H11L1 opto isolator does not pass signal to Arduino (correctly) I have made a very basic test circuit for the opto coupler H11L1 ('similar'datasheet: http://www.datasheet4u.com/datasheet/H/1/1/H11L1_MotorolaInc.pdf.html))

I use the test circuit:







simulate this circuit &ndash; Schematic created using CircuitLab



The LED and 470 ohm resistor is only for testing.
U1 is the Arduino (software serial pin 9/11) and U2 is the H11L1.

The LED is flashing similar to the RX led on the Arduino.

Also I tested before in an even simpler circuit that the signals from the H11L1 pass through pin 4 (although inverted, which seems to be normal).

However, I don't receive any data in the Arduino (I tried different Arduinos, different speeds, different H11L1).

The data I receive from the (hardware) serial/debug terminal:

0
0
0
0


etc.

My sketch:

#include &lt;SoftwareSerial.h&gt;

SoftwareSerial mySerial(9, 11); // RX, TX

void setup()
{
    Serial.begin(9600);
    mySerial.begin(300);
}

void loop()
{
    mySerial.write('A');
    while (mySerial.available())
    {
        Serial.println(mySerial.read());
    }

    delay(100);
}


How can I get the correct signal? (meaning 'A' are printed instead of 0's?

Update

New sketch (not having the duplex problem):

#include &lt;SoftwareSerial.h&gt;

SoftwareSerial mySerial(12, 13); // RX, TX

void setup()
{

  Serial.begin(115200);
  mySerial.begin(115200);
}

void loop()
{
  Serial.print("Write: A ");
  mySerial.write('A');

  while (mySerial.available())
  {
    Serial.print((int) mySerial.read());
  }

  Serial.println("");

  delay(100);
}


Output:

Write: A
Write: A
Write: A
Write: A


etc.

Update

Problem solved: I put it in an answer for clearity.
simulate this circuit &ndash; Schematic created using CircuitLab

I'm not sure I can help much with the MIDI problem but I can help with the schematic. I've reorganised it for clarity. Since the opto-isolator has an open-collector output any loads it is driving should be connected to supply + rather than common.