DIY chair occupancy sensor

Based on a contact sensor and a car seat pressure sensor

Introduction

For my home office I want a way to detect if I'm there behind the desk. To detect that, it's possible to use a motion sensor, to detect when you also sit still on a chair, it's better to use a presence sensor. The downside with one of those is, they will also detect animals or blowing fans and are more expensive. And they detect a wide range and not only the chair occupancy.

My solution

This “hack” uses a contact sensor connected to a car seat sensor to detect if a chair is occupied, exactly what I needed!

Other purposes for this sensor are:

In the couch/sofa/relax chair
Under your mattress to detect bed occupancy (only works if there is enough pressure through the mattress)
Under a mat on the floor (to detect if someone entering a space)

My solution
Automations
Required hardware
Home Assistant

Automations

With this new sensor, it’s possible to make all kind different automations, a few examples are:

When the light still needs to be on
Power up all the computer peripherals (monitor, lights, chargers)
Shutdown the computer and peripherals automatically when you don’t sit behind your desk for a while
When it’s time to take a break to stand up and stretch your legs
When it’s time to end your working day
Control the room temperature because it’s occupied
Send notifications with incorrect office health state values (CO2, temperature, humidity, PM2.5, VOC, or Formaldehyde)

Required hardware

This project only requires these two devices: A boolean sensor to detect true or false and a pressure sensor.

I have a Zigbee network, so I use a Zigbee contact sensors, but any other protocol sensor can also do the trick.

The first device needs to detect anything or nothing. This can be achieved with two different types of sensors. A water leak sensor or a contact sensor. Both work with a boolean (true of false) state.

1a. Aqara Water leak sensor (No soldering required)
The Aqara leak sensor has two metal screw contacts on the back of the sensor where you can connect direct the two wires of the pressure sensor behind.

1b. Or use a (Zigbee) Contact sensor (Soldering required)
A contact sensor is (mostly) cheaper than the water leak, but it requires soldering.
On this page I describe how it works with this sensor.

A Car seat pressure sensor (smaller or bigger versions are available)

To connect them together, and you use the contact sensor, you need also some soldering tools:

With the leak sensor, you only need to strip the protection layer from the pressure sensor cables, that it.

Wire them together

With an Aqara water leak sensor

With the Aqara water leak sensor, you only need to unscrew the screws, wrap the blank wires from the pressure sensor around it. Screw them tight again and done!!

With a contact sensor

A contact sensor is a boolean sensor, the circuit can be opened or closed.
That’s also exact what the car seat pressure sensor returns.

The only this that has to be done is connecting the pressure sensor wires to the (reed) contacts of the contact sensor.

First open the contact sensor. You can remove the reed contact, but you can also leave it like it is. Because there is no magnet nearby, those ends don’t make contact. The sensor is set parallel over this switch. Drill a hole in the side of the contact sensor so the cables can go inside. Solder the wires to each side of the (reed) contact. You can also use two pressure sensors if you want to cover more space with just one sensor.
You connect both sensors together on the same reed contacts ends. Now you can place the sensor inside a pillow on the chair of inside the chair itself if you can zip the seat open.

When you now sit on it, the state of the contact sensor will change.
Now you can create automations based on it!

Home Assistant

Create a new custom chair sensor

By default, the contact status is inverted as preferred. With this addition in the configuration.yaml file, it creates a new sensor that shows the correct status in the dashboard.

# Sourcecode by vdbrink.github.io
# configuration.yaml
binary_sensor:
  - platform: template
    sensors:
      chair:
        friendly_name: "chair"
        value_template: >-
          {% if is_state('binary_sensor.contact1_contact', 'off') %}
             on
          {% else %}
             off
          {% endif %}

homeassistant:
  customize: 
    binary_sensor.chair:
      icon: mdi:chair-rolling

Occupancy time sensor

With the history stats it’s possible to create new sensors which indicate how long something takes. In this case we want to track how long the chair is occupied on each day. The start/reset is on a new day and end time is the current time and within this timeframe how long has entity binary_sensor.chair_work the state on. This is the code to add in the configuration.yaml.
This will generate a new sensor called sensor.chair_occupancy.

# Sourcecode by vdbrink.github.io
# configuration.yaml
- platform: history_stats
  name: chair occupancy
  entity_id: binary_sensor.chair_work
  state: 'on'
  type: time
  start: '{{ now().replace(hour=0, minute=0, second=0) }}'
  end: '{{ now() }}'

Graphs

Now we have the data, it’s possible to present this on the dashboard!

Total time as text

Show the amount of time on the chair as an entity.
8 hours, 12 minutes and 36 seconds.

# Sourcecode by vdbrink.github.io
type: entities
entities:
  - entity: sensor.chair_occupancy

Occupied in time as graph

Or show the occupied time in a line graph over time.
It shows exactly where I took some breaks, the line is flat at that time.

A History graph Card is used for this.

# Sourcecode by vdbrink.github.io
type: history-graph
entities:
  - entity: sensor.chair_occupancy
logarithmic_scale: false
hours_to_show: 24
title: Office chair

Total time in a bar

Use the same History graph Card with binary sensors, then it’s presented as a bar.

Here I add the chair occupancy sensor next to the value of a pir motion sensor in the same room. As you can see, the chair sensor is much more reliable if you sit still!

History graph bar Card in Home Assistant

# Sourcecode by vdbrink.github.io
type: history-graph
entities:
  - entity: binary_sensor.chair_occupancy
  - entity: binary_sensor.motion_occupancy
hours_to_show: 24

Is occupied

Indicate if someone is sitting on the chair.

# Sourcecode by vdbrink.github.io
type: entities
entities:
  - entity: binary_sensor.chair_occupancy

That’s it, a very useful and reliable DIY Zigbee chair sensor (for me at least!).

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