Ideally, each camera in a CCTV system will have its own dedicated monitor – making it easy to watch and listen to what they’re all seeing and "hearing". But video monitors aren’t cheap and this approach is just too expensive for many of us.

Fortunately, there’s an alternative: use just one monitor, together with a gizmo called an "AV sequencer" or "camera switcher". This works like a multiplexer or scanner, automatically cycling around between the cameras so that you get the video and audio first from camera 1, then from camera 2, then from camera 3 and so on. Each camera’s signals are presented for a few seconds in turn, allowing you to keep an eye and ear out for anything of interest.

Fig.1: the block diagram of the Li'l Snooper. The audio and video signals are fed to multiplexing switches which are controlled by a sequencing counter. VR1 controls the oscillator to set the scan rate.

AV sequencers are available commercially, of course, but while they’re much cheaper than additional monitors, they’re still rather pricey. You’ll be able to build L’il Snooper for much less than a commercial unit and you will have the satisfaction of knowing that you built it yourself.

L’il Snooper can handle the video and audio from up to four cameras and its scanning rate can be adjusted over a range of about 10:1 to suit different applications. It has a row of LEDs on the front panel to show which camera is being presented at any moment and there’s also a "Pause" button. This button lets you stop the scanning and concentrate on just one camera if you spot or hear anything of interest from that unit.

Just about all of the parts used in L’il Snooper’s circuitry fit on a small PC board, making it very easy to put together. The completed board assembly fits snugly in a standard low-cost instrument case, with all input and output connectors along the rear.

A small DIP switch inside the unit lets you set up L’il Snooper for sequencing the signals from two, three or four cameras. The complete unit runs from a nominal 12V DC supply and draws less than 110mA, so it can easily be operated from a small plugpack or even a battery.