Not to be an idiot here, but this is not brazing, this is soldering. Brazing involves applying a brass filler metal to the joint.

Usually, that would be a photo of sweating but I am not assuming anything.

A classic application of brazing is bicycle construction.

a combination of fillet brazing (top tube, horizontal) and lug brazing (seat tube, vertical)
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The lug brazing is similar to sweating in that one tube is inserted in the other and the filler is at the gap but brazing gets its name from the brass filler, and sweating uses lead solder.

 

The very simplest and very safest scheme will be to use a separate, second, plastic electrical box that is designed to mount exclusively from the outside of the wall. Then there is no mains power to be dealt with and avoided, and a cover plate with the circular hole can be used to mount a momentary action push button switch. Because it is not mains connected it does not even require an inspection at all. And opening it to replace the button cell exposes no shock hazard at all.
Cutting a second hole in the wall near the original switch will be the only complicated part of the project. When the installation is no longer used, a simple blank plate can replace the plate with the button.

 

TS has said that there is no connection between the wall switch and the light fixture. One assumes that this is a new installation and the wall switch has no wiring to it.

Instead of a toggle switch, maybe a push button should be used.

 

One assumes that this is a new installation and the wall switch has no wiring to it.

I assume the opposite, that it is a refit and the switch used to control a light (or other thing) that is no longer there.

 

I assume the opposite, that it is a refit and the switch used to control a light (or other thing) that is no longer there.

Why would TS want to replace a wired installation with wireless control?

 

Why would TS want to replace a wired installation with wireless control?

If it is older wiring, the switch may have been controlling something no longer in use (say, a ceiling fixture) and he now wants to control, say, a floor lamp.

 

Looks like a new build to me.

 

Why would TS want to replace a wired installation with wireless control?

It's a younger generation thing. Every possible appliance 'has' to be controlled remotely these days. I'm surprised the TS doesn't want to use a phone app to operate the lamp!

 

Looks like a new build to me.

You can tell that from the picture?

 

OK, now the whole situation is clear. It is a room update and the switch controlled s light in the middle of the ceiling. So the wires do not go to the mirror, as it is a new installation. so the TS needs to modify the existing switch to provide a momentary closure when operated. That is a mechanical option, and as the momentary switch will only be controlling 3 volts, the safety rules do not apply. The other choice is remove the unused light switch and purchase a new cover plate, and install a momentary push button in the vacant side of the new plate. With the switch gone there will be plenty of room.
My guess is that the other switch controls a vent fan. Or maybe not.

 

There is a light in the ceiling. Perhaps the other switch controls that light.

 

Another option is to do it all mechanically. If the existing switch can be dismantled, taking out the electrical switch and removing its back, the lever could possibly be connected via a rod to the tact switch with a spring somehow pushing it back when released.