[IMSAI Guy] wants you to build a non-contact scope probe. The cost? Assuming you have a bit of wire and a regular scope probe, it won’t cost you anything. Why do you want such a thing? You can see what he does with it in the video below.
The probe is really just a coil with little slip-over coils that grab it. You can stick it on and remove it just as easily, so you don’t have to sacrifice the probe for normal use. It won’t give you high-accuracy readings, but if you want to sniff around a circuit without directly connecting to it, it will do the trick. If you are too lazy to make a coil, you can even clip a ground lead to the probe tip, although that won’t work quite as well.
There are many variations on this theme that are useful when you don’t want to actually contact what you are probing. For example, a thin insulating cap or coating over the scope probe can allow you to capacitively couple to the circuit under test.
This is a trick we’ve seen before, but — honestly — not lately. It is a great thing to have in your virtual toolbox and if you don’t have a real one and you need it, they take just a second to make. You might even be able to repurpose a ground spring with a little effort.
While these probes will pick up EMI, if that’s what you want to measure, you can do better. What we really want is a free contact scope probe costs nearly nothing
