Design and thinking…

To design a chair sensor, we have to find out what is different when someone sits down on a chair. In other words, how the sensor will know you are sitting in the chair.

So what changes?

Photo by jose aljovin on Unsplash

Upon sitting on a chair, here are some potential changes:

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Distance, your body blocks the back of a chair.

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Vibration, the chair moves and causes vibration as you sit down.

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Weight, as you sit down the weight of the chair increases.

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Crossing things off the list…

1. V̶i̶b̶r̶a̶t̶i̶o̶n̶

Although a vibration sensor might fit the bill, most chairs don’t cause vibration. Four-legged chairs (like the one shown in the photo) rarely move and are unlikely to trigger a vibration sensor.

2. D̶i̶s̶t̶a̶n̶c̶e̶

An ultrasonic sensor attached to the back of the chair is a good idea. However, this will not work with stools.

3. Weight

Lastly, we’ve come to the most consistent aspect of a chair: weight. A chair is designed for sitting. No matter what kind of chair it is, as you sit down, the weight will always change.

How?

Monitoring the weight…

The most straightforward way of monitoring weight is using load cells, which are oftentimes used in scales. In our use case though, there is no need for such accuracy. We only need a binary status of wether

Yes, you are in the chair. or No, you are not in the chair.

This could be done with some sort of simple binary pressure mat that triggers as you sit down.

That is exactly what I chose to use.

Battery vs Mains

There is a reasonably simple answer to this: we need it to be battery powered. Chairs often get moved around. More importantly, most chairs are in a place inaccessible to power outlets.

Combining the two…

After much thinking of how this is all going to work out, I stumbled across smart door window sensors. These sensors go onto your (as the name implies) door and window and allows you to monitor whether they are open or closed.

When you open the door/window, a magnet is moved away from the main sensor. Essentially, what the magnet does is triggering and un-triggering a reed switch. When the reed switch is triggered, two contacts inside the sensor connect. When the reed switch is not triggered, the two contacts are left disconnected. Let’s take a closer look inside one of these sensors:

The sensor I chose to use

What does this mean for us?

Taking the logic, if we manually connect the two points, the sensor will trigger. Once we disconnect it, the sensor will un-trigger. So…