Cellular Antenna Lights - I'm confused

[keymaker]

Some people say they work with the battery power, some say they work on RF..<p>What's the truth here?

[Ron H]

OK, here's my rationale: I don't know the specs for a cell phone receiver, but a good radio receiver has a sensitivity of a few microvolts (or less), so I'm assuming that cell phones are similar. With typical antenna impedances, this translates to a few picowatts of received power. An LED requires a few milliwatts to illuminate it. The ratio is in the range of 1,000,000,000. It seems logical to me that the power has to come from the battery, probably driven by same circuitry that controls the ringer (or vibrator, or whatever).<p>Ron H

[keymaker]

Very good...<p>But they never have a wire on them,,, And that's what confuses me.<p>MAYBE they have a watch battery in them and a sensor that triggers it when RF is preasent??

[russlk]

A neon lamp will light in the presence of RF voltage without any connections, but I am suprised that a cell phone has enough power in the transmitter to light a lamp. The lamp has to be near the top of the antenna because that is where the highest voltage is.

[Externet]

Hi !<p>The leds turn on when the antenna transmits, not with reception microvolts as mentioned above...<p>One miliwatt could be enough to emit light from a very efficient led; and a portable phone outputs about a watt to antenna, so there is enough energy to tap from.
I believe the portable telephones manage the power outputted accordingly to the signal strenght seen coming from the cell transmitter in order to save battery. But anyway, if power is reduced to 25% there would still be enough energy for the leds to rob and shine.<p>Miguel

[dacflyer]

i been wondering the same thing too...<p>the guy that mentioned the rf lighting a neon bulb,,impossible to do using a cell phone,, not enough rf power to do so..<p>anyway i am tempted to but one of them light thingies and crack it apart to see what makes it tick,,,
i am beliving that the LED gadget is powered by a small efficent coil,and driving the LED's and the circuitry as the phone transmits,,,and yes the phones intermittently transmit while on stand by..<p>well see you all later

[Ron H]

Well, I can believe that there is enough RF present during transmit to light an LED. In fact, I thought of that before I composed my last reply, but the fact that they apparently only light when receiving a call threw me off. I guess there is a handshake transmission that occurs before answering, and this is what lights the LED. There would have to be a return wire if they were lit by the ringer, and the fact that they are available for most models makes that an impossibility.
I wonder if cell phones transmit at times other than when receiving a call or when calling out. If so, this would also light the LED.<p>Ron H

[bodgy]

Cell phones transmit at regular intervals whilst switched on.<p>They have to do this so that the local base station computer knows where you are located, otherwise you wouldn't be able to roam and still receive phone calls. This is how an 'authoised agency' could track a mobile user thought it does take up a lot of resources. A digital mobile sends out the IMIE and Sim number to the base station tracker.<p>Analogues send out their serial number. <p>bodgy

[dacflyer]

hey all, i found out how they work now,, theres 2 different ones out there,
one actually uses the phones power,but.its just a boring lit led when you talk,,,<p>the 2nd one is the cool one,, it has 4 colors
red
blue
yellow
green micro micro LED's
and they get their power from the phones RF
via a signal dioad.. then that runs the circuitry to make the leds flash & blink,, i actually am using one on my 2-way radio i have at work,,lol<p>NOW YOU KNOW !