A lot of people have a spare Android phone lying around in a drawer somewhere. It doesn’t matter how old the phone is, whether it’s got a great camera, a cracked screen, or even what model it is — it’s a multi-sensor device that dedicated smart home hardware can’t quite match.
So if you’re looking for a way to reuse an old Android phone while getting access to some useful triggers for smart home automation, it really is the perfect device. All you need to do is connect it to your home assistant using an Android automation app like MacroDroid or Tasker, and put those sensors to work.
Your phone already knows how bright your room is
Using the ambient light sensor for smarter lighting
Your Android smartphone has a built-in ambient light sensor that it uses to automatically adjust screen brightness depending on your surroundings. But since we’re talking about an old phone here, that ambient light sensor can easily pull double duty to trigger your smart home lights when it gets too dark. Whether you leave it by a window to turn it into a daylight sensor or just in a random spot in your house you’d want illuminated when it gets too dark, it works the same way.
The Home Assistant companion app can share this sensor reading with your smart home hub, triggering automations like turning on a specific room’s lights when the brightness falls below a certain lux reading. Depending on what smart home appliances you have, you can trigger lights, blinds, or even air conditioners when you detect direct sunlight coming into a room. It’s a genuinely useful substitute for $30 to $50 lux sensors that you’d otherwise have to buy separately.
Detect movement without a camera
The accelerometer triggers motion-based automations
Every smartphone has an accelerometer — a tiny chip that detects movement and orientation changes. Apps like MacroDroid can expose this sensor directly as a trigger, letting you create automations when the phone is picked up, tilted, or shaken.
The immediate use case is for any surface that has some sort of vibration. For example, you could leave a phone on your washing machine and get a notification (or control a light) when your wash cycle ends; that is, when your machine stops vibrating. You can also use it as a quirky way to control lights, fans, or just about any other smart home appliance.
Yes, you can listen for events
Using the microphone for sound-based triggers
Smartphone or not, every phone has a microphone. These microphones aren’t just there to receive voice commands for AI assistants. You can use them to listen for specific audio events and use them for triggers.
For example, you can leave an old phone in your kitchen to listen for smoke alarms and send out a push notification to your main device. Or you could place it in a baby’s room that triggers the baby monitor or lights when it hears sustained crying or noise.
That said, setting up an exact sound to be used as a trigger can be a little complicated, especially considering you might have to resort to complex tools like Frigate. For simpler basic noise-level thresholds, you can trigger webhooks by reading the microphone’s decibel levels via MacroDroid.
It knows when something gets close
Proximity sensor tricks for simple presence detection
Your Android smartphone has a proximity sensor that’s used to turn the screen off when something comes near the display — that’s why your screen automatically turns off when you take a call. While its range is short, typically under five centimeters, that might actually work in your favor.
This will usually make a rather useful contact trigger. You can mount your phone face up on a shelf or table, and any item kept over it or hovering near the surface can trigger an event. You can also use this to tap or hover near the screen and trigger a smart light sequence, essentially turning the phone into a fancy smart light button.
It can even read air pressure changes
Barometer-based automations you probably never considered
Not every phone has one, but many mid-range to flagship phones from the last several years include a barometer. Originally added to improve GPS altitude accuracy, you can also use it to detect weather pattern changes before a single cloud appears in the sky.
A steadily dropping barometric pressure reading is an early indication of incoming rain or storms. You can pipe the sensor’s readings to trigger automations that automatically close your smart windows, roof vents, or blinds before it starts raining. The best part is that this automation is fully reactive and doesn’t rely on an internet-connected weather API.
You’ve already got the sensors. Use them
Setting it all up is also easier than it sounds. You can either use the Home Assistant companion app on your old phone and enable the sensors you want, or use apps like MacroDroid or Tasker to trigger automations without needing a hub. MacroDroid itself handles automations entirely on-device and provides over 80 sensor-based triggers.
You probably have an old Android lying around — here are 6 clever ways to use it
From a handheld gaming device to a wireless security camera, your old Android still has plenty to offer.
You will have to keep the old phone plugged in permanently and perhaps mount it somewhere sensible, but all of that is much more useful than spending on yet another smart home device, which can be a pain to set up. There are things your old Android phone does better than any dedicated gadget, meaning that seemingly useless old phone can be a surprisingly capable part of your smart home setup, without needing to spend a single dime.
