Also, be merciful to the handyman who might be called on to solve a problem with such a system. Or, yourself in two years. DOCUMENT IT.

Write up what the different parts are for. Be clear. Use full sentences. Assume the person reading this cannot read your mind. Make a schematic. Note the little twitchy things you had to be aware of. When you're done, pretend six months or a year have passed, then look at the documentation and add any thoughts that come to you during that review.

Seriously. This is not a difficult system but it's way different from PODs (Plain Old Doorbells). You don't want hatred directed at you while a service person tells your client you were an idiot for making such a bastard system. A manual will make it legitimate!