Probably yes.  The key is whether the sponsor receives “any substantial return benefit” in return for the payment.  If the event had an admission price and the sponsor were entitled to a couple of free tickets, the value of the tickets would most likely be considered something of “substantial benefit” and would have to be deducted from the payment to determine how much was contribution and how much was the value for which the sponsor paid.  Since the event is open free to anyone, that is not an issue here.

The IRS regulations also make clear that mere acknowledgment of the donor is not considered advertising and does not constitute a substantial benefit so long as it does not contain a “call to action” that would endorse or promote the purchase of the goods or services provided by the sponsor.  It sounds like you have avoided that issue as well.  (See Ready Reference Page: “IRS Finalizes Regs Covering Sponsorship”).

The irony of all this is that a sponsor that is in business doesn’t really care whether the payment is deductible as a business advertising expense or as a charitable contribution deduction.  They are both deductible and very few businesses ever reach their limit on charitable deductions.   It may make a minor difference for your charity in calculating your public support fraction, where donation income is helpful to increase your public support while sale of advertising is likely to be unhelpful.  But it is extremely unlikely that the calculation from a single event would be so significant as to cause you to become a private foundation.  (See Ready Reference Page: “Calculating Public Support Percentage”)