Many years ago I began using an inductive telephone pickup device to detect if a particular circuit board had power going to it. If the circuit being tested only had steady DC across every component the inductive probe would not be of much value, but in most modern circuits there are many sections where the DC is pulsed before it is rectified and turned back to DC at a higher or lower voltage. That being the case I find it helpful to do a quick inductive probe test across transformers and coils found on the circuit board. In some cases I may do so even before I remove the back of the TV.
One example is when a customer brings me a TV for an estimate, and I want to know if the power supply is attempting to turn on after I push the power on button. When I place my probe as close to the circuit board as possible I can often hear a change in magnetic fluctuations even though the back on.
The making of such a probe is quite simple. You can make a few turns of wire around the end of a pen and that’s all it takes. The larger the circle and the more turns you have the more sensitive it will be. I wanted something that wasn’t overly sensitive so I used a small coil. The coil I used was wound around a ferrite core which seemed to increase it’s sensitivity.
The diode is used was a 1N914. It’s a popular diode found at most radio shacks.