Delayed Start with Solenoid?

I need a short-duration timer for my chicken feeder.
I have a 120V. solenoid that when powered pulls open a slide and allows chicken feed to flow through a pipe to scatter about the coop. The power comes from a mechanical timer with a 15-min. minimum range. I don't want the solenoid to be charged that long a period (fear of burnout). 15-20 seconds is enough.
If the wall timer powers a 12 volt transformer that feeds a 555 circuit, that powers a relay that powers the solenoid, that would be perfect. The main stumbling block seems to be that any monostable 555 circuit is initiated by a manually-pressed momentary contact, and I want circuit initiation from an non-manual source. And, I can't find a way to make that signal not be continuous, which seems to me would continually send an initiation signal to the 555. Is that so? Thank you for this forum!
And, I found my solution.
The output is not the same, but by adding the 0.1uF cap. and the 2 39k resistors on the input I am able to keep the 555 from triggering twice, when fed by a continuous signal.

Thank You Delphi Forum!