The Blog of Ian Mercer.

HX711 Strain Gauge Pulsor Sensors

Cover Image for HX711 Strain Gauge Pulsor Sensors

I've been using strain-gauge sensors for some time now in my home automation system. They are invisible and highly reliable.

Some time ago I switched from the Sure Action boards to my own HX711 based processor boards and recently I added capacitive coupling between the bridge and the HX711 which removes any DC offset or temperature drift.

HX711 circuit

It works well. Next step is to create a PCB.

Related Stories

Cover Image for Time Series Data Compression

Time Series Data Compression

This new technique to compress the time series data collected by my home automation system seems to be working really well.

Ian Mercer
Ian Mercer
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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
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Bluetooth Tracking Project

My year long Bluetooth project that won the $20,000 HCI and Microsoft competition during lockdown has continued to grow and now reliably tracks how many people are in the house and outside and can locate any device down to room level.

Ian Mercer
Ian Mercer
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Digital Twins are never identical

Digital Twin are an online representation of a real world object, a copy of its properties in the digital world and a way to send updated and commands to it. In effect I've been making them for years but now they have a trendy name.

Ian Mercer
Ian Mercer
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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
Cover Image for Collinearity test for sensor data compression

Collinearity test for sensor data compression

One way to reduce the volume of sensor data is to remove redundant points. In a system with timestamped data recorded on an irregular interval we can achieve this by removing co-linear points.

Ian Mercer
Ian Mercer
Cover Image for 3d Printed ESP32 Brick

3d Printed ESP32 Brick

ESP32 provides a great platform for sensors around the house but by the time you've added a USB power brick, cable and enclosure it's quite messy. I wanted a device that I could just plug in with no exposed wires and no mounting needed so I designed one in OpenSCAD.

Ian Mercer
Ian Mercer
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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
Cover Image for Bluetooth Sensing for Home Automation

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
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Gas sensors

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

Ian Mercer
Ian Mercer
Cover Image for Humidity Sensors (DHT11, DHT22, AM2320)

Humidity Sensors (DHT11, DHT22, AM2320)

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

Ian Mercer
Ian Mercer
Cover Image for Light sensors for Home Automation

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
Cover Image for Microwave Doppler Sensors (RCWL-0516)

Microwave Doppler Sensors (RCWL-0516)

Microwave doppler sensors can be found in some alarm sensors but there are also available very cheaply as a separate component. They offer exceptional range but suffer from false triggers requiring a probailistic approach to people sensing.

Ian Mercer
Ian Mercer
Cover Image for Optical-beam sensors

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
Cover Image for PIR Sensors for Home Automation

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
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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
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Temperature sensors for home automation

Temperature sensors I've experimented with for home automation.

Ian Mercer
Ian Mercer
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Event blocks

Home automation systems need to respond to events in the real world. Sometimes it's an analog value, sometimes it's binary, rarely is it clean and not susceptible to problems. Let's discuss some of the ways to convert these inputs into actions.

Ian Mercer
Ian Mercer
Cover Image for Logistic function - convert values to probabilities

Logistic function - convert values to probabilities

Another super useful function for handling sensor data and converting to probabilities is the logistic function 1/(1+e^-x). Using this you can easily map values onto a 0.0-1.0 probability range.

Ian Mercer
Ian Mercer
Cover Image for ATAN curve for probabilities

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
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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
Cover Image for Probabilistic Home Automation

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
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Multiple hypothesis tracking

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

Ian Mercer
Ian Mercer
Cover Image for A state machine for lighting control

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
Cover Image for The Internet of Hubs (and things)

The Internet of Hubs (and things)

Maybe it should be called the Internet of Hubs instead

Ian Mercer
Ian Mercer
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World's Smartest House

Over 15 years of experimentation with home automation

Ian Mercer
Ian Mercer
Cover Image for The Grideye 8x8 camera sensor

The Grideye 8x8 camera sensor

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

Ian Mercer
Ian Mercer
Cover Image for CCTV Cameras as Home Automation Sensors

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
Cover Image for Pressure Sensors for Home Automation

Pressure Sensors for Home Automation

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

Ian Mercer
Ian Mercer