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Limit or proximity switch


Norman Pirollo

Jan 1, 1970
Need some advice..

I am designing a small feed system for lumber grading.
This feed system is part of a lumber grading system.

There is a motor drive feeding the individual boards along a
horizontal surface using rubber wheels or rollers.
The feed motor is started manually with a magnetic switch.

At a certain point the piece of lumber should activate a limit or
proximity switch indicating the lumber is correclty against the
horizontal surface of the grading component ( very important to
correctly grade)

If the board rises above the surface , even as little as 1/3 in., the
limit or proximity switch should shut the feed motor down using the
same magnetic switch used to start it..(severly warped board, etc..)

I wish to use one limit/proximity switch to do this.
The difficult part is that the switch is normally open while the board
is being fed through, then becomes closed, then open if there is a

The open switch then needs to shut the feed motor down...

I have been looking at metal roller ball type switches, preferably
dust-proof and rugged.

How to perform this trickery?



Jan 1, 1970
There is no proximity switch (that I know of) that can sense proximity
of wood. The application sounds like it might lend itself to the use of
photosensors, but it's not clear whether you're running one piece of
lumber through the system at a time, or multiple pieces. If multiple,
then obviously a photosensor isn't going to help much. There are
"smart" optical sensor systems which can perhaps be utilized. See the
site of DVT Sensors in Norcross, Georgia ( There
are plenty of competitors to DVT, and comparing one to another is not
easy. Apples and oranges. And the stuff is not cheap. I wish you good
luck in evaluating capabilities.

Otherwise, try to be a little more specific regarding the feed system.

But BTW, aren't you going to link a brake to the sensor as well as
tripping the power to the feed motor(s)?

Mark 'Sporky' Stapleton
Watermark Design, LLC


Jan 1, 1970
Look for a capacitive proximity sensor they sense all materials unlike the
inductive types.