Open Window Indicator

Published 20 November, 2016
Open Window Indicator

If you forget to close the window after using the bathroom in the colder days, you'll waste a lot of energy. If you don’t open your windows yours rooms will get sticky and possibly will mildew. Therefore I needed a reminder indicator when I forget to close the windows.

The RheinEnergie (a local energy provider) and the TH Köln (technical university) are doing a study on the usage of their smart home system. It is based on the Qivicon from Telekom.

For this reason I have got the possibility to check, how long our windows are open. I saw, that sometimes we forget to close them for over one hour. That’s quite a lot of energy waste! When the study started, we got window sensors, heating controllers and controllable power plugs. Everything I need is there, only a smart indicator is missing. The system is able to send push messages to our smartphones, but we don't like to get a ring every time the window is open too long.

UI Programming I can trigger a timer for the power plug once the window is open. The power plug now switches on after 15 minutes and off when the window was closed.

LED Strips Now I need a nice display. I made a small box out of beech wood and put a blue sheet of blue paper with a printed "open-window"-icon between two acrylic glasses behind the front plate. I liked to have a big blue icon, so I put three short strips of RGB-LEDs on the inner back and only connect the blue color. You can use RGB-LED Strips without a (remote-)controller. You only need a 12V power supply, connect the "plus" to the +12V of the power supply and the connections of the colors you want to use to the ground pin of the power supply. By wiring you can set red, green, blue, yellow, pink, cyan and white. I only connect the "plus" and the "blue" connection.

Circuit Diagram

The power supply is connected to the power plug and switches on if a window is open for longer than 15 minutes. Of course technically it is not a smart solution, because you need a switching power plug for every indicator. There are I/O-modules available for that communication standard, but it is not supported by the smart home base station yet.

The reminder works well and is used surprisingly well. Most of the time the windows are not open longer than 20 minutes what seems to be a good time for the bathroom (it can be set individually).

Some additional information: When you have a shower, humidity in your room rises (absolutely and relatively). The absolute humidity (weight of water in the air) is nearly the multiplication of the temperature, the relative humidity and a constant (the pressure is reduced, because of its constant in a small area).

You often get saturated air after a shower. Additionally, air temperature is higher than most walls and surfaces. When air is saturated, the dew point is equal to the temperature and steam condenses at them. Condensed and remaining water causes mildew which is toxic and carcinogenic. Therefore, it is important to air all rooms; especially the ones you sleep and shower in. If you open the window now, it is possible that the relative humidity inside and outside is equal, but the absolute humidity is not. When you heat the cold air from outside, the relative humidity drops and the drier air can admit more water.

If it is not enough when you open the window one time and reheat the air, you should replicate it. It is best to open the window after every single shower. Once you leave the house, open the door to the floor. Normally the air in all rooms gets too dry in the cold months, therefore it is possible for the other rooms air to admit more water. Attention: When the dew point drops below the temperature of the coldest surface, that surface will get wet and will mildew too! That will happen, if you have god a bad insulation (e.g. windows with only one glass layer), the room temperature is too cold or there is too much water in the air. Solution: Get a better insulation (It should be breathable! Never use polystyrene (well known as Styropor®)!), set the temperature at your heating controllers higher or let in more fresh (and drier) air.

It is no problem to get 0.5 liters of water in one towel (two liters in a four head family). Just make your own test: weight your towel on the kitchen scales before and after the shower. This water evaporates into the air and raises the humidity again. If it doesn't evaporate, your towel will not dry and start to smell. On the wall of the shower (and the cabin or the curtain) is water too, if you didn’t clean it. All of this water has to be put outside or spread inside! Otherwise your bathroom and the towels won’t get dry.

Dryer Sketch

Tip: If you use a hairdryer, try to hold the air input near to the bottom of the open window. The hot air inside rises to the ceiling. The cold air enters in the lower half of the window and falls. The heated air from outside can admit a lot of water and will dry your hair faster. Additionally, you can switch the temperature higher without destroying your hair because when the fan input is colder, its output is, too.