XTension Logo

An Online Tutorial

Different types of motion sensors

There are many different types of sensor that can be used in the home. Some of these are very simple, and others can be very complex and expensive.

This is the second of a series of pages about motion sensors. If you have not seen the first page, you might want to visit it before this one.

Here are some of the 'keywords' that you will discover as you browse this subject:
Area Sensors

Passive InfraRed
Proximity (rf field)
     Local Sensors

Active InfraRed (light beam)
Visible Light beam
Laser Beam
There is a fantastic webpage about all sorts of motion sensors.

Important qualities of all sensors

We want all motion sensors to indicate the same thing : that some condition has changed.

All sensors have some 'normal' state.

Some sensors only report when the 'normal' status is disturbed, others also report when the condition reverts to 'normal'.

Other considerations will be things like the sensitivity of the sensor, the range of the sensor, whether the sensor requires a special power supply, whether the sensor can directly or indirectly send X-10 signals, and whether you must download or create a special function which will monitor some special device.

Signalling method ( how do we know ? )

There are several different models of motion sensor which are available that directly send X-10 signals to the powerlines, and others which send wireless signals to a 'transceiver' which then re-transmit the command to the powerlines.

Addressing (Which sensor is talking ?)

If we have multiple sensors in the home, then it is very important to be able to identify which sensor is talking.

In the X-10 system, each sensor is 'named' with a unique address. This must always be the case, regardless of the technology. It is also just the information that XTension needs to be able to recognize the exact unit, and locate it in your database.

Free Dusk Sensors

All of the motion sensors produced by X-10 provide a free 'dusk' sensor. This is quite nice, but always involves another X-10 house/unit address. When you have a lot of these motion sensors, it's not necessary to have each of the dusk sensors enabled.

However, you can use this sensor as a verification of a light having turned on. Just make sure that you experiment with the placement, so that you know that the light sensor will not trigger for any reason other than the light turning on.

The 'light' or 'dark' sensor status is often integrated into the 'logic' of the motion sensor, which sometimes produces aggravating functionality for the home automator. But all of the X-10 devices can be easily modified so that they always behave as motion sensors, and never report whether it is 'dark' or 'light'.

Since we just don't need that many light sensors, we might make good use of that extra ability of the PIR sensors. If we clip the 'light sensor' inside the Hawkeye, and replace it with any kind of switch, it can become even more useful.

Special sensors and 'hacks'

Some really neat sensors must be handled differently, and may require that you be clever at programming, or that some good samaritan has already offered a free plug-in that will handle the 'odd' device.

Some common devices can be 'hacked' easily to make it behave differently, or to add some useful function.

Many of these 'hacks' have been discovered and described on other pages and websites. For a beginning, we will limit this article to things which only require a screwdriver ( and maybe wire cutter/strippers)

Basic types of 'motion' sensors

Passive InfraRed ( PIR )

The most frequent use of the PIR sensor is as an 'area' sensor. Whether it is to detect 'someone moving in the front yard', or 'someone moving in the bathroom', or 'someone moving through a doorway', or even 'someone opened the beer cooler', it is all technically the same sensor and logic.

There is a simple electronic device which is sensitive to 'heat', or rather the infrared light that is emitted by warm or hot objects (like humans).
In its simplest form, it looks like
an old metal transistor with a black plastic
'window' on the top.

The 'logic' of the PIR sensor is that it must detect 'significant change' of the normal level of heat within the 'field' of its view. The circuits that control it must be able to determine what 'normal' is, and then close a switch when the normal field changes, as when a human walks in front of it.

It must also be able to 'tolerate' slow changes within the field, and remember that as the new 'normal'. This is so that gradual changes like the sunlight changes throughout the day, don't cause a false alarm. This is a standard behavior of 'PIR' type sensors. (There's a lot more electronics there than just the black window...)

The most common (PIR) type of X-10 motion sensor is the "Hawkeye" model line from X-10. It is quite small, and although can be quite a pain, it is cheap (often a freebie), it can be very useful.
Currently, there have been 3 different 'upgrades' to this device. But the last two are functionally identical. The latest is simply more rugged for outdoor use.

Why does the sensor wear 'lenses' ?

You'll notice in all three pictures of PIR type sensors on this page, that they all have some sort of plastic 'lens' that covers the circuit board and the PIR sensor device.

This is a 'Fresnel' lens. It 'pinches' light that passes thru it. If you hold it to your eye, you can see that there are apparent distinct 'bars' of light as you move it across a scene. Some of these bars may be vertical, and some may be horizontally oriented.

The lenses that are made for most PIR sensors, tend to 'pinch' the light such that it is horizontally sensitive.

This means that the Lens/PIR will be more sensitive to motion of a warm body, horizontally 'across the field of view'.

Please note that this means that these sensors are most insensitive to warm bodies moving from a 'distance' and directly towards one of these common devices...!

