```html

Smart sensors are essentially auxiliary devices to your existing system when you set them up as triggers for other smart devices. For example, one that detects humidity can automatically turn on a humidifier if a baby's nursery becomes too dry. A contact sensor can turn on the lights when you open the shed door, for example, or set off an alarm when your teen sneaks into the snack cupboard. You can find a range of sensor types, but for this guide we have the most tested generally useful types in three categories: motion, temperature and contact. The Aqara Motion Sensor P1 is the best motion sensor because of its simple installation process, versatile stand and fast, reliable performance. The Centralite temperature and humidity sensor works well and is compatible with many Zigbee-based hubs. The Aqara door and window sensor is the best contact sensor, thanks to the best response time in its class and its compact size. Smart motion sensors Our choice The P1 offers excellent battery life, fast response speed and a particularly great mode. This requires the use of a branded hub. Compatibility: Amazon Alexa, Apple Home, IFTTT The second generation Aqara motion sensor P1 is easy to install and detects motion reliably and quickly. It's a great option for alerting you to movement in areas around your home that you want to keep an eye on, and you can also set it up with Alexa or Apple Home to activate other smart devices like smart switches or lights. The P1's swivel mounting stand makes it extremely easy to install discreetly almost anywhere. Most impressively, the P1's dual battery setup means you can measure battery life in years rather than months. Second place Although bulkier than our top pick, this sensor is responsive and works with many hubs. We also like the design. Compatibility: Amazon Alexa (Zigbee speakers only), Samsung SmartThings, Zigbee hubs The Centralite Micro Motion Sensor detects motion exceptionally well, even if it doesn't do much more than that. During our testing, this Centralite sensor was easy to pair and consistently detected motion at close range or from a long distance. Although this Centralite sensor does not have adjustable mounting options, the beveled edges give you flexibility during the installation process. Most importantly, this Centralite sensor works with many common Zigbee hubs, including many Amazon Echo speakers, so it can easily be paired with an existing smart-home setup out of the box. Smart Temperature Sensors Our Pick This temperature sensor mounts securely to a wall and works with most Zigbee hubs and Alexa speakers. Compatibility: Amazon Alexa (Zigbee-compatible speakers only), Samsung SmartThings, Zigbee hubs The Centralite temperature and humidity sensor quickly detected indoor conditions in our tests. Unlike our second temperature and humidity sensor, this one can be linked directly to a compatible Alexa speaker or other hubs that support Zigbee wireless. We weren't crazy about the reset and pairing process for this Centralite sensor, but it squeaked into our runner-up position due to its full Amazon Alexa support and wider compatibility with Zigbee hubs. The included screw-mount clip gives you additional options if you install the sensor in an indoor area with unpredictable temperatures or humidity levels. Runner-upAqara's temperature sensor offers best-in-class temperature sensitivity and an exceptionally compact design, yet compatibility is more limited and requires a branded hub. Compatibility: Amazon Alexa (partial), Apple Home, IFTTT The Aqara temperature and humidity sensor was one of the fastest and smallest sensors we tested overall. It provides near real-time temperature updates, and its small size makes it easier to hide if you mount it on a wall or window frame. But this Aqara sensor's incomplete Alexa support (you can't create automations for it in the Alexa app) will likely be a dealbreaker for anyone who has an Alexa-powered smart home. Smart Window/Door Contact Sensors Our Choice This affordable, responsive contact sensor is small enough to be almost unnoticeable on a door or windowsill. Compatibility: Amazon Alexa, Apple Home, IFTTT The Aqara door and window sensor responded immediately during our testing. It was also the most compact door sensor we tested (combined, both parts of the sensor take up less space than a matchbox), so it won't be noticeable if you need to place it in a prominent spot in your home. Unlike the Aqara temperature sensor, this model also supports Alexa automated routines and scenes. Runner-upCentralite's door sensor isn't as slick as Aqara's, but still offers solid build quality, easy installation, and wider compatibility with smart hubs. Compatibility: Amazon Alexa (Zigbee speakers only), Samsung SmartThings, Zigbee hubs The Centralite Micro Door Sensor isn't as compact as the Aqara Contact Sensor, but it's plenty small and shouldn't stand out in your home. In our tests, setup was easy and we paired it with an Amazon Echo speaker (which is also a Zigbee wireless hub). We appreciated the simple button on the side for reset and pairing. This Centralite sensor is an affordable alternative to the Aqara contact sensor if you already have a compatible hub that supports Zigbee.

```