Measuring current drop

Other Hardware General Electronics
1

Hello,
I have an external circuit with 12V DC, having 2 stages of current circa 2mA and 1mA. I need to detect these states. What would you suggest to use INA219 or something simpler with shunt resistor.
The circuit is sensitive if I introduce too much of additional resistance, I assume extra 100ohms will be easily tolerated.
thanks,
Kernie

2

KernieBay:
I have an external circuit with 12V DC, having 2 stages of current circa 2mA and 1mA. I need to detect these states.

please post complete circuit diagram and other info on this circuit.

The circuit is sensitive if I introduce too much of additional resistance, I assume extra 100ohms will be easily tolerated.

What is that assumption based upon? If it is indeed fine for the circuit, I'd say problem solved. 1 mA = 0.1V, 2 mA = 0.2V. That you can detect with an analog pin already. Or add a 10x OpAmp stage and you have 1V and 2V.

3

Please see attached schematics.
It is an alarm circuit in my house which I am going to re-use to read motion sensors to control the lights.
Thanks!

Schematics.png
Schematics.png2122×1074 172 KB

4

If it’s in your house :

  1. Most alarm circuits use 4 or 6 wires ( 2 for tamper) . Can you not reuse those ?
  2. You could probably not bother with the tamper connection ?

You may have problems with your design if you multiple circuits - there must be a common 0v line.

5

I would first measure voltage on the two wires (in respect to alarm ground) with a DMM.
Voltage on one of the wires will depend on the three states of the two switches.
Easier to measure that than current.

The whole alarm system might be matrixed (keyboards/switches/etc).
Might have to use a scope to see that.
That could make it much harder to measure voltage or current.
Leo..

6

Hey guys, thanks for your replies. there are indeed additional free cables, however motion sensor doesn't have an additional free dry relay which I could use and then wired straight from Arduino.
I don't want to mess with the tamper, as it will require to change resistors at the Alarm Switchboard end.
No Motion/Motion there is a current drop from 1.43mA to 0.95mA, which is a bit weird given that resistance doubles from 4K7 to 9K4 and current doesn't halve. I measured the voltage at the switchboard panel where COM and Zone cables from the motion sensors are coming to and it is 6.7V.

Am I having the only option of using INA219 with shunt resistor of 1ohm (for bigger voltage drop, given low voltage, low current circuit) OR I can have a simpler schematics? I need to wire up 8 motion sensors in the end.
Many thanks