Optocoupler with Arduino

Question

I'm trying to input 12v into Ardunio Digital inputs. But since Arduino digital input max voltage is 5v, I would like to use an optocoupler as a switch for 12v input into 5v from Arduino itself.

So I have got P621 Opto, But I' little concerned about the opto handle 12V??

Also for optos both in and out can withstand same voltage??

I'm only a beginner since not much able to grasp the tech jargons from the datasheets..

• awjlogan: Do you need the isolation the optocoupler provides? If not, you can just use a voltage divider.
• Sandeep Thomas: @awjlogan Honestly not. I tried with a voltage divider which is working fine. But thought about a different approach with Opto..
• awjlogan: @SandeepThomas Optos are slow, expensive, and large - if you don't need the full galvanic isolation (ie, no shared ground), but want to protect the Arduino inputs, use a buffer IC and/or put in some protection diodes.

Answer 1

Think in terms of current. You want your input voltage to cause about 10 mA to flow in the LED. The LED will drop 1-2 volts, so (12-2)/0.01 = 1k.

The output transistor will produce a current that you need to convert to a voltage with a resistor. For low speed signals, 10k is a good choice.

Note that this circuit will invert the signal, 12V in will cause a logic zero. This is easily handled in software. Or, you can reverse R2 and Q1 to get a non-inverted signal.

schematic

^{simulate this circuit – Schematic created using CircuitLab}

• Sandeep Thomas: Thanks a lot friend.. But I have a little question... Is that 10k is pulldown resistor??
• the busybee: Look at the schematic: It's clearly a pull-up resistor.
• Ohbhatt: can't we use arduino's built-in INPUT_PULLUP to eliminate the 10K Resistor?

Answer 2

But I' little concerned about the opto handle 12V??

enter image description here.

The electrical specifications states that it can accept upto 24V. So, 12 V is with in the valid operating range.

Also for optos both in and out can withstand same voltage??

The LED can be surely driven by the 12 V too. Important thing is to size the series resistor properly so that the recommended Foreward current of the opto LED is not violated.

Also, you can think of placing a diode in reverse direction across opto LED for reverse voltage protection

• Sandeep Thomas: Thanks friend.. A question... You have mentioned to size the series resistor properly.... But why the resistor required since it handle 24v?
• User323693: the LED current should be limited to some value, else the LED will get hot and burns out. Please refer to the datasheet to know the maximum LED current alowed
• Nebula: Which is 16mA as per the line below.