Back in the days of 2G the pre-wake-up pulses, aside from causing massive FM/AM interference like demented Morse code, would light up tuned antenna spark gaps on stickers for your Nokia phone. In Japan they sold cute phone tokens which had glowing eyes.
Energy harvesting the same way Theremin did for his passive wall bug.
I remember when you could tell you were about to receive a cellphone call based on radios and other analog electronics behaving weirdly. I got various people thinking I could see a few seconds into the future.
Tuned spark gaps? Or just an antenna hooked up to a pair of back to back LEDs? Just recently I saw something about light-up fake fingernails that would blink when you got a 2G phone call.
But it seems that the idea is still alive, from 2023:
"Relying more on ambient energy sources could prove monumental in automated warehouse inventory tracking, in medical instrument management and for deployment in airports, shopping centers and even individual smart homes. Nokia’s goal is to have energy harvesting technology in cellular networks that can support this massive IoT deployment."
I read the technical description of this and the whole work seems like magic. Antenna design, extremely low-power passive networks, etc. On top of that, it can tap into network signal bursts as a communication medium.
It feels like a project sent from the future.
I've always been curious what energy harvesting systems are capable of.
Also, what is the third type of energy harvesting besides light and 2.4GHz? I couldn't figure out what that might be.
For a while I had a very overpriced phone case from moeco that lit up when cell service was being used - https://www.moeco.jp.net/ - it came with a big warning not to use wireless charging with it (due to instead heating the case). Sadly, I wouldn't get one nowadays, I love wireless charging too much.
This is super neat. By modulating the timing of the data being sent to a websocket, (which is basically a /dev/null data sink) it implements a covert air-gaped side-channel data transmission mechanism.
> That last one consumed an hour of diagnostic time and involved using time-domain reflectometry (with a 20 ps rise-time pulser and 20 GHz scope) to locate the fault to within a region of a couple millimeters on one trace.
How does one even obtain the skills, much less the equipment to run such precision?!
A good EE degree with some RF specific course parts will teach you the concept. The scope .. well, you kinda have to borrow it from your employer as they're in the $10k range at that frequency.
There are plenty of amateur- to semipro-level youtubers out there in the metal and woodworking spaces with a total amount invested in their workshop that is way larger than that. I wouldn't be surprised if there are EE hobbyists with picosecond level scopes.
Source meters are expensive for normie hardware, but certainly not out of reach. A brand new keithley 2401 is ~$5k, so I imagine a used one could be picked up for a few thousand or perhaps less.
One question: what load is the matching network designed for? Did the designer find the equivalent small signal impedance of the diode network via simulation? Is the SPICE model even valid at 2.4 GHz? Is small signal even applicable?
Energy harvesting the same way Theremin did for his passive wall bug.
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