Capstone Project - coin classification identifying the 50p piece

Hello, Has anyone had any luck identifying the heptagonal 50p piece in the Capstone project part of the Python Bootcamp?  I can id and classify the round coins by using Haugh circles and IF statements but getting the program to ID the 50p piece has defeated me.  I have tried numerous edge detection methods, Canny, Sobel, Laplacian and even Hough lines but have not gotten a good result from any of them. I just need a hint... Thanks, in advance, for any help SJ