i-dropper

A nice idea, but if this technique is used for keyless entry outdoors then I would think that sunlight could saturate the LED.

Here's another clever idea from MERL, based on an idea from Microchip's old "Designing For Dollars" competition.

It should be easy to prevent sunlight from hitting the LED. You could make it the proverbial "light at the end of a tunnel". ;)

Here's another l>A nice idea, but if this technique is used for keyless entry outdoors then I

Ah, but then you would have to be almost on top of the sensor in order to communicate.

At that point it may be better to just touch the key to a mating pair of contacts. Or if ESD is an issue, use a wireless RFID like the one mentioned in the link below.

But if you're indoors, I would think that the LED system should work most of the time.

RF can be made to be low range by limiting the transmit power, or by reducing the coupling between transmitter and receiver. The passive RFID mentioned in my previous post is inherently short-range.