Detecting Motion
Circuit alarms are very effective for guarding the perimeter of a house, but they don't work so well inside a building.
This is because the intruder's actions are highly unpredictable -- you don't know where they'll go or what they'll touch. A specific "trigger" isn't very effective. To detect an intruder who's already in the house, you need a motion detector.
Basic motion detectors are fairly common these days. You see them all the time in automatic doors, for example. There are several different sorts of detectors.
An automatic door opener is an example of a radar-based motion detector. The box above the door sends out bursts of microwave radio energy (or ultrasonic sound waves), and then waits for the reflected energy to bounce back. If there is nobody in front of the door, the radio energy will bounce back in the same pattern.
But if somebody enters the area, the reflection pattern is disturbed. When this happens, the sensor sends a signal and the door opens. In a security system, the sensor sends an alarm signal when the reflection pattern in a room is disturbed.
The motion detector emits radio energy into a room and monitors the reflection pattern.
If somebody disturbs the reflection pattern, the motion detector sends an alarm signal to the control box.
Another simple design is a photo-sensor motion detector. These are the devices you might see in a store at a shopping mall. When somebody enters the store, the motion detector sounds a chime or bell. Photo-sensors have two components:
• A source of focused light (often a laser beam)
• A light sensor
In a home security system, you aim the beam at the light sensor, across a passageway in your house. When somebody walks between the light source and the sensor, the path of the beam is blocked briefly. The sensor registers a drop in light levels and sends a signal to the control box.
More advanced security systems include passive infrared (PIR) motion detectors. These sensors "see" the infrared energy emitted by an intruder's body heat.
When an intruder walks into the field of view of the detector, the sensor detects a sharp increase in infrared energy. Of course, there will always be gradual fluctuation of heat energy in an area, so PIR detectors are designed to trigger the alarm only when infrared energy levels change very rapidly. See this page for details.
All these motion detector designs can be combined in a house to offer complete coverage. In a typical security system, the control box will not sound the alarm immediately when the motion detectors are triggered. There is a short delay to give the homeowner time to enter a security code that turns the system off.
If the security code is not entered, however, the control box will activate various alarms. In the next section, we'll look at some of the alarm types you might find in a typical security system.
Sounding the Alarm
There are several things a security system might do when it detects an intruder. In an advanced system, the control box will be wired to several different components. Typically, it will activate:
• A siren or other loud alarm noise
• Flashing outdoor lights
• A telephone auto-dialer
The siren and lights serve three functions:
• They alert occupants and neighbors that someone has broken into the house.
• They drive the intruder away.
• They signal to police which house has been broken into.
The telephone auto-dialer can:
• Dial the police directly, and play a pre-recorded message giving the address of the house and any other relevant information. This message will usually play over and over so that the police will still hear it even if the call is put on hold for some time.
• Dial the security company (Argus) that installed the equipment. In this case, the control box can feed specific information about the intrusion -- which circuits or motion detectors were activated, etc. The security company then relays this information to the police.
Home security is a rapidly growing field, and there are new and improved burglar alarms popping up all the time.
For the most part, these systems are all built around the same basic structure. A central control box monitors several motion detectors and perimeter guards and sounds an alarm when any of them are triggered.
There are many different ways to create a motion sensor. Active sensors inject energy (light, microwaves or sound) into the environment in order to detect a change of some sort.
The "motion sensing" feature on most lights (and security systems) is a passive system that detects infrared energy. These sensors are therefore known as PIR (passive infrared) detectors or pyroelectric sensors. In order to make a sensor that can detect a human being, you need to make the sensor sensitive to the temperature of a human body.
Humans, having a skin temperature of about 93 degrees F, radiate infrared energy with a wavelength between 9 and 10 micrometers. Therefore, the sensors are typically sensitive in the range of 8 to 12 micrometers.
The devices themselves are simple electronic components not unlike a photosensor. The infrared light bumps electrons off a substrate, and these electrons can be detected and amplified into a signal.
You have probably noticed that your light is sensitive to motion, but not to a person who is standing still. That's because the electronics package attached to the sensor is looking for a fairly rapid change in the amount of infrared energy it is seeing. When a person walks by, the amount of infrared energy in the field of view changes rapidly and is easily detected. You do not want the sensor detecting slower changes, like the sidewalk cooling off at night.
Your motion sensing light has a wide field of view because of the lens covering the sensor. Infrared energy is a form of light, so you can focus and bend it with plastic lenses. But it's not like there is a 2-D array of sensors in there.
There is a single (or sometimes two) sensors inside looking for changes in infrared energy.
If you have a burglar alarm with motion sensors, you may have noticed that the motion sensors cannot "see" you when you are outside looking through a window. That is because glass is not very transparent to infrared energy. This, by the way, is the basis of a greenhouse. Light passes through the glass into the greenhouse and heats things up inside the greenhouse.
The glass is then opaque to the infrared energy these heated things are emitting, so the heat is trapped inside the greenhouse. It makes sense that a motion detector sensitive to infrared energy cannot see through glass windows.
