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Topic: Digital Input from an LED (Read 685 times) previous topic - next topic

tocpcs

Interfacing to another PCB, I'm looking to pick up that a pump is being driven on.
The back of the PCB has 4 LEDs commoned up on one side (I assume the ground side - anode of the 4 LEDs)
On the other side of the LEDs, after a resistor is where I've picked up what should be a digital signal - high when the LED is being driven on, and low when LED is off.

Metering this, I was expecting (perhaps incorrectly) that the voltage would be 5v (the LED would be driven from the onboard PIC).
What I got was 0.5v when the LED was off, and 1v when the LED was on.
No problem, still a usable method of determining it.

My question based on the above is- I want to wire that into a digital input. So for it to read 'high', I think I need a transistor. If I use an NPN transistor from 5V to the digital Input, and feed it's base direct from the wire described above, will I get the desired behaviour?

fungus

What about on the other side of the resistor?
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

tocpcs

Same signal - it was 0.5V off, and 1V on.

cjdelphi

0.5 might be enough to trigger the transistor (off would be on)

fungus


Same signal - it was 0.5V off, and 1V on.


Why not use an analog input? The Arduino has 6 of them.
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

cjdelphi

#5
Apr 24, 2013, 05:36 pm Last Edit: Apr 24, 2013, 05:38 pm by cjdelphi Reason: 1
actually no....

how much of a voltage drop is the LED causing? cut them off completely....

tocpcs

I would use an analog input, but as the arduino only has 6 of them, I wish to use them for other purposes.
The intent is to pick it up as a digital signal, so I'm after a method of doing that.

What I don't yet know is whether direct to a transistor base pin is good enough or so

Cutting off the LED isn't ideal.
I've found a better output from the PIC pin which looks like it drives when needed.

MisterResistor


(I assume the ground side - anode of the 4 LEDs)


I think you meant cathode not anode. Rather than cutting out the LED, how about replacing it with an optoisolater?

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