Some alternative suggestions:


Have the button operate a relay which latches a light above the bed and/or
door and rings for the nurse.

Have a resistor between each button on the circuit and measure the
resistance (gives wrong answer is more than one button pressed at once -
limited number of buttons in practice).

But those are rather clunky. I really think your best idea is to use Dallas'
1-wire bus with their digital id chips. You can power the id chips from the
bus if I recall correctly.





----- Original Message -----
From: "MILTON MEDICINTEKNIK KB" <milton.medicinteknik@TELIA.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Thursday, October 26, 2000 7:33 PM
Subject: (PIC) Hospital ward alarm


Hello everyone out there...

I wonder if anyone of you have come across the following:

Presently a "nurse-call alarm" consists of one button in every room in a
large ward. There is only one twister pair across the whole clinic, as,
until now, it doesn't matter where the alarm comes from (just wake up the
nurse...). The new use is an indicator to tell which button was pressed. I
don't want to climb around to install new wires everywhere. The idia is to,
for instance, send a code from a 12c508 just inside the button. I suppose
powering this over the twisted pair, a diode and a buffer capacitor might be
an approach, and temporarily sink the line with ID data (YES a protection
diode is necessary on the output pin).

One problem is the noice and stray currents induced in the long wires.
And --- this "must be fail-safe"

Anyone been in this corridor before?

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu