cell-phone-powered LEDs

Hi folks,

Kinda new here so my apologies in advance if this is the wrong place to post this, but after a fair bit of searching I can't seem to find what I'm looking for so this seems like as good a place as any to start. /ubbthreads/images/graemlins/smile.gif

I'm sure we've all seen any of the various cell-phone powered LED antennae, charms, keypads, and whatnots out there that light up when near a cellphone at power. I know they're basically low power LEDs fed by the RF energy coming out of the phone. Some even have little switcher circuits that blink and do patterns and such. Sometimes a fair number of LEDs can be powered this way (I've seen upwards of 8 or more).

I've searched here, Googled it to the best of my ability, and checked the LED resources online that I know to no avail. Since this circle is fairly new to me I admit I've probably missed some good resources, and I'm pretty sure at least sometimes I'm not uttering the proper Bugs Bunny "magic woids and phrases".

Does anyone have any reference on the actual electronics involved to build a small circuit like this? Given the size and minimal cost of the products I know it can't be complex circuitry, I'm just drawing a blank on finding how to build a small circuit like this.

I saw someone else's post here recently on LECs (Light Emitting Capacitors) and while I have to admit I have more more experience with SEDs and LERs (Smoke-Emitting Diodes and Light-Emitting Resistors) than LEDs, I think nonetheless I could do what I'm thinking about doing if I could find a push in the right direction on how to power a small LED circuit from RF energy.

All I'm looking for is a small RF powered circuit (ideally tunable, or peaked at 1900MHz) to power 4-6 small low-power LEDs. Nothing fancy, but the little devices I've purchased to tinker with really aren't all that open to disassembly (they're little epoxy blobs with wires running out).

If anyone has any guidance, experience, or pointers to some documentation I'd greatly appreciate it!

Joe

joe,

it seems you have a pretty good idea there. theres one big problem though. there is no such thing as an RF powered LED. the little gizmos your talking about consist of a battery powered led (sometimes tied into the phones Li-on battery) with an RF detector tuned to the frequency range of cell activity. some also have vibration switches that activate when the phone rings. the only device i've ever heard of running off RF is a good old fashioned crystal radio set, which uses yards and yards of antenna just to give you faint crackly talk radio.

if it were possible to harness the RF radiation floating around and amp it up enough to power an led to the point of decent brightness then we'd all be walking around with collector antennas attached to our hats.

thats a neat idea but your talking less science and more magic.

Wow, thanks for the quick reply!

If these things are battery powered I have no idea where the battery would be stored in some of them. I have a few small "antenna toppers" that light when in range of a cell phone and they have no really discernable battery in them at all (like I said though, they're mostly just epoxy blobs so I suppose there could be a miniscule battery in there, but wow I've never seen one that small).

Assuming I'm smoking rope and these are indeed battery powered circuits, then I guess I'll change my query to looking for some kind of small RF detector and go from there. I'll go hit Google again. Thanks for the quick reply.

Joe

Those things are indeed powered by the radiated energy from the antenna. They are avialable in different shapes and grades of annoyance. Usually they contain smd-leds.

BBL, thanks for the reply.

I know some of these things are battery powered - the more complex ones like the blinking keyboard skins and so on, but those little antenna toppers, man if they've got batteries in them they've gotta be microscopic ones.

I agree that they're all extremely annoying, but I have a slightly less-annoying implementation of something I'd like to try, but I'm just coming up short on the circuit. I think from the LED side I'm okay, I'm sure I can find sufficiently low-power SMDs to do what I need, it's just powering the blasted things that's the stumper.

Joe

i guess i misunderstood your explanation. these things dont just pull signal from the air. they have to have contact with the phone. and if they are attached to the antenna lead then the phones signal is greatly degraded because some of the energy meant to be radiated is rerouted into the leds. so the device does not run on radiated energy, it runs on energy that has not yet been radiated. i'm not sure what your upto but it seems the best way to make it work would be to wire your device into the phones battery.

See, that's just what I'm trying to *avoid* having to do. The little jobbies I'm talking about don't need physical contact with the device or the antenna - whats why I'm assuming they're RF powered. They don't require electrical connection to the device or antenna.

A similar item is a cell-phone alerting pen, with an LED on the end that lights up when near a ringing or off-hook (so to speak) cellphone:

http://www.fcon.com/adpromotions/newpromotionalitems.htm (not a recommendation, just an example).

The little antenna-toppers that I'm referring to don't need to be physically attached to the device either. It can be sitting on my desk a foot away and blink when I make a call.

Oh well, I'm still Googling. If I can find something more specific I'll follow up.

Joe