Alarm sensors - which do you need?
07-29-2004, 10:14 AM
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Alarm sensors - which do you need?
(Another Car Audio article...)
Looking at all the different sensors available for alarms today can be overwhelming. Proximity, impact, “glass”, motion, shock, “radar”…what do they do, and what should you expect from them? Here’s the scoop:
Impact sensors (also called shock sensors): These sensors are intended to detect damage to the vehicle’s sheet metal, primarily in the door lock cylinder area. In this case, damage means “what happens when you punch the locks out with a hammer and chisel”. Better sensors will also have a shot at detecting damage to the trunk lock, but trunks are so well isolated in some newer cars it’s not always a sure thing (the best way to cover your trunk is an impact sensor and a trunk pin switch). If someone tells you that your impact sensor is also going to detect breaking glass, take it with a grain of salt – breaking tempered glass creates a completely different vibration signature than striking sheet metal, and if your sensor detects both, I’d expect too many false alarms.
Many of these sensors operate on some sort of sympathetic resonance principle – while the exact technology differs from design to design (well, now it does, after some nasty patent litigation), the basic idea is the same.
How I test them: With the door closed, the windows up, and the alarm armed, I hit the door lock cylinder firmly with the side of my closed fist. If my hand hurts and the alarm does not react, the setting should go up. If barely touching the door gets me a full-blast alarm, the setting should drop. The sensitivity should be even from the left side of the car to the right side (reaching this goal may require sensor relocation).
Glass breakage sensors (also called audio sensors): This is a microphone with a basic analyzing circuit attached. It has only one job – reacting to the sounds of metal on glass and cracking glass. Any damage to sheet metal…well, that’s not glass, now, is it? Also, glass sensors hate open windows and sunroofs – if you leave the windows down, expect falsing. Back in the day, this sensor was the real deal (shock sensors of this period were basically a drywall screw and a ballpoint-pen spring, so glass microphones were “high-tech”). Alpine even had an alarm (the venerable 8122) with one microphone for each front door – the installer had to pull the door panels and install them inside! To be fair, those mikes were also intended to “hear” slim-jims before the door was opened. I still love glass mikes, as I believe they allow an installer to lower impact-sensor settings to a reasonable range, thus avoiding many false alarms.
How I test them: Use a set of keys – one key rapped against the front window should not set off the alarm, but three all at one time should.
Proximity sensors (also called “radar”and microwave sensors): These use a low-power microwave field to detect moving mass. The mass forces the field generator to work harder, as it were – and this shift in field generating difficulty is sensed. The bigger the mass, the farther away it is detected (hence, UPS trucks are detected from farther away than, say, your head). These are great for convertibles and Jeeps, and can play an important role in any full-fledged security system. If you choose a microwave sensor, make sure your alarm system has some sort of pre-warning feature – nobody needs to hear sixty seconds of full siren just because they walked past your car!
The early sensors could only elicit one reaction: full-blast siren. Most newer systems allow for multiple levels of reaction. Personally, my advice is that if your system will not support more than one level of response, you should not add a proximity sensor to it! Instead, buy a new alarm, preferably one which is designed to accommodate these sensors.
A few things to remember about microwave sensors:
· Many brands of window tint have metal flakes which effectively stop a microwave field. If you have tinted windows, keep this in mind.
· These sensors can be affected by radio installations at airports, transmission towers, some radar detectors, and microwave ovens.
· Since the field size can vary from summer weather to winter with some designs, check your sensitivity regularly.
How I test them: Note that while an ideal installation has even coverage on both sides, with some cars you just can’t always get what you want. Proximity sensors are very installation-intensive: if you want them to work just so, expect to pay for some additional installation test and trial time.
With all the windows rolled up, placing my face up against the front window, as if I were checking out your in-dash, should trigger the system’s warning response. I should not be able to trigger the full alarm response with the windows up –if I can, I would turn it down.
With one window rolled down, reaching into the interior far enough to get one shoulder inside should give me full siren response – this test should be tried on both sides.
