A mist free mirror

I don't know if you're like me, but I find it very annoying after a shower or bath to have to wipe down the bathroom mirror 36 times with a towel just so I can see myself shaving (or putting on make-up for the girls).

How do you solve this tricky problem? First of all, by looking at the cause of the problem: when you take a shower, the "steam" generated fills the room. When it encounters a surface colder than its own temperature - a mirror, for example - it condenses, creating the notorious fog.

The solution that immediately springs to mind is to find something to prevent the fog from settling on the surface you want to protect, i.e. the mirror.

There are at least two ways of doing this:

By rubbing the surface of the mirror, when it's dry, with a piece of dry soap. In this way, the water droplets that form the fog are drained off and run down the mirror. Disadvantage: you'll need to do it again very regularly.
By making the mirror at least as hot as the "steam" generated by the shower or bath, droplets can't condense on it. Typically, it's just over 25°C or 77°F in a hot bathroom, so setting the mirror to around 30°C or 86°F works.

So the second solution was chosen. The idea is to build a mirror heated on the back by an electric resistance that keeps it at around 30°C or 86°F.

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Last modified on: September 5^th 2024

Materials required

A mirror: yes, it's pretty obvious, take a medium-sized mirror that's "naked", i.e. unadorned and with the back directly accessible => no lining or adornment - you need to see the back of the mirror, which is a gray, brown or dark purple coating. The shape doesn't matter; a rectangular mirror is just a little easier to dress (see below).
Resistive wire: this isn't easy to find, but you can try electronic component stores, stationery shops, paint or art stores and model-making stores. This wire (sometimes called "constantan") is sold by the metre and comes in various diameters. We use 4/10ths but it doesn't really matter.
Aluminum chimney tape: this is a kind of wide, heat-resistant aluminum foil tape. You can find it in DIY stores in the fireplace section.
A piece of styrodur: or expanded polystyrene, 1cm thick and the size of a mirror. This is intended to thermally insulate the bottom of the mirror once the resistor has been fitted, to protect the wall against which it will be mounted.
A current source: Anything that delivers a voltage of around 15V. It doesn't have to be direct current; if you can find an old transformer, that's fine too. I use an old PC power supply on a 12V output (yellow and black wire), through a switch like a room light. As for recycling PC power supplies, see this excellent site on the subject.
Miscellaneous: electrical wire, spray glue, an electric domino and a coat hook to hang the mirror once it's finished...

How it's done

Once you've got the idea down, you can do it your own way.
Here's one way, inspired by the one at home.

Clean the back of the mirror well with a little degreaser (trichloroethylene, acetone) to facilitate adhesion of the tape and glue. To work comfortably, lay the mirror flat on an old piece of carpet.

Snake the resistive wire along the mirror, as evenly as possible. The wire should protrude a little at the start and "arrive" near the start, also protruding a little, to allow connection to the current via a domino. A bit like the diagram opposite, but "tighter".

As you lay the wire, hold it against the mirror using pieces of aluminum tape. Note that there should be no crossing of the wire.

Once all the wire has been laid, glue the styrodur plate to the mirror using a glue gun, putting a little between the wire's passages.

Ideally, the plate should be a little larger before being glued, and then its size should be matched to the edge of the mirror with a cutter or, better still, a filicutter.

Detail of a cut seen from the side

Connect both ends of the wire to a domino to facilitate connection to the power source.

This can be done before or after installing the styrodur.

The mirror itself is finished, all that's left to do is add a hook to hang it from (I used an ordinary scrap of plastic glued together with a glue gun), and possibly improve its aesthetics by framing it with mouldings to your taste.

Then connect the domino to the chosen power source to heat up the mirror.

A normal mirror after a good shower...

The anti-fog mirror in action!

That's all there is to it. Now you can look at your hoe in a dry mirror, even after a shower at 50°C or 122°F...

Second version

After almost a year of good service, I've tried to improve the mirror by making a slightly less cobbled-together version with a larger, circular mirror. Here are the technical changes:

The PC power supply is replaced by an old DELL laptop power supply, which delivers around 20 volts. This is ideal in terms of power and voltage, plus the power supply has an indicator light and its own protection mechanism against short-circuits and overheating. I just added a switch on the mains input.

Conclusion

Well, there's certainly room for improvement, but it's up to you to decide what you need. Here are a few suggestions if you feel like it:

Install a temperature control system, so that when the temperature reaches 30°C, the mirror switches itself off.
There's certainly a calculation to be made between the voltage of the electrical source (around 20 volts at no load) and the length of the resistive wire (U=RI?).
If you have a large mirror in your bathroom, why not consider heating it, thus giving it a dual role: anti-fog and room heating? The difficulty will undoubtedly be to lay a large quantity of resistive wire on a large mirror that's already in place, but I still think there's an idea to be explored here. And I wonder if, for a large surface like that, a wire is really suitable? It's a shame we haven't heard more about the famous "heated paint" of a few years ago, as its use would undoubtedly have been ideal for this...

Of course, if you'd like more information or to make a contribution, please don't hesitate to contact me.