Measure

hello every 1 :-),

i have question measuring resistor on 12 voltage , gorund arduino uno
please see picture schematic attachment understand mean.

i want connect existing smoke detector arduino uno.
the smoke detectors 12 volt , have build in resistor of (1k2) in case of smoke alarm resistor go
between 12v + , ground. last smoke detector has end of line (5k6) resistor, if remove 1 of smoke
detectors circuit open.

i want know how read out vaults , display in serial monitor.
 
if have suggestions please let me know.

thanks,

john
 

use voltage divider make 12volt 5volt or bit less , monitor analogread()

for voltage divider need 2 resistors

12v ---[20k]---x---[10k]---gnd

measure analogread @ x , voltage around 4 volt  (


30k @ 12v =>    = 12/30k = 0.4ma   =>    0.4ma * 10k = 4 v

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