What's Wrong with This?
Wally `Larman installed a burglar alarm system for a small jewelry store located in a local shopping mall. As requested, he designed a two-area system, one area for the premises and the second area for the safe, and installed the devices as shown in this diagram. When Wally was finished, he carefully tested and adjusted the system and found that everything worked as expected, but when the customer began using the alarm, frequent false alarms were caused by the front microwave detector. Wally readjusted the microwave and even replaced it, but the false alarms continued. Can you see what Wally did wrong and what he must do to correct the problem?
5-Minute Tech Quiz Answers
Here are the answers to What's Wrong with This? and the 5-Minute Tech Quiz that appears on Page 45.
- b — Due to the extreme conditions encountered when installing outdoor detectors, only those devices specifically designed for outdoor installation should be used.
- c
- d
- d — All of these conditions can affect the overall coverage of an ultrasonic detector. Temperature/humidity affect the sound waves’ ability to travel through the air, and furniture and wall/floor coverings can absorb or reflect the ultrasonic energy.
- a
- d
- a — If the receiver is installed so that other sources of light, such as sunlight or lighting fixtures, can be seen by the receiver, it is possible that it will not detect motion along its entire length. This is why you always should test photoelectric detectors at many points along their lengths.
- a — Microwaves can penetrate most common building materials except for metal, which will reflect the energy.
- b
- a
- c
- b —Infrared detectors detect a change in infrared energy. Ultrasonic and microwave detectors utilize the Doppler shift principle.
- b — Infrared detectors are passive, hence the name passive infrared detector, and as such do not transmit any energy.
- b — Contacts that open on alarm should be wired in series, and contacts that short on alarm should be wired in parallel.
- b — If each detector draws 35mA, then the total for 15 devices would be 15 x 35mA = 525mA, which can be represented as 0.525 amps.
Answer to: What's Wrong with This?
Wally’s problem is caused by where he located the front microwave detector. Because microwave detectors can penetrate materials such as glass easily, the false alarms are caused by people walking near the show window in the mall after the store is closed. Wally should relocate the microwave detector as shown in this diagram and re-test the system. As an alternative, he also could replace the microwave with a PIR, which will not detect motion outside the store. It is important when designing a system to fully understand the capabilities of the devices used and carefully look for potential sources of false alarms.
