AC Proximity Detector

[[email protected]]

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.

Many thanks,
Tim.

 

[Jamie]

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.

Many thanks,
Tim.

Electrostatic charges are on the surface of the wire.
Those simple probes you're thinking about are just hi-z
input sensing circuits via a couple of diodes to rectify
charge on the surface so it can drive a CMOS or FET type
sensing circuit. Normally input load are tailored to work in the
desired range and maybe a clamping protection diode to save the
day.
Electricity produces charges on the surface of the conducting
material.
If you would like a PDF file on the subject of how electricity flows?
I think I may have something designed for students. You can email me if
you wish.

 

[MooseFET]

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.

 

[Don Lancaster]

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.

Many thanks,
Tim.

http://www.tinaja.com/glib/muse89.pdf , of course.

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Joel Kolstad]

http://www.tinaja.com/glib/muse89.pdf , of course.

Calling that circuit "basically a 60Hz e-field radio receiver" is kind of an
insult to all the Real Radios out there, Don.

 

[Robert Baer]

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.

Many thanks,
Tim.

Very simple, they use CMOS logic where the first (inverting) gate is
biased in the linear region with a high value feedback resistor.
That makes the input sensitive to small signals that get amplified by
that gate; the output is then passed to the second stage which amplifies
it more and saturates (signal swings from ground to supply voltage).
That signal swingcan be detected by another gate, which can drive an LED.
I have seen three variations of this, and one of them had an
adjustable sensitivity.

 

[Richard Henry]

http://www.tinaja.com/glib/muse89.pdf, of course.

Your battery symbol is upside-down.

 

[Spehro Pefhany]

Your battery symbol is upside-down.

Yeah, I've always thought that it should be the opposite, since the
can on carbon-zinc batteries is the (-), but that's the standard..

Best regards,
Spehro Pefhany

 

[Rich Grise]

Yeah, I've always thought that it should be the opposite, since the
can on carbon-zinc batteries is the (-), but that's the standard..

So, then, Don's symbol is "right" in that the thick black one, the
carbon, is plus, and the thin line, the zinc envelope, is minus.

That's the way I've _always_ done it, and I don't care if everybody
else in the world does it wrong. ;-)

Cheers!
Rich

 

[ehsjr]

So, then, Don's symbol is "right" in that the thick black one, the
carbon, is plus, and the thin line, the zinc envelope, is minus.

That's the way I've _always_ done it, and I don't care if everybody
else in the world does it wrong. ;-)

Cheers!
Rich

As long as you label the side _you_ designate as +
it won't be misunderstood, even if you do it *right*.


Ed