If you look carefully at the animation on the
first page of this series, I've tried to show the man running 'across' the field of view of the sensor.

What is UP with these sensors ?

All of the most common X-10 PIR sensors can be mounted in any position you can imagine.
But just how and where you mount it will have a lot to do with how satisfied you are with that sensor.

The first thing to remember is what is 'horizontal' orientation.
Most of the common devices will have some sort of label, and even all sorts of hints about what is "Top".

But that may not be the best orientation for this sensor.

You can 'stick' them to just about any surface, and hide them in bookshelves or even your bedroom drawers.

I have been very satisfied with the idea of orienting the sensor according to what I know is it's 'most discriminating' position, and what I intend to accomplish by installing this sensor.

The picture of the DM10 below, might not pass the spouse approval test, but I used it for years to determine whether I was moving in my bed. The DM10 is a little better at this because it only sends an ON command every 25 seconds or so, as long as motion continues. It never sends an OFF command.

This sensor could only see 'across' the head of the bed, and thus any movement would register an ON command to XTension from that address.

I used the 'time stamp' features of XTension to determine the reasonableness of ON commands from other sensors in the house. For example, if I get up in the middle of the night, and the Kitchen motion sensor turns on, then I could easily determine that this is me, having gotten up in the middle of the night, so do turn the light off when the 'bed' motion sensor again signals 'ON'... or maybe a few seconds after...

I particularly like to put Hawkeyes in doorway locations. I orient them so that they are rotated 90 degrees from their 'normal' viewing position.

I find that in this position, and pointed towards a neutral wall, they are very good indicators of 'someone walking thru this doorway'. When I tried this in the 'normal' position, it would register any one moving near the door, and that was bad because there is a clothes closet near that bedroom door...

Are there any Bad positions ?

You must consider the basic Sensor and the Lens whenever you position a new PIR sensor.

But remember that it is a simple device. It can be fooled.

The most basic mistake is that you position the sensor so that it can 'see' too much !

Surely, you might want a sensor that could detect every car that moved on your street, or every bird that moved in your yard.
But that's not normally very useful in a home automation system...

Another thing to be careful about: Most PIRs don't detect a body that is moving TOWARDS the sensor until the last few few...

Again... position PIR sensors so that the most likely motion in their field of view will be horizontal to the 'normal' position of the sensor unit.

But there are physical limitations to the PIR technology.

One thing that seems to befuddle PIR sensors, is a brick wall that is struck by the sun during some part of the day. The brick (cement, close neighbor etc.) wall, becomes hot, and even a human moving in front of it is not as 'noticable' as the background heat of the wall.

Another limitation of these devices is that depending on the 'apparent background', they may take 'some time' to detect motion.

This can occur at different times, with different sensors and locations. Different PIR sensor products may be quicker at deciding, but the common devices may sometimes take a couple of seconds to 'trigger' when you wave your arms...

Wireless ! the Hawkeye series and the DM10 send notification of motion and dusk by a Radio-Frequency wireless channel. Normally these signals are received by a plug-in device like the RR501 or TM751 RF to X-10 transceivers.

Don't point PIR sensors at large surfaces that can become 'hot' periodically.
Area sensors are more prone to false triggers. Try to rely most on 'local' sensors.

Don't make a big bother.

Another thing to remember is that you don't want to get excited every time motion is detected in the living room, if it's reasonable that the family should be there.

And don't create situations where the lights in a popularly occupied room go off at times that those rooms might reasonably be occupied.

The "Hawkeye" series of X-10 motion sensor insists on sending an ON command every 10 seconds, as long as it continues to sense motion.

This can be a real pain if you don't have the MR26 wireless receiver, but in any case, you will want to have a clause in your motion sensor ON scripts that ignore continuing ON commands.

Perhaps the next most common motion sensor is the DM10A which is also a PIR type, but is much larger than the Hawkeye, and not very attractive. It does however have larger batteries, and better range of 'field', as well as transmitter distance.

It never sends an OFF command, but does send an ON command every 25 seconds or so as long as motion continues.

And it definitely has a Dusk sensor that uses the 'base address' + 1.

Perhaps the oldest, and most loved/hated, is the PR511. It is basically a PIR/Dusk combination sensor that was really designed to sense driveway or walkway motion, and based on whether it was 'dark', can turn on up to 4 other X-10 addresses.

It is very nice to have this unit turn on it's lights directly sometimes.

It is cheaper to buy this device with the floodlights rather than without. (?!)

Light Beam type 'Motion' Sensors

Body Proximity Sensors

Magnetic Vehicle Sensors

Contact Switches

Commonly available PIR 'Motion' Sensors

Back to the Tutorials


Copyright 2007, Sand Hill Engineering Inc. All rights reserved.

Mac Made

Last modified: April 29, 2002
Michael Ferguson, webmaster@shed.com