long time ago i have heard and read some info on tempest ( electromagnetic radiation ) and now i heard about "New kind" of tempest.
optical tempest - there are saying the you can trasmit via LED information throw walls even to the end of your street...

now my questions .

1. what is the diffrence between those types?
2. how can i send/recieve information from the transmit LED?

that's it for now but i'm sure after i get some answer more questions to come .

thanks in advance


Posted on 2004-10-04 19:18:07 by eko
http://applied-math.org/optical_tempest.pdf - has an example code for sending information trough keyboard leds and its overall great research work on the subject.

I doubt though that those optical emanations can be intercepted through walls. Glass window maybe. It's an optical afterall :-D
Posted on 2004-10-04 19:53:35 by arafel
thanks for the info
now the 2nd question , how can i recieve the transmation data?



Posted on 2004-10-05 07:47:25 by eko
Through a photodetector, maybe?
Posted on 2004-10-07 04:18:10 by AmkG
As AmkG said.
The above papers explains it all. In experiment they used special photodiode+amplifier connected to oscilloscope.
Posted on 2004-10-07 14:30:54 by arafel
I believe some special photodiodes can be used that have a fast reaction time so as to be able to receive data at fast rates. Not sure if phototransistors have faster or slower times. Photoresistors I believe are only good for detecting ambient light level, I haven't heard of a photoresistor that has a reaction time of less than a second.
Posted on 2004-10-07 22:01:12 by AmkG
thanks for reply .
does any one has better way the photoresistor ?
Posted on 2004-10-09 19:04:21 by eko
Photoresistors, I believe, are really really not used for communications. Photodiodes or phototransistors are what are commonly used.
Posted on 2004-10-11 19:19:57 by AmkG
Light does not transmit through walls. Heat detectors have been used where infra-red is common to see warm blooded objects through walls or detect a rat in a hole up to six feet onder ground. Another method is radio scaler waves which is broad band (spread spectrum) for physical imaging through surfaces. You cannot tranmit modulation or intellegence through walls with light at any end of the spectrum.
Posted on 2004-10-27 15:06:06 by mrgone
Quote from the PDF:
Herein lies the problem: if certain LED indicators are visible to an attacker, even from a long distance away, it becomes possible for that person to read all of the data going through the device.
(In other words, if no walls are in the way).

The modems that are vulnerable are the ones whose manufacturer was a bit lazy to put a diode and capacitor to prevent the LED from blinking thousands/millions times in a second :) .
PhotoResistors are never used in communications. Well, some 40 years ago one could transmit voice with them (up to 3kHz), with no modulation. That is iirc - I've seen the schematics of the transmitter (using a lamp) and receiver (using photoresistor).
Phototransistors are slower than photodiodes, but offer beta times more sensitivity.
Iirc, photodiodes are so fast that they're used as receivers in the optical lines. Electronics is no more a hobby of mine for 4 years already ^^"
Posted on 2004-11-14 05:25:40 by Ultrano
I just started playing around with some IR communications... very low data rate stuff... very commonly available transmitters abound for this stuff.

They are called TV remote controls.

Seriously, they do work very nicely. Codes abound, but basically they transmit approx 40 KHz burst for a ONE, and nothing for a zero. Radio Shack sells one receiver, photo diode, lens, amp, detector in a 3 pin package that demodulated the signal back to the ones and zeros.

Fire up a receiver, connect to a scope, point your favorite remote control at it and read off the codes.

I now have a PIC processor decoding all the commands my X-Box transmitter can produce and writing the button on an LDC display. (Took a couple of hours to get all the code working using Oshon Software PIC Basic compiler)

Isn't technology wonderful!
Posted on 2004-11-18 16:41:18 by Ernie
In your case the data rate is so low only because the modulation frequency is 40kHz. With simple IR LED + IR phototransistor I was able to make digital transmission of music - taking 3 or 5MHz bandwidth iirc :)
IR is more fun than radio for me, because it's easier to make :)
Posted on 2004-11-18 16:59:39 by Ultrano
Yeah I've done that Ernie. If you buy the cheaper IR detectors they have a broader bandwidth and lower sensetivity. Sensetivity is S/N or signal to noise ratio. In other words they cant's receive more than say 6 feet. The broader the bandwidth the more white noise received on adjacent frequencies.
Posted on 2004-11-18 17:45:50 by mrgone