Flashing LED mobile phone stickers

Helen C

I met a guy the other day who had a clever little sticker on his mobile phone that contained LEDs that flashed when the phone rang or a call was active.· As I understand it the power comes from the RF signal.· Does anyone out there know how these work?

Cheers,

Helen

jakjr

My mom used to have something like that. I believe hers operated on a small button battery.

Apparently it just flashes when it detects any signals being sent/received because ever time the pone would switch between towers the LEDs would go blinking.

shandar

I think the physics/electronics is more or less the same as in this batteryfree radio:
http://www.scitoys.com/scitoys/scitoys/radio/homemade_radio.html
It powers an earphone from the energy of the radiowaves, allowing you to listen to different radiostations for as long as you want without ever bothering about batteries. With some modifications you should be able to construct a device that is powered by the RF waves and, when close enough to the rf-source, flashes a LED.

DntGvaSht

Wouldn't absorbing the RF energy to power the LED cause the signal to be deminished?· I mean, if the LED is powered by the RF signal, it must be through inductance, which drains power from the source.

With a common LED draining 20ma, and a typical cellphone RF output rated at·600ma, the signal loss would be about 3.3%.· Not accounting for efficiency of inductor or driving circuitry for the LED(s) if any.

I dunno, just blabbin off the top of my head·

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