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(I am professionally a software guy working in the GCC compiler with MELT, so I am fluent in software, but much less in hardware - I did use a solder for a 74LS00 nand gate based stuff in the 1990s. This is my first Arduino project)

My first Arduino project would be to automate the lights and the fan in my inhouse WC toilet in Paris (France) suburb. I bought several Arduino Nano v3 boards. I'm thinking of driving with a relay a low voltage 12v LED lightbubble. I want to detect human presence in the toilets. I'm thinking of some PIR sensor, but I am afraid that PIR won't detect anything if the temperature is above 37°C (or about 100F), which is unusual in Paris (France) but may happen a few days per year (I've got no air conditioning). Is my worry correct, or can a PIR still work with very warm outside temperature? Should I use an ultrasound detector instead?

  • I thought about the same thing. My ideal system would be to detect if the door is open using a hall-effect-sensor or reed-switch, and to detect if the door is locked (not quite sure what would be the ideal sensor for that). That way you can detect when people enter (open; close; lock), and when they leave (unlock; open; close). – Gerben Dec 29 '15 at 16:47
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PIR sensors work fine and are probably easier to program for than ultrasonic sensors. For example, this tutorial on a PIR sensor sold by Adafruit has a datasheet at the bottom that says:

Operating Conditions Operating Temperature -20 ˚C to +70 ˚C

Storage Temperature -30 ˚C to +80 ˚C

Operating Voltage 3 to 10 V at Rs = 47 kΩ

You can try to find and check the datasheet for the sensor you intend to purchase to determine if it will work.

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One thing to be wary of with PIR sensors is that they only detect moving heat. That means that your light would turn on as you enter the toilet, but would turn off again when the sensor fails to detect movement. That will be once you're sat on the toilet and reading the newspaper. To keep the light on you would need to flail your arms around to keep the sensor on, and thus the light on.

So from that perspective alone the ultrasound would possibly be a better option since it can detect the presence of anything within its cone of sound - as long as that something is big enough, like a human body.

A third option that you haven't mentioned is infra-red beams. Depending on how big the toilet is you may be able to just have one IR beam passing through the room, and when someone is in the room that beam will be broken. Only works in small cubicles though. For larger rooms you may need either a mesh of beams to cover the whole room, or a dual-beam in-out counting arrangement on the entrance (two beams, one before the other. If beam A breaks before beam B then someone has entered - if beam B breaks before beam A then someone has left. When there are 0 people in the room then turn the light off).

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