Looking at all the different sensors available for alarms today can be overwhelming. Proximity, impact, “glass”, motion, shock, “radar”…what do they do, and what should you expect from them? Here’s the scoop:
Impact sensors (also called shock sensors): These sensors are intended to detect damage to the vehicle’s sheet metal, primarily in the door lock cylinder area. In this case, damage means “what happens when you punch the locks out with a hammer and chisel”. Better sensors will also have a shot at detecting damage to the trunk lock, but trunks are so well isolated in some newer cars it’s not always a sure thing (the best way to cover your trunk is an impact sensor and a trunk pin switch). If someone tells you that your impact sensor is also going to detect breaking glass, take it with a grain of salt – breaking tempered glass creates a completely different vibration signature than striking sheet metal, and if your sensor detects both, I’d expect too many false alarms.
Many of these sensors operate on some sort of sympathetic resonance principle – while the exact technology differs from design to design (well, now it does, after some nasty patent litigation), the basic idea is the same.
How I test them: With the door closed, the windows up, and the alarm armed, I hit the door lock cylinder firmly with the side of my closed fist. If my hand hurts and the alarm does not react, the setting should go up. If barely touching the door gets me a full-blast alarm, the setting should drop. The sensitivity should be even from the left side of the car to the right side (reaching this goal may require sensor relocation).
Glass breakage sensors (also called audio sensors): This is a microphone with a basic analyzing circuit attached. It has only one job – reacting to the sounds of metal on glass and cracking glass. Any damage to sheet metal…well, that’s not glass, now, is it? Also, glass sensors hate open windows and sunroofs – if you leave the windows down, expect falsing. Back in the day, this sensor was the real deal (shock sensors of this period were basically a drywall screw and a ballpoint-pen spring, so glass microphones were “high-tech”). Alpine even had an alarm (the venerable 8122) with one microphone for each front door – the installer had to pull the door panels and install them inside! To be fair, those mikes were also intended to “hear” slim-jims before the door was opened. I still love glass mikes, as I believe they allow an installer to lower impact-sensor settings to a reasonable range, thus avoiding many false alarms.
How I test them: Use a set of keys – one key rapped against the front window should not set off the alarm, but three all at one time should.
Proximity sensors (also called “radar”and microwave sensors): These use a low-power microwave field to detect moving mass. The mass forces the field generator to work harder, as it were – and this shift in field generating difficulty is sensed. The bigger the mass, the farther away it is detected (hence, UPS trucks are detected from farther away than, say, your head). These are great for convertibles and Jeeps, and can play an important role in any full-fledged security system. If you choose a microwave sensor, make sure your alarm system has some sort of pre-warning feature – nobody needs to hear sixty seconds of full siren just because they walked past your car!
The early sensors could only elicit one reaction: full-blast siren. Most newer systems allow for multiple levels of reaction. Personally, my advice is that if your system will not support more than one level of response, you should not add a proximity sensor to it! Instead, buy a new alarm, preferably one which is designed to accommodate these sensors.
A few things to remember about microwave sensors:
· Many brands of window tint have metal flakes which effectively stop a microwave field. If you have tinted windows, keep this in mind.
· These sensors can be affected by radio installations at airports, transmission towers, some radar detectors, and microwave ovens.
· Since the field size can vary from summer weather to winter with some designs, check your sensitivity regularly.
How I test them: Note that while an ideal installation has even coverage on both sides, with some cars you just can’t always get what you want. Proximity sensors are very installation-intensive: if you want them to work just so, expect to pay for some additional installation test and trial time.
With all the windows rolled up, placing my face up against the front window, as if I were checking out your in-dash, should trigger the system’s warning response. I should not be able to trigger the full alarm response with the windows up –if I can, I would turn it down.
With one window rolled down, reaching into the interior far enough to get one shoulder inside should give me full siren response – this test should be tried on both sides.
07-29-2004, 10:16 AM
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Something not mentioned here is that the more sensors you have, the lower each one should be set, making false alarms LESS likely, rather than more...
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