Red or purple cellophane will work. In fact, red ink from a magic marker "painted" onto the old lense should work. The IR emited by the key is not a "steady" beam of light. It is modulated and coded. The modulation is tuned to a specific frequency as is the wavelenght of the IR light. The red plastic lense allows only IR wavelenghts to pass to the sensor transistor. Sunlight getting through a worn red plastic filter is de-sensitising the photo transistor. BTW If you buy a "learning" remote control for a TV set, it can "learn" and store the code for your key remote. Not practical, but interesting. If you have a junk TV remote, you can use the red plastic on that to make a new filter too. There is really nothing special about that red plastic.

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Resistance is Futile.