current. The brightness is proportional to current, but the perceived brightness probably is not. I'd try 4k (1/4) and go from there. The LED will be drawing very little current (compared to the resistor limits), so any resistor size will work fine.

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Use a variable resistor, something like 10k, in series with the existing 1k resistor. Then you can adjust it to get the brightness you want, disconnect it from the port and measure the actual total resistance with a meter. Pick the closest value fixed resistor you can find to replace them both.

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Change to larger resistor. You can find the correct value only by trial and error. Try e.g. 1.5k resistor. If it is too bright, try 2k. If it is too dim, try 1.2k. Etc

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