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I am trying to build a (solar powered) device that heats up a mirror a bit until all steam/mist that was on it is gone. In the winter, the water is frozen. This is to remove any visual obstructing water.

I have so far strapped some thin wires on the mirror and when I run some current through them, they heat up and after a while the mirror is clean. This is basically the same way things in a car work like.

Now since heating eats up lots of precious energy, and since I would like to run this from solar mostly, my plan is to run this only if there actually is something.

When there is enough moisture, I can misuse the heating wires to measure resistance, but this won't work so well when things are frozen, or when the mirror is already partially dried.

I was thinking about some capacitive sensing, but I am not sure if it will work, especially when things are frozen.

This thing will run based on some microcontroller, so calibration data or so won't be a problem.

What is a good way to reliably detect "water based visual obstruction" on a mirror?

Note: The mirror in question is in some area outside where there is very little space to drive (so you can see around the corner) so I have very little space in front of it (otherwise I would hit it with the car). So stuff that looks at the mirror from a meter away or so is not wanted.

For the more visual people, it is similar to one of these:

enter image description here

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  • \$\begingroup\$ Solar thermal may be good enough - can you add a black solar oriented absorber thermally linked to the back of the mirror. Or even a PV panel linked to a resistor on the mirror back near the botton with a thermal insulating layer over it to ensure heat moves into the mirror to escape. \$\endgroup\$
    – Russell McMahon ♦
    Jan 6, 2015 at 11:33
  • \$\begingroup\$ @RussellMcMahon: It is already thawing quite well when in sunlight. I need to remove the stuff in the morning, when after a cold night the mirror is all steamed up, and can't use it anymore to look around the corner. \$\endgroup\$
    – PlasmaHH
    Jan 6, 2015 at 11:35
  • \$\begingroup\$ Are you sure you need solar for this? Isn't there any building close by that has a mains connection? \$\endgroup\$
    – Ilmari Karonen
    Jan 6, 2015 at 16:36
  • \$\begingroup\$ @IlmariKaronen: Yes, I am sure that I need solar. It is attached to a wall, and the only mains that I would have access to is across the pathway. \$\endgroup\$
    – PlasmaHH
    Jan 6, 2015 at 16:38
  • \$\begingroup\$ Hmm... can you run a wire over or under the pathway? Or through / around the wall? Presumably whoever owns the wall is not categorically opposed to the existence of the mirror, or of random electronics attached to it, so they might also agree to let you use a few dollars' worth of their electricity (and to let you run a cable through their wall, if necessary). Even if you need to drill a hole in the wall and arrange permission for it, that might still be the cheap and easy solution, compared to a solar panel and a fancy ice/fog detector. \$\endgroup\$
    – Ilmari Karonen
    Jan 6, 2015 at 16:52

6 Answers 6

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You could actually achieve this pretty simply with a reflectance sensor setup. Essentially it is a light and a photodiode that detects the reflected light. This kind of sensing solution is used on many line follower robots.

In your application, as snow, or freezing ice, accumulates on the window or transparent surface, the more blocking of light it will be. So if you put a light behind the glass, turn it on, and then measure the amount of blocked or reflected light, you'll be able to tell how much snow is there. You will have to do some tinkering to get your desired setting on when to turn it on will be but this is a good solution if you want to keep it out of the outdoors.

I think this could work with fog, although I can't say for sure right now.

Sparkfun has two solutions, one with an analog output, and one with a digital output for relatively cheap and can both be used with a something as simple as an Arduino.

Edit: (New information added to question) Since you're looking for a solution on your side mirrors and not your windshield, you could look into placing a similar solution used to detect rain onto the mirror. It would essentially be a couple of wires, traces to be exact, that are not connected initially, although when water is frozen on the surface, the wires would be connected since they conduct electricity. Sensing it would essentially be polling the wires, setting one as an output and the other as an input and toggling the output to see what you get on the input.

There are a few videos on youtube and such explaining how to get circuits onto a surface such as glass, although I'm not sure how it would work for a mirror surface. If you decide to go down this route, test it out on one a junky makeup mirror or something first.

