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.
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.
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.
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