Re: AC Proximity Detector

On Sep 3, 2:02 pm, monolo...@xxxxxxxxx wrote:
Hi,

Out of curiosity - how do those hand held AC proximity sensors work ?
(e.g. the pen sized devices that you hold near to a live mains wire
which light up when AC is present). Is there an off the shelf IC that
you can purchase to make one yourself ? Anyone have a circuit diagram
for one ?

Reason for asking, I want to build an electronic circuit to detect the
presence of 240v in a wire - without disturbing the wire.

The simplest form of that uses an NE-2 neon glow bulb. They don't
work very well, but they don't need any batteries. It just uses the
AC electrostatic field and the fact that you are providing the
connection on the other side of the bulb.

The modern ones use a little bit of electronics. You don't need
much. Just a high input impedance and enough output current to drive
the LED. There is no need to rectify and level detect or anything
like that. The amplified 60Hz is basically applied to the LED
directly.

If it wasn't for the ESD problem, this circuit would work:

D1 470R
C1 !!-----!<-----/\/\----+9V
PROBE <---!!--+---!! TN0601N3
! !!----
20V /-/ !
ZENER ^ !
! !
---------+----------------9V Return and your fingers

C1 is about 1000pF and rated at 3KV.

D1 is a green super-bright LED

The Zener attempts to protect the MOSFET etc but it gets destroyed
after a few minutes of fooling around with it. This circuit will also
detect people combing their hair and turning pages in books etc. It
works over quite a wide band width.

If I was building one for real, I'd use some sort of low powered op-
amp. They make quite a few that can run for a long time on a 9V
battery. With an op-amp you can make a narrow band filter to reject
everything but the 60Hz.

.

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