A few weeks ago, I had to install some burglar alarm wiring deep within a brick cavity wall. The architraves and door jambs were not yet fitted, so I had quite good access to the cavity.

But the view inside the cavity was well and truly hidden by a bunch of power cables.

The camera we selected is a "Super Micro B/W" from Oatley Electronics and it is micro - just 17mm wide, 16.5mm deep and 60mm long! At right is a front-on view of the same camera. We take advantage of the two bosses (on each side of the camera) to not only hold the unit in place in the slots cut in the conduit but also to mount the two infrared LEDs.

I tried all the usual tricks of "fishing" for the alarm cable – yellow tongue, thin dowel, a telescopic wand, straightened-out coathanger – but nothing worked. Not only were the power cables completely blocking my view, I was at least a little wary of a power outlet one of them connected to (especially when poking around with the coathanger!).

"If only I could see inside the wall..."

It was about this time that I remembered seeing not one but many advertisements for small, relatively cheap TV cameras. Could I use one of these to be my eyes inside the cavity? If so, how?

Searching inside a pitch-black cavity for an elusive cable is never easy. Our spy camera puts the odds way back in your favour. The very bright spot on the monitor is a timber joist right in front of the camera, illuminated by the two infrared LEDs fitted to it. Looking inside the cavity it was still pitch black - but the camera responds very well to infrared light.

I wanted to know more about these cameras. They are available from many SILICON CHIP advertisers, with a range of prices as wide as the range of models and types. You can get colour or black & white, various shapes and sizes, with various lenses and various resolutions, or quality.

Most of the cameras, though, would be too big for this application. The majority are built into relatively small metal cases, usually around 32 x 32 x 20mm.

Given that the average wall cavity is only about 25-35mm wide, these would either be a very tight fit – or no fit.

Then there are the "micro bullet" cameras – longer but certainly thinner – mostly around 22mm diameter and about 60mm or so long. Now these were starting to look interesting! Of course, we’d need to come up with some type of mounting arrangement so as to get the camera right into the thick of the action, where it was needed.