Since you did not agree on a price before starting, the price is free! If you want to keep the friend, be sure to do all in writing, including what you will deliver to the customer and the fee for doing the job.
Next time estimate the hours you need to do the job and use the local hourly wage of a local restaurant worker.
The time they came to me asking to help their project they also promised me that they will pay my service by making the codes in their arduino project. And now i finished their project they ask me how much my service. Since, that is my first time to have a client thats why i am asking here to have an idea how do i evalute my price.
So, Thank you, you have a good suggestion on how do I rate my service.
@Paul_KD7HB is absolutely correct. Since you're asking for business advice you'd be best advised to consider his advice as well. Price should always be agreed upon in some form before the commencement of a project.
About pricing, how much time did you spend on it?
What do you consider a reasonable rate per hour?
How good of a friend is this, do you want to give them a discount because they are your friend?
After you figure those three questions out, consider if you think that the total price is fair for your friend.
Bear in mind too, that with no price firmed up you should accept whatever your friend is willing to give you, even if it's less than the price that you calculate.
All the other questions and remarks are relevant. I would challenge somewhat this though If you have a formal computer science degree
That’s of course not an engineer or software developer usual fee, that would be a very friendly rate
sell your skills at the market level if you don’t want to give them a discount.
Codes might have bugs and your friends might have changing requirements, you should also agree on a hourly/daily rate for any modification (and decide how you handle bugs once they signed off on the initial deliverables)