Circuit alarms are very effective for guarding the perimeter of a house, but they don't work so well inside a building.
This is because the intruder's actions are highly unpredictable -- you don't know where they'll go or what they'll touch. A specific "trigger" isn't very effective. To detect an intruder who's already in the house, you need a motion detector.
Basic motion detectors are fairly common these days. You see them all the time in automatic doors, for example. There are several different sorts of detectors.
An automatic door opener is an example of a radar-based motion detector. The box above the door sends out bursts of microwave radio energy (or ultrasonic sound waves), and then waits for the reflected energy to bounce back. If there is nobody in front of the door, the radio energy will bounce back in the same pattern.
But if somebody enters the area, the reflection pattern is disturbed. When this happens, the sensor sends a signal and the door opens. In a security system, the sensor sends an alarm signal when the reflection pattern in a room is disturbed.
The motion detector emits radio energy into a room and monitors the reflection pattern.
If somebody disturbs the reflection pattern, the motion detector sends an alarm signal to the control box.
Another simple design is a photo-sensor motion detector. These are the devices you might see in a store at a shopping mall. When somebody enters the store, the motion detector sounds a chime or bell. Photo-sensors have two components:
• A source of focused light (often a laser beam)
• A light sensor
In a home security system, you aim the beam at the light sensor, across a passageway in your house. When somebody walks between the light source and the sensor, the path of the beam is blocked briefly. The sensor registers a drop in light levels and sends a signal to the control box.
More advanced security systems include passive infrared (PIR) motion detectors. These sensors "see" the infrared energy emitted by an intruder's body heat.
When an intruder walks into the field of view of the detector, the sensor detects a sharp increase in infrared energy. Of course, there will always be gradual fluctuation of heat energy in an area, so PIR detectors are designed to trigger the alarm only when infrared energy levels change very rapidly. See this page for details.
All these motion detector designs can be combined in a house to offer complete coverage. In a typical security system, the control box will not sound the alarm immediately when the motion detectors are triggered. There is a short delay to give the homeowner time to enter a security code that turns the system off.
If the security code is not entered, however, the control box will activate various alarms. In the next section, we'll look at some of the alarm types you might find in a typical security system.
Sounding the Alarm
There are several things a security system might do when it detects an intruder. In an advanced system, the control box will be wired to several different components. Typically, it will activate:
• A siren or other loud alarm noise
• Flashing outdoor lights
• A telephone auto-dialer
The siren and lights serve three functions:
• They alert occupants and neighbors that someone has broken into the house.
• They drive the intruder away.
• They signal to police which house has been broken into.
The telephone auto-dialer can:
• Dial the police directly, and play a pre-recorded message giving the address of the house and any other relevant information. This message will usually play over and over so that the police will still hear it even if the call is put on hold for some time.
• Dial the security company (Argus) that installed the equipment. In this case, the control box can feed specific information about the intrusion -- which circuits or motion detectors were activated, etc. The security company then relays this information to the police.
Home security is a rapidly growing field, and there are new and improved burglar alarms popping up all the time.
For the most part, these systems are all built around the same basic structure. A central control box monitors several motion detectors and perimeter guards and sounds an alarm when any of them are triggered.
There are many different ways to create a motion sensor. Active sensors inject energy (light, microwaves or sound) into the environment in order to detect a change of some sort.
The "motion sensing" feature on most lights (and security systems) is a passive system that detects infrared energy. These sensors are therefore known as PIR (passive infrared) detectors or pyroelectric sensors. In order to make a sensor that can detect a human being, you need to make the sensor sensitive to the temperature of a human body.
Humans, having a skin temperature of about 93 degrees F, radiate infrared energy with a wavelength between 9 and 10 micrometers. Therefore, the sensors are typically sensitive in the range of 8 to 12 micrometers.
The devices themselves are simple electronic components not unlike a photosensor. The infrared light bumps electrons off a substrate, and these electrons can be detected and amplified into a signal.
You have probably noticed that your light is sensitive to motion, but not to a person who is standing still. That's because the electronics package attached to the sensor is looking for a fairly rapid change in the amount of infrared energy it is seeing. When a person walks by, the amount of infrared energy in the field of view changes rapidly and is easily detected. You do not want the sensor detecting slower changes, like the sidewalk cooling off at night.
Your motion sensing light has a wide field of view because of the lens covering the sensor. Infrared energy is a form of light, so you can focus and bend it with plastic lenses. But it's not like there is a 2-D array of sensors in there.
There is a single (or sometimes two) sensors inside looking for changes in infrared energy.
If you have a burglar alarm with motion sensors, you may have noticed that the motion sensors cannot "see" you when you are outside looking through a window. That is because glass is not very transparent to infrared energy. This, by the way, is the basis of a greenhouse. Light passes through the glass into the greenhouse and heats things up inside the greenhouse.
The glass is then opaque to the infrared energy these heated things are emitting, so the heat is trapped inside the greenhouse. It makes sense that a motion detector sensitive to infrared energy cannot see through glass windows.