I have connected a resistor of 120 Ohm to a simple led as described in Arduino Playground - LEDSensor.
The led is continuously blinking (very dimmed). However, the led does not seem to turn on when the actual plastic is touched/covered. Only when I touch the wires on the kathode side the led turns on.
What am I doing wrong here?

If the LED is just poking out into the air, merely placing a finger over it will not block much light on the LED die itself.

Try placing an opaque tube around the LED (way below the leads, light gets in through the bottomm, too) and then wave your hand over it.

Hi, you say that the LED blinks dimly, which it seems it will do, uncovered.
The LED will blink even dimmer or go out when covered or you put your finger over it.
If the LED is a coloured LED and not clear, then it will be dull in normal light as the colour dye in the LED will filter light before it reaches the LED junction.

So if the sketch compiled okay and your LED is clear, then it could be the characteristic of the LED that it is causing it to be dull.

Tom..... :slight_smile:
ps, interesting property, I have seen some projects around that use this characteristic to make cheap simple solar panel trackers.