To get an idea of how this solution works, check this out

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  • \$\begingroup\$ Quite right, depending on where OP wants to place the device (wasn't specifically stated), the sensors will need to be warmed up into proper operating temperatures should the ambient temp fall below \$\endgroup\$
    – Funkyguy
    Jan 6, 2015 at 10:47
  • \$\begingroup\$ Unfortunately this would not work well for me. Since it is a mirror and not just some glass where I can put stuff in front of and back, I also would need to put the sensor and light quite far away from it to capture some bigger area. I don't have much space there (will add that to the question) and a stick that makes the thing look like a satellite dish isn't going to work well. \$\endgroup\$
    – PlasmaHH
    Jan 6, 2015 at 10:50
  • \$\begingroup\$ @tomnexus We seem to think alike, was just looking at that \$\endgroup\$
    – Funkyguy
    Jan 6, 2015 at 11:01
  • \$\begingroup\$ Regarding your addition: the mirror is not installed in a car, it is outside, so people can look "around a corner". Also note that as stated in the question I already have considered (re/mis)using the heating wires, but resistances when things are frozen are really high, and when water is partially evaporated near the heating wires, there are still stripes of water that a better solution should detect. \$\endgroup\$
    – PlasmaHH
    Jan 6, 2015 at 11:38
  • \$\begingroup\$ Oh one of those bulby things, that would make the update more difficult then. (BTW in the future, you should state right away what your application is when you are asking for a solution so the question doesn't get this cluttered). I'm honestly out of ideas, that's quite the application that I've never really thought about, they've always been ice free and clear when I see them. \$\endgroup\$
    – Funkyguy
    Jan 6, 2015 at 11:42
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As others have answered the question asked, I will focus on the actual problem. You have an external "around the corner" mirror. You said it's mainly a problem in the mornings, likely before any sunlight can help defrost it. So why bother with the more complex monitoring when you can use a "needs based" application.

A real time clock, and a temperature sensor. You likely only need the mirror between say 6 and 8 am, and 5 to 6 pm. Using the rtc as a timer, have the defroster turn on any time between those hours, if the sensor shows a below freezing temperature. Once the mirror and sensor are brought up to the right temperature, reduce the defroster current enough to keep the mirror warm. A second temperature sensor would also help (one on the mirror face, one away from the mirror for comparison).

If odd hours are a concern, you can periodically cycle through the defrost all day instead of just keeping it on, based on the temperature.

Also try applying Rain-X or similar hydrophobic solution to the mirror. Works great to make a bathroom mirror fog free when taking a hot shower. Probably helps with snow and ice too, especially with the defroster circuit.

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  • \$\begingroup\$ It is not only me, it is other people too that need the mirror. Earliest possibly 4am, and from then throughout the day. On bad days with not much sunlight, the mirror is unusable all the day. Since this should run on solar power, I have to be very careful with energy consumption. On the days I need the defrosting most, I have the least energy available (bad weather+short days).I didn't do any calculations yet on how much I need, but keeping the solar panels as small as possible would be good. Also I could then harvest all days long the solar energy for the batteries and use it in the morning. \$\endgroup\$
    – PlasmaHH
    Jan 6, 2015 at 16:21
  • \$\begingroup\$ Regarding Rain-X: I found this here, I guess I can't wait long enough for a good product ;) dailymail.co.uk/sciencetech/article-2288268/… \$\endgroup\$
    – PlasmaHH
    Jan 6, 2015 at 16:21
  • \$\begingroup\$ @plasma the Rain-X works well enough that it should reduce the amount of work your defroster will need. \$\endgroup\$
    – Passerby
    Jan 6, 2015 at 18:40
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Is the mirror actually getting fogged? If it's outside at the same temperature as everything else then the only time it should get condensation is when it's foggy outside... ie the dew point is lower than the temperature.. or to say it another way the humidity is 100%. So maybe an outdoor humidity sensor would solve your problem.

Editing answer: (Good point in comments by OP)
You are right you can still get frost in the morning. (Though I'm not sure if this is related to the humidity or not.)
OK if you can't use the mirror itself to do a reflection measurement, how about another (little) mirror, with light source and detector. And assume that when one mirror is frosted the other is too.

