if I keep each donation value below 500 do I need a signature on the 8283 form from the donation center?

Yes.  If the overall total is over $500, then yes, you still need the signature.  You cannot make 20 donations of $499 and avoid using the form. 

Individuals, partnerships, and corporations file Form 8283 to report information about noncash charitable contributions when the amount of their deduction for all noncash gifts is more than $500.