Can W1209 be run with 5V DC only
It looks like It would work from 5V if you remove and bridge over the voltage regulator &quot;IC2&quot; to let the 5V through  and replace the relay with a 5V part.
It may also be necessary to reduce the resistance of R7 to increase the current to Q1 because the 5V relay coil will use more current than the 12V one so a stronger base drive may be needed.