Hello,
I have a project where i want to converge two systems.
Currently i have a number of LED's, common positive and the ground changes to make different ones light.
I want to read when the LED is lit (Ie. Digital Input to my Arduino Nano)
The LED's are 12V (Red, Orange & Greens)
Is it safe to sink 12V @ 20mA or something?
My proposed schematic diagram is here:
Let's assume your sink has resistance to limit LED current
Your main problem is that your arduino input is varying from +12volts (LED off) to roughly +10volts (LED on) (based of a forward voltage of 2 volts)
To read these values you will need a potential divider to limit the arduino input to 5 volts (or 3.3)
So now you are looking at 5 volts (off) to around 4.2 volts on)
Whilst this is "do-able" it will be subject to the stability of the 12 volt supply and the forward voltage drop of the LEDs. With lower forward voltages it will be even harder to discriminate reliably between off and on state.
Okay, so the input to the Arduino Nano has to be 3.3 or 5v?
I see the problem now, any alternate suggestion to the schematic?
High side switching by your control circuit, then your arduino will be looking for +5volts (LED off) and about 0.8 volts (LED on)
However if this isn't possible, you can still "see" the switch using low point switching since you've reasonable discrimination between say 1023 counts (LED off) and say 860 counts (LED on)
Ah yeah, could use a FET and high side switching...
Thanks!
OK just put brain into gear
Use high side switching (FET or transistor) but with current control resistors on high side of LED (LED cathodes connected to ground)
The arduino inputs now look directly at voltage on the LED anodes.
This will vary from 0volts (LED off) to somewhere between 1.2 and 3volts (depends on LED characteristic) for LED on.
That way you do not need any potential dividers.