Why Are Thermostats Still on the Wall?

A couple of weeks ago I noticed the heat was staying on in my office pretty much all day. I have a boiler heating system with 4 zones and the thermostat that controls the front of my house is right there in the office. I wasn’t cold in there, but the thermostat wasn’t reporting that the temperature ever reached what I had set.

I pulled the Nest off of its mounting bracket, put my hand near the hole in the wall, and I could feel cold air. So I grabbed an instant read meat thermometer and stuck it through the hole. The reading inside the wall was 10° lower than a foot away from the wall.

For a simple fix, I stuffed a bunch of insulation through the hole and covered it with foil tape.

In order to monitor the effectiveness of the fix, I put together a quick temperature sensor instead of having to turn the meat thermometer on and off.

It worked!

Two or three years ago I had the opposite problem with this heating zone; it was always cold in the office. By feeling the wall I came to the conclusion that the thermostat had been installed right next to one of the pipes sending hot water to the upstairs registers. Brilliant! The fix that time was moving the thermostat over between the next set of studs.

After these two issues with the placement of a thermostat, I starting thinking. Why are we still basing our heating on measurements taken from a set position on the wall? With the Internet of Things we can do this much smarter.

Imagine each zone in the house having one or more mobile temperature sensors. Like the simple circuit pictured above, but in a small case. These could be battery-powered or plug-in. Windows, wind, and location of the sun can all affect the heating of different areas of a house. Being able to move the temperature sensor with you as you make dinner in the kitchen or watch a movie from your recliner would be awesome.

These temperature sensors would wirelessly report the temperature back to the home automation system. I use Home Assistant, which would make it easy to set the heating schedules for each zone. If a zone needed to go on or off based on the sensor’s reported temperature and the schedule’s target temperature, it would wirelessly trigger a relay module at the furnace or boiler. The relay would wire in to the furnace/boiler system in place of the wires that come from each thermostat and it would never know the difference. None of these pieces are hard to build and the parts are cheap.

This is all just something that ran through my mind as I was fixing my heating issue. I don’t have plans to build such a system, but if I did I could ditch my 4 Nest thermostats. For someone who works at home, often at random times of the day, I think Nest thermostats are overrated anyway because the learning and auto scheduling system doesn’t do much for me.

HC-SR04 as a Motion Sensor

The HC-SR04 ultrasonic sensor uses sonar to determine distance to an object like bats do. It offers excellent non-contact range detection with high accuracy and stable readings in an easy-to-use package. From 2cm to 400 cm or 1” to 13 feet. Its operation is not affected by sunlight or black material like Sharp rangefinders are (although acoustically soft materials like cloth can be difficult to detect). It comes complete with ultrasonic transmitter and receiver module.
Complete Guide for Ultrasonic Sensor HC-SR04

You can get the HC-SR04 from Amazon or various electronics shops for $3-5 or even under $2 if you buy packs of them. I got 2 of them in a parts kit I bought on Amazon and used one for Blog in a Box Paparazzi.

I was using the sensor to sort of detect motion, or more specifically when someone walked into a room. My prototype was set on a desk at about chest height about 1-2 feet after the doorway. While working on the project I ran into several challenges:

  • Accuracy of readings.
  • Other activity on the Raspberry Pi.
  • Sampling over multiple readings.
  • Rogue readings vs actual motion.
  • Timing between readings.
  • Bailing if an echo takes too long.

I wrote my code in Python and heavily based it on ModMyPi’s blog post HC-SR04 Ultrasonic Range Sensor on the Raspberry Pi. Here are the important pieces…

def read_ultrasonic() :
	# Make sure the trigger pin is clean
	# Recommended resample time is 50ms
	time.sleep( 0.05 )
	# The trigger pin needs to be HIGH for at least 10ms
	time.sleep( 0.02 )

	# Read the sensor
	while ( True ) :
		start = time.clock()
	while ( True ) :
		diff = time.clock() - start
		if ( GPIO.input( ULTRASONIC_ECHO_PIN ) == GPIO.LOW ) :
		if ( diff > 0.02 ) :
			return -1

	return int( round( diff * 17150 ) )

def is_ultrasonic_triggered() :
	global prev_ultrasonic

	# Take 6 readings
	for i in range( 6 ):
		ultrasonic = read_ultrasonic()
		#Shift readings
		prev_ultrasonic = ( prev_ultrasonic[1], prev_ultrasonic[2], prev_ultrasonic[3], prev_ultrasonic[4], prev_ultrasonic[5], ultrasonic )

		if ( is_light_enough()
				and prev_ultrasonic[0] != -1
				and prev_ultrasonic[3] < ULTRASONIC_DIST and prev_ultrasonic[4] < ULTRASONIC_DIST and prev_ultrasonic[5]  ULTRASONIC_DIST and prev_ultrasonic[1] > ULTRASONIC_DIST and prev_ultrasonic[2] > ULTRASONIC_DIST ) :
			#print 'Ultrasonic: {0}'.format( prev_ultrasonic )
			return True

	return False

while ( True ) :
	if ( is_ultrasonic_triggered() ) :

It worked alright, but triggered a little too often. About a week later I came across the Python package gpiozero, which makes it easy to work with a bunch of common Raspberry Pi GPIO components. I wrote an alternate version of BIAB Paparazzi using this package, which worked a bit better. It was so much simpler with gpiozero because it has built-in support for the HC-SR04. All I had to do was initialize the sensor and tell it what code to run when something in range was detected.

ultrasonic = DistanceSensor(
	max_distance = ULTRASONIC_MAX,
	threshold_distance = ULTRASONIC_DIST )

ultrasonic.when_in_range = take_picture

The neat thing about the gpiozero package is when you initialize a sensor it automatically starts taking readings, keeps the values in a queue, and does comparisons against an average. My code attempted to do something along those lines, but was much more rudimentary. As nice as this version sounds, it still triggered too often. You can find the complete code for both versions in the BIAB Paparazzi repo on GitHub.

I think I was pushing the limits of what the HC-SR04 is meant for. Most of the examples I’ve seen are people using these to detect approaching walls on a robot. The biggest issue I ran into was the inaccurate readings. For example I’d be getting readings of about 160cm and then out of nowhere it would return a distance of 80-90 cm, even several in a row at times.

At the end of the day there are reasons it’s such a cheap sensor. 😉 For a couple of dollars, what do you expect? I’m curious to try my code on a more powerful Raspberry Pi 3 and see if it works any better. Was the less powerful Pi Zero causing problems?

Garage Monitor Updates

I made some updates to the Garage Temperature Sensor & Monitor. I didn’t like how the desired temperature was set via the app’s configuration file, so I moved it to a slider control in Home Assistant and updated the LCD to always show the value. Only being able to enable/disable monitoring via the device’s button also wasn’t great. I converted the binary sensor I was using to flag monitor mode in HA to a switch control and moved the actual monitoring logic from the Python app to HA automation. Everyhing is updated on GitHub.

Info and controls in Home Assistant
Custom temperature and humidity monitor