Optoisolator confusion - Arduino

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Joined Aug 3, 2009
I am building a circuit that will work in a car. The intent is to have something on the car (12V) trigger on the Arduino input pin. I'm using the below circuit and have it working, but I am confused about something. I expected that with no signal on the 12V side, the Arduino input pin (connected to #5 below) would be Low (or near 0V). However, I get 5V on #5 until the 12V input is applied (then it drops to near 0v). Also note that I did not connect the ground between #2 and #4 (but have tried with and without and the result is the same). D2 (diode) is also not being used as this is a DC circuit. I can work around this in code by simply reversing what I am looking for (LOW), but thought that optoisolators worked by triggering the output when input voltage is applied - in this case it is opposite this? What am I missing? I am using a NTE3041 optoisolator (NPN based).



Joined Dec 26, 2010
Your post will be closed, because you referred explicitly to this being a car project, which is barred from discussion on this forum. I can't therefore really discuss it, but perhaps some general remarks can be made about opto-couplers.

An LED / photo-transistor opto-coupler provides an approximate current transfer, so that passing current in LED makes the photo-transistor capable of passing current.

If an NPN photo-transistor is connected with its emitter to ground and its collector resistor via a pull-up resistor to a positive rail, then lighting the LED will pull the output down.


Joined Apr 5, 2008

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