I'm looking for a 120VAC proximity switch, SPST NO, something like alarms
use on doors and windows. Has anyone seen anything like this?
The application is to determine if an x-ray shield is in place.
Unfortunately there is no AC in the area to be able to use a relay and low
voltage sensors. There are just the two wire that need to be shorted when
the shield/door is in the correct position. The previous solution was a reed
switch, but the geometry of the shield/door makes this a very difficult
solution and the sensor get's destroyed if the user doesn't manipulate the
shield correctly. I'm hoping to change to a proximity senor to keep this
from happening again.
Perhaps an optical sensor using a reflective tape target, an ultrasound
sensor which measures a nearby shield's reflection / echo, a ferric /
magnetic sensor if the shield has any iron / ferric content, or some other
form of non-contact approach or, instead, a weight sensor which relies on
the mass of the shield / door using a solid state or other strain gauge?
Do you know how much current you need to control? If it's not too
much, say 200-400 ma, you can use an AC prox switch.
www.mcmaster.com search for AC proximity Switch. They are available
in NC and NO and change state when close to a metal object.
More $$ than a reed or mechanical limit switch, but very reliable.
If you need more current, just use a mechanical limit switch.
There are industrial limit switches that will handle
the voltage. You might look at a limit switch that
has a nylon rod or flexible spring actuator. I've
used the nylon rod type and the rod can bend all sorts
of ways without damage. Google "nylon rod limit switch".
Thanks everyone for thier input!
The general problem with a regular type switch is that it has to be
triggered when the shield is swung into it's correct place, but has to let
the shield over swing it's correct place and not trigger.
TDD get the cookie prize for this one. Both Grainer and MSC have a number of
whisker tipped limit switchs that I think will do the job. I have to call
MSC on monday to make sure their less expensive model has a normally open
condition. (It seems normally closed is normal for limit switches, at least
that's what Double D's catalog implies) If they do, then that's the chose.
If not, then its the more expensive Telemecanique model from Grainger, but I
figure it's worth waiting till monday to maybe save $50.
Thanks again everyone, it's great to have a resourse of thinking minds to
Not knowing your profession, I'll point out that there's a whole world
of 'industrial controls' from people like Omron, Square-D,
Allen-Bradley. (It's been a long time since I was involved, so the
names may have changed.)
But, it sounds like you want a non-contact, line-voltage, proximity
switch that operates without power. (ie, the absent neutral.) It's not
my area of expertise, but I think the pickings will be slim - maybe just
Finally, I don't know if this is a cross-post, so I'll also suggest
sci.electronics.design. Not their primary focus, but there's a lot of
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