The wall was a good example of what some artists might call a blank canvas. It was a featureless and, dead flat, matt painted, made from plaster board fixed over a standard wooden frame. Most times, you can look at these walls at certain angles against the light and spot where the plaster board has been nailed down. Or you can tap at it with a screwdriver handle or a rubber hammer handle and hear the sound change as you tap on a hollow section or right on a wooden stud. If I could find them, I'd stand a better chance of nailing the picture hook in a spot which could reliably support the heavy picture frame.
This little mini metal detector was the solution I came up with in less than an hour.
Three basic types of metal detectors exist - Beat frequency oscillator (BFO), field balance, and pulse induction. Other more esoteric types also exist, including capacitive field and 'radar' detectors, but those three are the ones most commonly build by enthusiasts.
This circuit is one of the BFO variety. A fixed oscillator is mixed with a second oscillator whose frequency varies when metal objects approach this oscillator's search coil. The mixing process results in a difference tone, or "beat" frequency, being heard. The unit is built around a single CMOS IC operated from a 9V battery. It draws about 8mA during use, but since that is limited to the odd 5 minutes three or four times a year, I've yet to replace the battery.
To cut off a piece like this, score around the ferrite rod using a small triangular file. It needs to be more than a scratch - probably about 0.5mm deep will do the trick. Then, while holding the larger end of the ferrite rod in a vise, lightly tap down on the end of the rod with a small hammer. The rod will break (almost always) quite cleanly at the scored line.
I find (recover) most of mine from musical greeting cards, although quite a few of mine also were used as ringers in discarded cordless phone handsets.
I built my version of this on a scrap of PCB measuring 20mm wide by 50mm long, in "ugly bug" style with the chip soldered "pins-up" onto the PCB. The other parts were soldered into place using the chip and the PCB to hold things together. (This really was a "quick and cheap" project!) No parts are at all critical - The values shown are just those which I had right on hand at the time. All capacitors are disc ceramic types, although C9 and C10 can be mylar ('greencap') types.
The battery was mounted onto the back of the PCB with a nylon ziptie. I used a pushbutton for the on/off switch. It must be continually pressed to keep power on the circuit. That way, I can't accidentally leave it turned on.
The search coil was left to dangle by its leads from one end. I've never bothered to build it into any better or tidier fashion. I just don't use it often enough to be bothered, but for those making greater use of the device, it can be built into a small plastic box.
Once built, the variable frequency oscillator is adjusted using the 60pF trimmer capacitor. This is very sensitive, and a wide range of "tones" (I use this word with some care since it describes a set of audible sounds which musicians would not usually classify as tones) can be produced as this trimmer is adjusted. I found it easiest to adjust using a plastic screwdriver. Just adjust it for a reasonably loud audible tone. Moving a metal object (such as a small nail or the end of a metal screwdriver) near to the search coil will cause this tone to change wildly.
In service, the search coil is moved across the painted wall, about 10mm from the surface. The tone will abruptly change as the coil passes close to a hidden nail. The small diameter coil allows the location of the nail to be found with satisfactory precision.