Clashing concurrent Arduino devices: camera and telescope drives

Question

I have an Arduino driven telescope mount based on Kevin Ferrare's device here:

https://hackaday.io/project/4386-arduino-st4-telescope-control

Which seems to work nicely.

I have added a DSLR remote shutter control using a similar optocoupler design, which also works nicely on its own.

However, when I run them together, any movement of the mount motor while the shutter is open closes the shutter.

I have completely stripped down the code to this:

    // This file is part of Arduino ST4.
//
//
void setup()
{
  // Flash pins on startup

  pinMode(2, OUTPUT); 
  pinMode(3, OUTPUT); 
  pinMode(4, OUTPUT); 
  pinMode(5, OUTPUT); 
  pinMode(7, OUTPUT); 
  digitalWrite(2, HIGH); // Red
  digitalWrite(3, HIGH);  // Blue
  digitalWrite(4, HIGH);  // Yellow
  digitalWrite(5, HIGH);  //Green
  digitalWrite(7, HIGH);

}


void loop()
{
}

With the result that only three pins illuminate 2, 4 and 5. Otherwise it's fine.

Is there something about concurrent pins that I need to know, or do I have a wiring problem?

Thanks for any input.

Steve