I am trying to add a secondary box to add some features to a project. Basically I have a stand alone 4 LED driver that opens and closes the negative leg of the LED to turn it on or off. I need to be able on a second arduino completely independent to measure that opening or closing and then do something else based on that.
Is measuring current across those legs the easiest way? Im looking for probably 20-50ma of current. Is there an easier way? Can I just put an analog pin and hook it up to the resistor (180Ohm or so) and just get a reading from that? It doesnt need to be anything accurate as far as a definitive value. Just needs to know if the LED should be on or off.
Yea the led is low side switched correct. I dont know the LED full specs other than there are 8 total leds 4 signals, 2 leds each. there are 1 set of green leds and 3 sets of reds. I will have the actual product tomorrow so i can measure across it and get an idea. The voltage too the led's is either a 9v or 12v signal. I need to confirm that.
so, you have a power source to the LED.
somewhere, we assume, is a resistor.
either before the LED or after it.
there is a transistor/FET that brings one leg to ground and we assume completes the circuit and the LED lights.
or opens the circuit and the LED goes off.
the Transistor gets a signal.
the LED has current flow
the resistor has a voltage drop.
the potential on the collector is 9 or 12 volts until the transistor/FET changes state and then goes to ground plus whatever drop across the transistor.
so, you can :
detect the base of the transistor
measure the current in the line by adding a shunt resistor
measure the voltage after the LED/resistor
it will be interesting to see the device and how much you are allowed to modify it.
someone asked the color, you could also put a sensor on the LED to see when it is emitting.
I was able to get my hands on it today and I was able to trace and get the specs for the LED driver. Im able to just tap off of the pins and get the information I need.