Re: Proximity Switches

From:         dave lawson <>
Organization: Dave Lawson
Date:         07 Jul 96 14:15:55 
References:   1 2
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Ed Hahn wrote:
> In article <airliners.1996.1046@ohare.Chicago.COM> Al Hrovat <> writes:
> ah> I understand that modern aircraft use a magnetic sensor called a
> ah> proximity switch to determine the position of landing gear, doors,
> ah> etc.  Does anyone know why this arrangement is used instead of a
> ah> simple mechanical switch?  Are the magnetic pick-offs more
> ah> reliable than a simple switch?
> The prox switches (in theory) are much more reliable than the older
> mechanical switches, as they are sealed from the environment and need
> no adjustment to ensure proper operation.  (As opposed to a mechnical
> switch which must be adjusted to ensure that the switch reads "closed"
> at the appropriate location in the mechnical travel, depending on the
> application.)
> The principle of operation is very simple; the magnetic pickup can
> sense an change in inductive impedance when the appropriate metal is
> touching the surface over the pickup.

The key here is to remember that the output is analog, not digital.  That
is to say, it is a sensor, NOT a switch.  The working range of the sensor
is fairly narrow and careful adjustment is required for predictable
results.  Futher, unlike a switch, additional electronics is required to
obtain a useful output (ie. latching a relay, lighting a lamp, etc.)

Dave Lawson