RF Indicator

[Kim Sleep]

I bought a bunch of RF indicators, small COB PC boards that would flash a
led whenever a cell phone, or any other source of RF radiation was nearby.
These are used to indicate acell phone ringing in a noisy situation where
you cannot hear the ringer, or the ringer has been silenced.
They are actually VERY sensitive, and will light up even when a electric
drill starts up a few feet away.
Anyone seen schematics for a equivalent type of circuit for such a thing?

 

[Dan Major]

I bought a bunch of RF indicators, small COB PC boards that would
flash a led whenever a cell phone, or any other source of RF radiation
was nearby. These are used to indicate acell phone ringing in a noisy
situation where you cannot hear the ringer, or the ringer has been
silenced. They are actually VERY sensitive, and will light up even
when a electric drill starts up a few feet away.
Anyone seen schematics for a equivalent type of circuit for such a
thing?

I used to make something similar with an LM386 audio amp chip. Input was a
short telescoping antenna, one of those replacements for a cordless phones,
going through a high-pass LC filter (100KHz IIRC). Had a speaker and low
power LED.