How do home alarm system motion sensors workOne of the most important components of a home alarm system is the motion sensor or detector. Its purpose is to detect, sound the alarm and notify when there is someone in your home when they shouldn’t be there.

When it detects motion it sends a signal to the control panel to sound the alarm, send out a notification to those monitoring the system or take any another action that has been programmed – such as turn the lights on or start a camera recording. (For more information on this see my post on how alarm systems work). The system can be set to do this immediately or a delay can be added in to allow time for a pass code to be entered in to disarm the system to avoid false alarms being caused by people that are permitted in the home.

In home security systems the sensors are primarily used to cover important internal rooms and spaces after an intruder has entered the premises. Normally as minimum you’d cover at least 2 important ground floor rooms and the access way to your bedrooms.

Motion sensors can be used for other aspects of security too. Some of the common uses are for turning on outdoor lights when someone approaches your house and used in conjunction with a CCTV camera so that when motion is detected it starts to record what is happening.

There are different types of motion sensors used in alarm systems. The most common types are:

Passive Infra Red

This is the most common sensor used by home security systems. They are passive in that they do not emit waves. They detect changes in infra red energy given off humans and animal i.e. they see body heat
The field of vision horizontally for these sensors is normally between 90 degrees to 110 degrees. To avoid blind spots they are usually placed at about 7 feet high and in the corner. In this way they can cover all the room. Their range of vision is typically 30 – 40 feet.

The field of vision horizontally for these sensors is normally between 90 degrees to 110 degrees. To avoid blind spots they are usually placed at about 7 feet high and in the corner. In this way they can cover all the room. Their range of vision is typically 30 – 40 feet.

Some of these sensors are pet immune. This means they are set to ignore the animals that weigh under a certain weight normally somewhere between 50 – 80 lbs depending on the system. This makes it possible to set your system so all sensors are armed (normally known as away mode) when you are away from home and the pets shouldn’t trigger false alarms. The alternative is to leave the alarm off or set it in home mode (only entry sensors armed and motion sensors unarmed) meaning your home is not fully covered.

As they detect changes in body heat they become less effective as the air temperature gets closer to body skin temperature of 93 Fahrenheit. They also have some other issues – when someone is walking directly towards them they have difficulty in detecting changes in temperature, if they are pointed directly at a window flashes of light can be seen as movement and a heating vent system can cause a false alarm with too much hot air being blown into a room.

Other Sensors


Computer Vision
. This is where the sensor compares digital images from frame to frame using an algorithm to determine whether there has been movement. It is currently in the early stages of being used by alarm systems. Computer vision is the technology that Canary Home Security System is using to trigger the camera to start recording when motion is detected. I’m sure this technology will be used more in the future in particular with digital CCTV and IP cameras. It currently works up to 20 feet away.

Microwave or UltraSound. The sensor emits microwaves or sound (ultrasound) and that listens for changes in the signal bouncing back, as an object moves it will change the frequency of the wave coming back. These are often used with passive infra red sensors to confirm that movement is real. They have lot more range then PIR but will detect movement through walls. They are expensive and not used that often in home alarm systems.

Photosensor. This is a beam of light that goes from a light emitter to a light sensor. When the beam is interrupted by someone or something crossing its path it detects movement. These are typically used at doorways or in areas where someone need to walk though. They aren’t a common in home security sensor.

Entry Sensors. These detect the opening of a door or window or someone coming through an entry point. See my post here on more detail on these special type of sensors.

Used In Home

There are 2 main armed home alarm modes used in home security systems. These are away mode and home mode.
In away mode all the sensors are typically armed. If the motion sensors detect movement it sends that information to the control panel and the siren is sounded and notifications sent out.

When in home mode the system does not respond to motion being detected. It is ignores this. This is so when you are home you can continue to move around your home without setting the siren off every time you move pass the motion detector.

Some systems do come with the ability to change these settings by zone and sensor. A zone is the settings for one or for a group of sensors telling them how to respond in certain situations. This means you could have it set so that a sensor in a particular room senses movement but you just get a notification to your phone and the alarm doesn’t sound. However in a room you aren’t going to use it could be set to sound the alarm as well as send out a notification such as you might use with your garage to protect your cars at night.

Installation of these sensors is straightforward these days as they are normally wireless in their communication with the control panel. You can stick them to the walls in the best place using double-back tape or screw them in place. They are battery powered and in the better systems you are informed when the batteries need replacing.
In setting up the sensors you want to put them in areas where a burglar is likely to pass through and protect valuables. The most common points of entry are the front door, back door, sliding door or garage door so it is important to cover their path from these point to have the best chance of detection. Criminals also make their way to the master bedroom as that is where most people store their most valuable items, so an area leading up to that point is important to cover also.

Motion sensors can play a number of important roles in home security and it is important they are maintained and placed in the best positions to detect movement. Maintenance includes replacing batteries and ensuring they are kept clean from dirt and dust.

As well as detecting movement in the home and triggering the alarm they can be used to turn lights on and off in the home as you move through the house to save on cost, used in conjunction with cameras so they only record when movement is detected, let you know when someone is approaching your front door and also turn on the lights outside.

Filed under: Buying Advice

Like this post? Subscribe to my RSS feed and get loads more!