Hi,
the 16C62x series have a built-in comparator. Or, you can measure the loop
balance value with 16C7x as A/D.
Imre


On Thu, 13 May 1999, Alan King wrote:

> Byron A Jeff wrote:
> > The toughest part is going to be the sensors. Door loops are subject to quit
e
> > a bit of noise. In my prototype alarm system (which used a button and beeper
> > interface, i.e. three beeps to activate, two to disarm, 4 for status) the
> > door loop falsely triggered until I debounced with a 200ms debounce routine.
> >
> > 2) Attached the 120V interface wires to an optoisolator and a reversed biase
d
> > led through a 10Kohm 2W resistor.
>
>   Just a note on the door loops, it's best to use a say 4.7k
> resistor at the sensor end, and a resistor at the alarm
> end.   Then use a 339 set up for a window comparator around
> 2.5V.  That way, even if they short or open the wire, it
> still goes off.  Any halfway competent thief can bypass a
> cheap system without this by shorting the wires.  And a 339
> and a few resistors is cheap anyway.
>   Also, I know you're talking simple here, but never trust
> an all wireless system.  Amazing how many expensive alarms
> can be walked through by setting a signal gen to ~318 MHz
> and cranking up the power from outside the house.  Good ones
> will still go off after a few minutes of no reports, but do
> this a few days running and the police stop coming..
>   Still seems just as easy to hack back before the triac,
> but at least you did #2!!  Amazing how many commercial
> devices violate the PRV spec, even though most magically
> keep working anyway..
>
>