Here is your sensor.
https://www.google.ca/search?q=hall+sensor&tbm=isch&tbs=rimg:CSiP0gqIaCzEIjhWeQpHmaVvnYl-OPUGUW7IZ-XYYRimlRJQwF7U27zXffZh4iYeaDdf_1TbH0aYwZdJHF_1rwvmlVnSoSCVZ5CkeZpW-dEbI705zN2sm4KhIJiX449QZRbsgRQB9-j0te0FUqEgln5dhhGKaVEhHo4OLSDofe7ioSCVDAXtTbvNd9EUBS7zM_12Ea8KhIJ9mHiJh5oN18Rw7D3J8dM1qYqEgn9NsfRpjBl0hEyys_1diAhJCSoSCUcX-vC-aVWdEfvCxv6mQFTR&tbo=u&sa=X&ved=2ahUKEwiivbezvsjaAhWm5YMKHVB8DKQQ9C96BAgAEBs&biw=1440&bih=787&dpr=1
and magnet
https://www.google.ca/search?q=small+magnet&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjfg6L3vsjaAhVB34MKHdVCCIIQ_AUICigB&biw=1440&bih=787