Hi everyone, thanks to you all for getting back to me & apologies it's been a few days. I installed the bell on Saturday using my exisiting AC transformer and chime (these probably date from the 90's by the look of them) and everything worked fine first time. I'm still not 100% on how the bell utilises its two connections for both power and to signal the chime but it works so I've put my curiosity to one side for now. Got to say I'm really impressed with the image from the bell camera and how it integrates into the Hik Connect app I already use for my IP camera system. Thanks again for your input.