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How can we design a cellphone detector similar to the one shown in the YouTube video, provided we set the following restrictions:

  • purely passive, (no external sources like battery)
  • power should come from RF signals from the cellphone

Notice that the brightness of the LED must be high. What components/diodes will we need and is there any schematics you could point out?

PS: based from the comments, they use RF diodes but didn't indicate any part numbers. Also, somebody tried it using RF diode/microwave diode but they failed to make the LED light up as brightly as in the posted video.

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  • \$\begingroup\$ Useful background here on harvesting RF including from mobile phones. Not much proximity required, can be several inches away. Hand-rolled solutions work too, if you don't want to buy a ready evaluation board - see my comments to the accepted answer. \$\endgroup\$ – Anindo Ghosh Nov 27 '12 at 7:01
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Despite the contrary opinion from my most learned colleague, I suspect that you may be able to achieve a usable result.

The units shown used tuned antennas which are resonant at the cellphone frequency.

The total length of wire required is ~~~ L = 150 / F
L = length in metres of a HALF wavelength.
F = Frequency in MHz.

If you stare into the beam (with remaining good eye) a modern LED will produce visible light at 1 mA (and less), reasonably bright output at 5 mA and very bright output at 20 mA.
1 mA = ~~ 3 mW
5 mA ~~~= 15 mW 20 mA ~+ 60 mW.

To get even 3 mW range will need to be small.

The video footage MAY be faked (it being on You tube) but seems more liable to be real.

Schottky "hot carrier diodes" should work OK.

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    \$\begingroup\$ It had to be tried, so I did. I'm able to reliably power an MSP430 and 2 LEDs (power-good and blinking) off a handmade coil of enameled wire and an LTC3108, two inches away from my mobile phone. It's apparently not wavelength sensitive, I've tried several coil configurations. \$\endgroup\$ – Anindo Ghosh Nov 27 '12 at 6:44
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You simply can't receive enough RF power from ordinary transmitters to light a LED. RF power transmission is possible, but both the transmitter and receiver have to be designed carefully for that. Cell phones are designed to transmit signals, and use only the minimum power necessary for that purpose.

Unless the receiver is right up against the cell phone and can somehow intercept most of the radiated power, there just isn't going to be enough. Even at only 1 m away with a receiver also the size of a cell phone, so little of the radiated power will be intercepted that it would be useless to light a LED in any ordinary sense.

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    \$\begingroup\$ There are little cell-phone "stickers" available, which do not need a battery and have not just an LED that lights up, but sometimes multiple LEDs that switch around, every time the mobile is being used for a call. So it is definitely possible. \$\endgroup\$ – ExcitingProjects Nov 25 '12 at 15:50
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    \$\begingroup\$ -1, It appears that there's a knowledge gap here - (a) I'm able to power an LED or two plus an MCU reliably off a coil near a cellphone. (b) Yes, I see those stick-on blinkies for mobiles everywhere. \$\endgroup\$ – Anindo Ghosh Nov 27 '12 at 6:46
  • \$\begingroup\$ @Anindo: You said in a comment to Russell's post that you got this to work at two inches from the cell phone. That's what I was referring to as "right up against" the phone, and is about 20 times closer (390x more power available), than the 1 meter I was talking about. The OP asked for a "cell phone detector", which sounds like something that needs to be more than 2 inches from a phone to usefully detect it. \$\endgroup\$ – Olin Lathrop Nov 27 '12 at 13:25
  • \$\begingroup\$ @OlinLathrop My scale of reference is the video posted by the OP, which has the cellphone in contact with the device. It's a cellphone activity detector all right, and the OP has not specified a greater desired range to contraindicate that. \$\endgroup\$ – Anindo Ghosh Nov 27 '12 at 14:09
  • \$\begingroup\$ @Anindo: I didn't look at the video. Important information to the question shoudl be in the question. I don't see how something that needs to within a inch or two of a cell phone can be considered a "cell phone detector". If you're that close, it should be obvious the cell phone exists. \$\endgroup\$ – Olin Lathrop Nov 27 '12 at 16:03

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