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I came upon a Yard Sale...The woman has a system For Sale for six cameras, each one either Battery (short life) or each could be run off a Transformer (supplied) on a short leash to a 110 outlet. I got the lot for $100. This was a system modern enough to be connected wireless or via a USB from the receiver back to your computer or even direct to each camera.
There are better more expandable systems today. Costco and other National Retailers often have then on sale through their store or sometimes even cheaper via their website.
Further, as Security Cameras go....There are even more modern systems which can upload to the Cloud as they record..or can be accessed via your internet system via your Smart Phone. There are a variety of these newer systems on display at Best Buy along with perhaps their own Sales Rep. Some of them can have Voice...so you can also speak over the Camera System.
However, while I believe such systems are perhaps excellent for Model Railroading...there are some downsides to using them for Security Surveillance. They can prove to be more frustrating ...as a Burglar can be seen, face plainly visible as he takes down and steals your camera. Then ? The police do little to help you, even though they have requested video footage. An ex-neighbor of mine along with leaders of the Church across the street from his now ex-home...went to City Council Leaders for help. I sat quietly in the room, nearly snickering to myself as the conversations became entangled. With video in hand and from across the street at the Church, this all caused them more consternation. My ex-neighbor sold his house, closing escrow last month at a loss of near $80k. Nice house too, kind of sorry I didn't buy it.