I had the exact same issue where only 3 circles were identified instead of 8. I found the answer to my solution by reading a stack overflow post explaining what param1 and param2 do, you can read it for yourself here.

Essentially, you need to change the values for param1 and param2. While even I don't understand the full effect these have on the image processing, what I do understand is that param1 controls sensitivity with higher numbers equating to more circles being found and vice versa, and param2 has to do with the number of edge points needs to declare a circle.

I personally was able to get the 3 circles (coins) to register when using param1=60 and param2=15. Also, in case any other changes I made affected my outcome, here is the whole line of code:

circles = cv.HoughCircles(img,cv.HOUGH_GRADIENT,0.9,120,param1=60,param2=15,minRadius=70,maxRadius=130)

This should help get your code to register all of the coins as circles, but if my exact solution doesn't work for you, continue playing around with those values until you are able to get different results.

Capstone coins classification

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