Hi MikeML,Let me try it again...
Assuming that you can tie the 0V end of the 24Vdc supply that runs the smoke detectors to the 0V of the 5V supply that runs the PIC, the circuit I posted back in post #7 is all you need. Here it is again, this time showing two Sensors going off at the same time:
Each sensor draws 40mA if active. I am showing what happens at the PIC A/D input if first one goes on, and then the second one goes on, then the first goes off, and finally the second goes off. Note that with a 33Ω load resistor, you get 0V with none on, 1.3V with one on, 2.6V with two on, 3.9V with three on, and 5.2V with four on. I'm guessing that the likely-hood of four going off at the same time is unlikely. The AD has more than enough resolution to determine if it is one, two, three or four...
The upper plot pane shows the voltage into the AD input, and the lower pane show the current through the two simulated sensors.
View attachment 80765
Yes it is very unlikely that 4 go off at the same time. But the standard I am trying to build it uppon (EN54 - parts 2 and 4) require a maximum of 32 sensors on each line. So that means that the panel needs to be prepared for 32 sensors on one line and the possibility that a lot of them go off at the same time without damaging the panel itself.
Another question I have here is, what is the alarm threshold? Would it be the 33 Ohm resistor?
I really appreciate your help.
Best regards