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  • \$\begingroup\$ Even if it has been outside forever, the mirror is not necessarily at ambient temperature. Astronomy folks will quickly point out that optics exchanging radiation with the sky often fog even when their surroundings do not. Granted, the poster's mirror is probably oriented more horizontally. \$\endgroup\$
    – Chris Stratton
    Jan 6, 2015 at 15:36
  • \$\begingroup\$ It wouldn't solve the problem because even if this was the case (btw. 100% humidity is not quite correct, which you can easily see here each morning when you have to remove the ice from your car... The dew point is reached there often only near surfaces (hoar frost). It might not be detectable as humidity atthe sensor ) as soon as its starting to get brighter, the humidity decreases, but the ice will still be on the mirror for many hours. \$\endgroup\$
    – PlasmaHH
    Jan 6, 2015 at 15:37
  • \$\begingroup\$ @ChrisStratton: This can be due to them being colder than their surroundings (due to radiating heat which is not added from elsewhere like at day), forming a very thin layer of cold air where the dew/frost point is reached. Especially in astronomy moving air is undesired and often measures done to prevent it, so there is little to no convection going on, causing these layers to stay intact. \$\endgroup\$
    – PlasmaHH
    Jan 6, 2015 at 15:39
  • \$\begingroup\$ @PlasmaHH, good point... I've edited my answer. \$\endgroup\$
    – George Herold
    Jan 6, 2015 at 16:17
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Perhaps you could detect the frost by looking for a reflection (or scattering) where there should not be one. I wonder if you could remove the silvering by polishing it off in a small area and use a reflective sensor looking out through the mirror. If nothing is there, the light disappears into the aether. If the surface is frosted you would get a reflection from the thickness of the mirror.

You'd probably have to modulate the light, optically filter, and electrically filter the demodulated result to get immunity from daylight and other external effects.

This would have the advantage that the sensor would be in back with the heater and could be protected without concern for extra wiring and such like.

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  • \$\begingroup\$ Probably the filtering could partly be done in a microcontroller. This idea is intresting and I have to look if I have access to the back to do that. This also makes me wonder... There is some IR range which isn't much present even at daylight because it is absorbed by the athmospheres water. I will check if its possible to build a sensor based on that, that should make it more robust. \$\endgroup\$
    – PlasmaHH
    Jan 6, 2015 at 16:47
  • \$\begingroup\$ There are optical rain sensors available for car windscreens, so yes, the principle can work. \$\endgroup\$
    – JimmyB
    Jan 6, 2015 at 17:21
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Just to confirm, we seem to be talking about this kind of mirror: http://www.safeoptions.co.uk/safety-mirrors/outdoor-convex-safety-mirrors.html

Interesting project. I can see two basic approaches that might work:

a) Sense moisture on the surface with wires. You've ruled this out in your original post, but I think it could work very well if the mirror is an insulating glass surface. However it may have false positives unless protected from rain. You're also concerned about partial sensing and want to know if the mirror is 100% clear.

b) Actually attempt to determine whether you can see in the mirror or not with an optical technique. A clear mirror will give a clean reflection, where a fogged mirror will not. You talk about not wanting to look from a distance of one meter, but what about 20m? The convexity (am I right that it's convex?) is a problem for laser based techniques, and also for techniques involving lights on one edge and sensors and another.

If you can mount something across the other side of the road looking from some distance then you might attempt a camera+image recognition system: does the (unchanging) image of the road+surroundings in the mirror look right? Potentially quite power consuming itself.

Other than that we're back to dead-reckoning based on temperature of the mirror. You could try something of the form "if the mirror has been below A and has not yet got above B", where A=0C and B=5C, then tune it to give desired results.

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  • \$\begingroup\$ Yes, it is such a mirror, just rectangular. And yes, it is slightly convex, probably too much to have anything laser based work. The camera idea is intresting (though this would be getting more expensive than I ever thought, since I would have to add two solar panels and some wireless communication. Nevertheless some fun thing to think about); it wouldn't be that much more power consumption, since 1) it can/must have its own solar panel+battery and 2)it isn't necessary to react in realtime, checking every few minutes is fine. Unfortunately no one commented much on capacitive possibilities yet. \$\endgroup\$
    – PlasmaHH
    Jan 6, 2015 at 15:57
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    \$\begingroup\$ If it's convex, you just invert your logic: On a clean mirror, the beam from a given point will be reflected to some other point; on a frosted mirror, the beam will be scattered and part of it will be detected at almost any arbitrary point. So put the light detector somewhere where the clean mirror will not reflect the beam. When the beam's reflection is detected you know something (frost) is scattering your light. Modulation of the beam is mandatory because you will definitely want the relative amount of light reflected measured, and (static) ambient light sources need to be factored out. \$\endgroup\$
    – JimmyB
    Jan 6, 2015 at 17:27
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What you're looking for is Dew Point

You need 3 sensors, 2 thermal, and one relative humidity.

The humidity and one of the thermal sensors are measuring the ambient air. The second thermal sensor should be bonded to the mirror to give a temperature of the surface of the mirror.

The microcontroller will need a lookup table of temperature/humidity to dew point similar to below : dew point lookup from https://wx4cast.blogspot.com/2020/07/dew-point-and-relative-humidity.html

if the temperature of the mirror is below the dew point, condensation is likely.

when both dew point and mirror surface are below freezing, then you've got icing

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