The Blog of Ian Mercer.

Microwave Doppler Sensors (RCWL-0516)

The RCWL-0516 microwave doppler radio board is a very cheap sensor for home automation. It responds to even slight motion anywhere within a considerable range (~10m) and it works through walls and other obstacles. It's not really "doppler" but it does detect changes in the interference between a weak transmitted microwave signal and the returned signal.

The downside to these properties is that it's no longer telling you about occupancy in a single room and it's susceptible to random triggers for no apparent reason. I tried the various hacks to reduce the sensitivity but it still triggers occasionally when it should not.

Given these properties I was forced to come up with a new approach in my home automation system that can handle unreliable sensors like this and sensors that span multiple rooms. My probabilistic model solved both these problems. By assigning a low probability to the radar sensor type and by associating each radar sensor with a percentage coverage in multiple rooms I can incorporate the useful data it provides whilst ignoring the spurious triggers.

In effect the radar sensor can confirm that someone is still in a room and it can detect someone has moved to a room (after multiple triggers each one increasing the likelihood of a move), but it cannot on a single trigger determine that someone has entered a room, nor even which room is being triggered.

The sensors are cheap and worth including in your home automation plans. Some commercial sensors combine technology like this with PIR to provide greater false rejection for both sensors which seems like a smart approach.

Related Stories

Home Automation

I've been working on home automation for over 15 years and I'm close to achieving my goal which is a house that understands where everyone is at all times, can predict where you are going next and can control lighting, heating and other systems without you having to do or say anything. That's a true "smart home".

Ian Mercer
Ian Mercer

Home Automation Sensors

An overview of the many sensors I've experimented with for home automation including my favorite under-floor strain gauge, through all the usual PIR, beam and contact sensors to some more esoteric devices like an 8x8 thermal camera.

Ian Mercer
Ian Mercer

Filtering techniques

Filtering raw data is essential for a reliable home automation system. Here are some of the many ways you can filter sensor data.

Ian Mercer
Ian Mercer

Bluetooth Sensing for Home Automation

Bluetooth sensing for home automation is a great proxy for people counting as it can detect and locate each cellphone in the house. iBeacons attached to tools, cars and pets can provide a 'find my anything' feature too.

Ian Mercer
Ian Mercer

Gas sensors

Gas sensors come in many different flavors including CO2, VOC and particulate sensors.

Ian Mercer
Ian Mercer

Humidity Sensors (DHT11, DHT22, AM2320)

Humidity sensors are great for controlling extractor fans in bathrooms and other damp spaces.

Ian Mercer
Ian Mercer

Light sensors for Home Automation

Having at least one light sensor is critical for any home automation system that controls lightng. Lights need to be turned on when it's dark not at specific times of day, especially here in Seattle when it can be dark and cloudy at any time of day.

Ian Mercer
Ian Mercer

Optical-beam sensors

Optical-beam sensors are reliable and can cover a long-distance such as across a garage or aisle-way. When they include multiple-beams they have good false-trigger rejection.

Ian Mercer
Ian Mercer

PIR Sensors for Home Automation

PIR sensors are cheap and easy to use but they suffer from slow response times and low repeat rates.

Ian Mercer
Ian Mercer

Strain-gauges

Strain-gauges are my top-rated sensor for home automation because they are invisible, reliable and can be tuned to detect people and ignore pets.

Ian Mercer
Ian Mercer

Temperature sensors for home automation

Temperature sensors I've experimented with for home automation.

Ian Mercer
Ian Mercer

ATAN curve for probabilities

In a home automation system we often want to convert a measurement into a probability. The ATAN curve is one of my favorite curves for this as it's easy to map overything onto a 0.0-1.0 range.

Ian Mercer
Ian Mercer

Bluetooth

One of my inventions recently won a $20k global competition for applications that could help in a pandemic. It uses Bluetooth to count people.

Ian Mercer
Ian Mercer

Probabilistic Home Automation

A probabilistic approach to home automation models the probability that each room is occupied and how many people are in that room.

Ian Mercer
Ian Mercer

Multiple hypothesis tracking

A statistical approach to understanding which rooms are occupied in a smart house

Ian Mercer
Ian Mercer

A state machine for lighting control

An if-this-then-that style rules machine is insufficient for lighting control. This state machine accomplishes 90% of the correct behavior for a light that is controlled automatically and manually in a home automation system.

Ian Mercer
Ian Mercer

The Internet of Hubs (and things)

Maybe it should be called the Internet of Hubs instead

Ian Mercer
Ian Mercer

World's Smartest House

Over 15 years of experimentation with home automation

Ian Mercer
Ian Mercer

The Grideye 8x8 camera sensor

Experiments with an 8x8 IR camera for privacy-preserving people detection using cameras.

Ian Mercer
Ian Mercer

CCTV Cameras as Home Automation Sensors

CCTV cameras are an option for detecting people but within the home there are privacy concerns that need to be addressed.

Ian Mercer
Ian Mercer

Pressure Sensors for Home Automation

Pressure sensors can detect HVAC system operation and could potentially detect clogged filters.

Ian Mercer
Ian Mercer