Hello Bordodynov,

I have rebuilt the optocoupler-arduino logic side and tested each one individually. They all work perfectly and the voltage at the transistor drain terminal follows the on/off of the arduino pin.

However, when I connect them in parallel to the Arduino 5V power supply. Only the first circuit works properly.

 

UPDATE 1:
My breadboard is broken, I guess that's why I found it at the bottom of the draw. The power line has a break in it, so only half my circuit was getting power...

I am very happy to have found that issues... Now I am sure it will all work ok.

UPDATE 2:
No success....

 

Update 3:

I removed the Mosfet on the logic side and connected pin 1 (LED side of the opto-coupler) with the Arduino output pin and connected pin 2, through the 470 ohm resistor to ground. All is working fine.

I have decided to slow the PWM down to a period of 60 secs and to vary the Duty cycle in steps of 1 second. For something with a time constant of 1-5 mins, i think this is enough...

THANK YOU ALL!
Alex
Time to get drunk... Have a good weekend.