Dave Liquorice pretended :
You say workshop, implies mains is available. Have you tried any of the
"wireless" door bells that use mains wiring for sending the call signal
rather than RF? (I think they exist...). Workshop sounder and place where
door is would need to be on same phase.

The mains signalling units tend not to work very well where there are
lots of filters around, filters as fitted to the likes of PC's,
printers and displays etc..

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk