magic led necklace question

[TheBeam]

I went to Fred Meyer and found a little $3.47 necklace on clearance in the toy section. This thing lights up when when your cell phone rings. It must be quite close to work, about a foot or so.

Does anybody know how this thing works?

 

[Negeltu]

I recall a small keychain light someone reviewed but I can't find it... it sensed the signals with some type of sensor and turned itself on...something along those lines... One of the reviewers on here would definitely know.

 

[silvercloud]

I had a Nokia phone with the lighted keypad that lit-up when in use. Probably something to do with the micro waves.

So on that note I went to the microwave oven with the phone off, the phone a foot to two feet away from the door, turned the oven on and the keypad lit-up like some alien craft was hovering over the house!

It seems the keychain light is using the same sensor. Try it near a microwave.

The microwave oven in the house doesn't get much use from me. Either the keypad is supersensitive or the oven is got some serious leakage issues!

 

[silvercloud]

I forgot to mention that the keypad that was in the phone was an LED version that I pick-up at a computer show. Of course most cell phone keypads light up when in-use.

Cut me a break, I'm just on my sixth cup of coffee. /ubbthreads/images/graemlins/blush.gif

 

[TheBeam]

I tried the microwave trick. It lit up. By itself, I think this thing is useless and too girly for me. Maybe I'll get one more to disect.

 

[TheBeam]

One more thing. I had it sitting right by my monitor, it lit up when I logged on.

 

[Xrunner]

[ QUOTE ]
TheBeam said:
One more thing. I had it sitting right by my monitor, it lit up when I logged on.

[/ QUOTE ]

It likes CPF. /ubbthreads/images/graemlins/grin.gif

-Mike

 

[Steelwolf]

There are a few types of circuits that cause some LEDs to light up whenever it senses a transmission signal. One version doesn't even require a battery. It is a small PCB with 2 sets of tracks, long enough to act as an antenna/inductor. One track connects to the gate of a SMD transistor, the other track connects to the source terminal of the transistor. The drain terminal connects to the LED which appears to connect back to the first track. This version needs to be practically sitting on the phone's antenna to work. As the phone transmits, the 2 tracks charge up. When a certain threshold is reached, the first track has enough energy to activate the transitor and allow the second track to discharge in to the LED. This can happen so fast that the LED appears to stay lit.

I have something like the necklace, except mine is a hand-shape to hold the phone when you're at home. It uses a battery, but I suppose the basic principle is similar to the first circuit, the battery being there to provide power to make it more sensitive. And also, there are 5 LEDs which blink in sequence, which, I'm guessing are also powered by the battery.

I haven't tried the microwave thing yet, but I can already imagine what my wife is going to say. /ubbthreads/images/graemlins/crazy.gif

 

[carbonsparky]

Cell phones are designed to send out a signal to turn on remote devices to indicate incoming calls. You can buy vibrating call indicators that use this same signal. I have seen pens and other devices that work from this feature.

I don't know the details on how this is done, I just have seen the products.

Steelwolf may be right about how the thing works, but if it just lights up when the phone is ringing, there must be a bit more to it.

 

[PhotonWrangler]

I've seen a little stick-on LED thingy
that was being sold at a discout chain.
It was powered by a coin cell battery,
and was sold as a cell-phone indicator.

I've found that I could trigger it with
plain ol' static electricity as well as
from nearby RF or EMI sources. I suspect
that it's input sensitivity is fairly broadband.

BTW, most microwave ovens seem to leak
at least a little bit, sometimes from the back.
But they can emit energy in various bands
besides the 2.45ghz frequency of the magnetron.
Some of the radiated energy seems to be from
the electronic controls, just like any other
device that's using TTL level signals >1mhz.

Put a portable radio next to your computer
and you'll see (hear) what I mean. For that
matter, put an AM radio next to a calculator,
tune to a blank spot on the dial, then turn on
the calculator and press some keys. You'll be
able to "hear" it thinking. /ubbthreads/images/graemlins/grin.gif

 

[Steelwolf]

[ QUOTE ]
Steelwolf may be right about how the thing works, but if it just lights up when the phone is ringing, there must be a bit more to it

[/ QUOTE ]

Not just when it is ringing, but also while I'm talking. Sometimes it will blink for no apparent reason. Sometimes it will blink awhile before the phone actually rings. I'm guessing that when it blinks for no apparent reason, it is actually sending out a signal to the nearest base station, perhaps to check for a stronger signal.

 

[TheBeam]

I just found out this thing has even more magic. It lights up when placed several inches from my cordless (not cell) phone antenna even though it's not in use.

Does that mean my cordless phone is constantly sending out a signal, or that it is haunted?

I cannot see inside a thicker part of the necklace, I'm assuming there are some button cells in there.

 

[PhotonWrangler]

[ QUOTE ]
TheBeam said:
I just found out this thing has even more magic. It lights up when placed several inches from my cordless (not cell) phone antenna even though it's not in use.

Does that mean my cordless phone is constantly sending out a signal, or that it is haunted?


[/ QUOTE ]

I have a 900mhz cordless phone that sporadically
transmits a short burst of RF, even when it's
on-hook and not doing anything. I think it's
constantly re-registering itself with the base
station and re-checking for the best available
channel, should it need to go online. This would
tend to trigger one of those blinky things.

Many compact fluorescent lamps radiate a signal
in the 20-25khz range; this might also trigger
it, if it's near a CFL lamp that's electronically
ballasted.