Do you have a meter do get some test readings?
Something is definitely defective or wired incorrectly.

Yes, I have a multimeter. By the way, I just uploaded a simple sketch to control the relay, and it works with the flyback diode. I’m thinking of step by step adding functions to see when it starts failing.

 

A meter is a faster way to do this.
What's a sketch?
Is this a sim or an actual constructed circuit we are talking about?

 

A meter is a faster way to do this.
What's a sketch?
Is this a sim or an actual constructed circuit we are talking about?

It's quite bulky, I’m not sure it will be easy to read. Do you have something specific in mind?
What readings with a multimeter do you suggest to take?

 

Start with a voltage reading on IO16 when the relay is supposed to be OFF.

 

Start with a voltage reading on IO16 when the relay is supposed to be OFF.

It's 2.4V. I suppose it should be close to zero? What does that tell us?

 

Code or software issue possibly.
Can you disconnect that IO16 pin from resistor R15.
Then read the voltage again on IO16.

 

Code or software issue possibly.
Can you disconnect that IO16 pin from resistor R15?

Sure. I disconnected it, and IO16 voltage is 2.46V.

 

Is there a pwm signal on that IO pin?
Do you have a scope?

 

How to check

Is there a pwm signal on that IO pin?
Do you have a scope?

Unfortunately, I don't have a scope, and my multimeter doesn't take readings of pulses.

 

Does the meter have a frequency counter?

 

Does the meter have a frequency counter?

I suppose "KHz" mode market with a yellow outline should do the job?

By the way, I just uploaded simple code to control the relay with the IO16 and now it's 0V when LOW.

 

I suppose "KHz" mode market with a yellow outline should do the job?

Should work see what it reads.
Suspect the problem is in the code.

 

Should work see what it reads.
Suspect the problem is in the code.

I tried measuring the frequency, but it shows nothing. Alright, I guess I need to check my code again. Thank you very much for your pieces of advice! I will post an update if I find a solution.

 

I tried measuring the frequency, but it shows nothing. Alright, I guess I need to check my code again. Thank you very much for your pieces of advice! I will post an update if I find a solution.

It was a software issue and now it's resolved. The pin stays at 0V when LOW, and the diode functions correctly. After several days of testing, the problem seems to have disappeared, and the PCB is working reliably.