this circuit is not working properly as expected ,signal from the arduino is given to the pstart,relay stays on all the time,can any help me diagniose?????

 

Please explain in a few words what you would like the circuit to do.

 

It may not work as you expect it to, but, if the relay is on always, it works as I would expect it to.

The Arduino can output either zero or five volts. In both cases current flows from the +12V through the LED in the opto coupler to the lower voltage.

 

signal from the arduino is given to the pstart

Is the signal a positive-going pulse ∧ or a negative-going pulse ∨?
Welcome to AAC!

 

As explained above the circuit cannot work.
You could connect P_start to gnd and disconnect R6 from the 12V supply and connect it instead to the arduino output pin.
This will reverse the operation so the relay will be on when the arduino output is high.

 

As explained above the circuit cannot work.
You could connect P_start to gnd and disconnect R6 from the 12V supply and connect it instead to the arduino output pin.
This will reverse the operation so the relay will be on when the arduino output is high.

And, unlike the original circuit which defeats the purpose of the opto coupler, that gives you isolation.

 

If the P-start signal is a nominal 0 V to 5 V, You can replace R4, and U1 with a 8.2 V zener diode.

ak

 

Make R4 smaller (to get the same current), and connect it to the +5V of the Arduino!