From: "j.b. miller"
References: <email@example.com> <3DA314EA.E28536DC@earthlink.net>
Subject: Re: Sensing Reed relays
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
Date: Wed, 9 Oct 2002 07:29:25 -0400
NNTP-Posting-Date: Wed, 09 Oct 2002 07:34:14 EDT
Organization: Cogeco Cable
The 'standard' way of sensing door contacts is to use a 'window comparator.
The normally closed door puts the magnet next to the switch closing it.
One side of the switch contact goes to ground, the other to the input of the
comparator circuit.It's rather simple,needs two op amps. A 'google' search
will provide you with the schematic, or a data book like National's
You'll need an R-C filter on the front end,plus MOV to ground. The voltage
ladder is usually setup for 1/3, 2/3 of Vcc. This allows for a fairly wide
swing in the loop to prevent false triggers. The outputs of the comparator
are 'wired or' and pulled up with a 10K, this feeds into a I/O pin of the
Classic, simple, RELIABLE. The key word here is reliable!
Another scheme is to use a fixed resistor across each contact. By trial and
error you could come up with a resistor ladder that could divide a reference
voltage and when the door opens ,the micro reads the input voltage. Each
door is thus 'coded' for a certain voltage. More complicated, more prone to
I've used method one for over 30 years, NEVER had a false trigger. Build it
right the first time.