Donald H Locker wrote:
> Hi, Steve.
>
> Detecting changes in current is quite easy. There are lots of other
> ways to do this, but most of them would cost more (use digital signal
> impressed on top of supply current,) so my guess still stands -- looks
> at the current and decides what the state of the system is. Ditto the
> up/down/light pushbuttons -- just put some resistors in series with
> each button, and detect how much current flows. That is actually a
> good clue to the technique.
I'm coming into this thread halfway, but I'd postulate that they
might use a modulated signal and a few diodes to multiplex two signals
on two wires. You can also use resistors to generate another set of
signals, but then you become vulnerable to the resistance of long
exposed bell wires and terminal corrosion.
My alarm system has circuits that work this way, you can combine
normally open with normally closed sensors on the same loop.
All the normally open circuits are in parallel, all the normally
closed circuits are in series with a 600 ohm resistor. So the alarm is
tripped by the resistance going to near-zero or infinity.
--
Trevor Boicey, P. Eng.
Ottawa, Canada, tboicey@brit.ca
ICQ #17432933 http://www.brit.ca/~tboicey/
"You might want to crane your necks upwards now to save time." - Homer
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