I am new to this community and I apologize for my bad English. I am interested in

Home automation

I am working on my new project. Where the requirement is to use a sensor for

Automatic Light on/off in washroom

I did some research on PIR sensors but PIR doesn't work in the case of a static human (not in motion). So please suggest a suitable sensor for static human detection. I have attached an image of the bathroom in the project.

Washroom drawing

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    – anonymous2
    Jan 15, 2021 at 12:52

1 Answer 1


Detecting presence (rather than movement) can be a challenge. There are many ways of doing, each with pros and cons.

  • PIR sensors can still be used, coupled with a long timeout after movement stops. But this has limits: either you set a very long timeout and you are keeping lights on unnecessarily, or the timeout is too short and you need to move to switch the lights back on.

  • Thermal detectors can be a solution in many environments, but they require more power (so are usually not suitable for battery-operated sensors, you'll need some cabling), and they may be fooled by unusual temperatures, which are common in a bathroom (though you could consider that if there's a hot bath there must be someone in it).

  • Cameras with face or shape recognition will require quite a bit of processing, and raise privacy issues in a bathroom.

  • Break-beam detectors (a focused light on one side, a light sensor on the other, if someone – or something – comes between them this is detected) could be used, but they can only detect presence along a line, so you would probably need several strategically placed. Placement should additionally ensure the beam can only be broken by someone and not something, e.g. a door or a bottle of shampoo in the wrong spot... Probably requires external power as well.

  • Time-of-flight (TOF) sensors (a signal is sent, and a sensor detects how long the signal takes to come back, which gives distance). Depending on the technology (light or sound) the detection angle can be very narrow or a bit less, but still, you'll again most probably need multiple sensors to cover the whole area. And it requires calibration of all the sensors. Again, external power probably required.

  • Pressure sensors (floor or seat) are probably not suitable here. Likewise for vibration sensors.

  • There are other solutions based on detection of RFID tags, wireless activity, noise, and so on, but they're probably not adapted here.

  • I think things like CO2 sensors have been considered, but from what I understand they're pretty slow to detect presence.

So, as you can see, there are no obvious solutions. I think in this specific case I would still go for a PIR sensor (or several, actually), with a pretty long timeout (at the very least 5 minutes, probably even 10 or 15). They're cheap, they can run on battery, they can be quite sensitive.

Make sure you understand how they work (they detect movement in a plane orthogonal to the axis only, and many of them only/mostly in a single direction). The more surfaces there are on the Fresnel-like lens the better (each is a detection area). The closer the sensor to the movement the better.

I would probably place sensors over the sink, over the bathtub, over the shower, and over the toilet. Make sure they are suitable for use near water, humidity and heat.

One alternative could be to combine multiple types of sensors.


I somehow forgot that PIR sensors also have issues with high temperatures such as those of a hot bath, so they’re probably not the best option in the area of the bathtub and possibly the shower. Maybe a Doppler radar may be an option there. You probably need to combine multiple technologies to achieve the best result.

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