In an obscure Randall Garret SF novel I read back in the 60s or 70s
(it *might* have been "Unwise Child" but it's been too long to be
sure) he had vibroblades.

He even described how they worked. Basically, the blade was of
sommething really tough with diamond(?) edged microserrations (think
Ginsu knife only finer). The blade was attached to a piezo electric
cystal in the hilt.

When switched on, the blade oscillated at a high frequency as the
power-pack applied the frequency to the crystal.

So it made like a high speed oscillating saw.

They were *highly* illegal. Not hat that stopped gang members and the
like.

The protagonist had a highly illegal gizmo he'd designed. It'd detect
an operating vibroblade within a fair sized radius (100 meters?) from
the RF emissions.

That was barely legal (Earth had an overly high law level, with the
sort of courts that'd be all understanding of the "deprived youth"
criminals and really down on citizens who fought back).

But it also had a switch that made it just as illegal as the
vibroblades. Flip the switch and any powered -on vibroblade that got
close enough (a meter?) got an induced feedback that caused the
powerpack to fail messily (think "exploding Li-ion laoptop battery"
sort of thing)

This was not good for the person holding the blade when the powerpack
went off.

As I said, highly illegal, even though it's defensive device (can't
hurt anybody unless they are coming at you with a vibroblade). Also
something that (unlike the vibroblades) *isn't available on the black
market. You have to build your own, which requires a fair bit of
knowledge and skill.

Our hero is leaving a building after meeting someone, and it being a
bad part of town, he has the detector in his pocket. It lets him know
there are several vibroblades operating nearby.

It's fairly obvious that the group of young men heading his way are
the source.

So he flips the switch and tries to dodge them.

Cue several badly injured "juvenile delinquents" as they try to
attack him.

I forget if someone else called the cops or he did.

The cop asking him about the incident notes that it's awfully lucky
for him the knives failed.

He replied with something about "Well, illegal weapons don't have the
greatest quality control" or some such.

The cops actions pretty much broadcast "I know you did something, but
I can't prove it and can't search you".

Our Hero, of course, is all "I have *no* idea what you are implying".

All done without anything explicit being said. Great set of scenes.
The info about the knives and counter is given as asides that aren't
at all info-dumpy, and the interaction between the cops and our hero
was great too.

--
Leonard Erickson (aka shadow)
shadow at shadowgard dot com