Sensors
There are many types of sensors available to HomeKit: motion, vibration, temperature, humidity, air quality, contact, leak, smoke, etc. Many sensors can detect more than one condition, for example humidity and temperature. You can use sensor information for a variety of things in HomeKit, including automations such as turning on a humidifier when the humidity drops below a certain point, or turning on a hallway light when somebody approaches a hallway. Sensors may come in standalone sensors, or as part of another device, such as a thermostat, doorbell, or security camera.
Sensor Connections
Bluetooth: Bluetooth has limitations such as speed and distance, but it is inexpensive to implement and can last a long time on battery power, which can make them smaller and easier to install, making them well suited for small sensors.
Zigbee: Zigbee devices can respond quickly and can also last a long time on battery power. Zigbee sensors will need a bridge to connect to HomeKit.
Wi-Fi: Wi-Fi offers has advantages over Bluetooth, including connectivity over longer distances, but will also require a power source for practical usage. This typically makes Wi-Fi sensors larger.
Motion Sensors
Some motion sensors only detect motion, but most of them contain other sensors as well. You will often find a motion sensor paired with a light sensor; this allows you to tale different actions when motion is detected based on the light level. Another common sensor pairing is motion, temperature, and humidity.
Temperature/Humidity/Air Quality Sensors
Most sensors that are measuring air quality values measure more than one thing. You will typically find at least temperature and humidity in a single sensor, and more often than not, they will also measure particles in the air and give you a "air quality" score.
Vibration Sensors
Smoke Detectors
Leak Detectors
Contact Sensors
What I Use
The Ecobee SmartSensors provide temperature and occupancy detection
Onvis Smart Motion Sensor - The Onvis is a handy little Bluetooth 5.0 motion sensor that also measures temperature and humidity. It stores 31 days of data, and uses two AAA batteries that last close to two years. I have found them to be reliable, although they need to be relatively close and line-of-sight to a HomeKit Hub.
Koogeek Smart Environment Monitor - The Koogeek Smart Environment Monitor specifications state: "6 high precision sensors and 360° open air circulation design ensure the accuracy and sensitivity". It has a 2.8'' LCD Screen that displays all of the readings it can measure: relative humidity, particulates (PM2.5 and PM10), as well as motion detection, light intensity, sound levels, the current time, charging status, and battery levels. It connects over Wi-Fi and uses a rechargeable battery or can be plugged in using the included adapter. A couple of operation notes: This unit has a fan in it (the manufacturer indicates that this is to get more accurate readings) which makes a quiet white-noise type of sound. I would not recommend it on a night stand or near a reading nook due to the noise. I have also seen the motion detector trigger when I know that nobody was in the room. If I were to purchase another device like this (something that measured temperature with a display that could be plugged in) I would give the Eve Room Indoor Air Quality Monitor a try.
First Alert OneLink Safe & Sound Smoke Detector
