Arduino Forum

Using Arduino => General Electronics => Topic started by: h_vestbo on Sep 17, 2012, 08:54 pm

Title: Nec 2501 photocoupler
Post by: h_vestbo on Sep 17, 2012, 08:54 pm
Need some help here. I have never worked with photocouplers before, so this is totally new for me.

Ive connected a nec 2501 to control an input to arduino from my car. it should read a car switch (12-15v) on/off.

Connected like this:
Car 12 -> anode
cathode -> 1k resistor -> GND

Arduino 3.3v -> Collector
Emitter -> arduino pin 2
Emitter -> 10k resistor ->GND

The voltage from the car varies from 12V to approx. 14,7V
When its 12V i measure about 1.something volt on the emitter
When car voltage to anode is 14.7Volt, I got 3.4volt on my emitter.

Whats the point of a photocoupler if the voltage varies like this?

Or is it badly connected?
Title: Re: Nec 2501 photocoupler
Post by: MarkT on Sep 18, 2012, 12:26 am
Double check the pinout, what you've described ought to work from what I can tell.
Title: Re: Nec 2501 photocoupler
Post by: Docedison on Sep 18, 2012, 01:19 am
You might also try to ground the Emitter of the Photo transistor and place the load resistor in the collector side from Vcc to the Collector and measure there, A low would indicate that the Diode is turned on, A high would indicate that the voltage on the Anode is below 1.5V. I've used the NEC2501/2/4 in that manner many times with great success. If you need a high to indicate a condition, invert your sketch logic.

Title: Re: Nec 2501 photocoupler
Post by: lefstin on Sep 18, 2012, 03:43 am
I've also used to common emitter configuration described by Docedison with the 2505.

I would be interested in a discussion from knowledgable folks about why one might choose the common emitter versus common collector, if all you are interested in is generating a digital signal to be read by an I/O pin.

(you might also confirm that you are getting the right amount of current through the LED - should be about 10 mA @ 12V with the 1k resistor.
Title: Re: Nec 2501 photocoupler
Post by: Docedison on Sep 18, 2012, 05:24 am
Common Collector circuits are emitter followers, based on the photon energy being the same as base bias... If the emitter is at 3 volts then the base internally is at 3.6V... This is the reason why (MY testing notwithstanding) that IMO an Emitter follower is not a good circuit for an Opto-Isolator . Possibly a cheap way to provide positive logic but basically unworkable...