The circuit is derived from the LM555 timer data sheet and Rob Paisley's grade crossing bell.
The Bell will only work on 24Volts, there will of course be a 24 volt supply from the batteries.
I had hoped it would work from 12 volts. The circuit was already built to power the bell directly, when I found this out. I tried smaller bells but they were not loud enough.
The TIP120 is probably overkill now and C3 isn't required.
The bell contacts are bypassed and D3 is internal to the bell.
It all works just fine
