These used to be all the rage in the 90’s at the peak of Nokia’s phone dominance. They were stickers that you could put on your phone, and they would collect some of the RF energy and use it to make some LEDs flicker. It was quite interesting, because it showed when your phone was randomly checking in with a remote tower, even when not being used for a call.
The circuitry is very simple, but the diodes may be a high speed RF type. The diode packages I’ve spotted so far have been labelled C1, C3 and C2L.
In an echo of the anti-5G hype there was also anti-2G, 3G and 4G (and in the future the same people will be anti-6G). Sometimes these stickers were aimed at those people and sold as radiation absorbing stickers, but in reality INCREASED the amount of RF they were exposed to.
I’ve not had any luck getting the stickers to work on a modern phone. That may be because of the different frequencies, or simply the much reduced power that is needed to communicate with the local low-power beacons that have mostly replaced the older high power masts.
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