If you live in an earthquake zone, this vibration sensor can save your property or your life.
not, it can be used as a bur-
or many of us that live in a seismically active region, an earthquake is a very real and constant threat. Even moderate earthquakes that are not strong enough to hurt anyone can cause a significant amount of damage to the contents of your house if you are not prepared. Although earthquake prediction has made great strides over the years, it is still very much an art form rather than a science. Earthquakes can strike without warning, and when they do, you do not have time to lock the kitchen cabinets or turn on any emergency lighting, much less find suitable shelter. If those steps can be done automatically at the first sign of an earthquake, property destruction and personal injury can be avoided or minimized. The Earthquake Detector de-
glar alarm, intrusion detector, o r a game of skill.
ROBERT BULLOCK scribed here is the heart of such an automatic system. The author uses this device to automatically latch all of the kitchen cabinets when an earthquake strikes. The circuit can also be used as a burglar or impact alarm when mounted in a car. When mounted to a garbage can, It can sound a loud alarm if an animal tries to get into the container, A self-contained alarm unit can also be used as a simple game to see how far and how fast someone can carry the device before it triggers. How It Works. A seismograph, which most people are aware of, is a device that measures and records the amount of seismic activity on a strip or drum of paper using vibrating pens, The sensors for that device usually consist of some type of suspended weight with a way to measure how much it moves when the sensor is shaken. The same method is used for the Earthquake Detector. A weight, in the form of a screw, is suspended on the end of a spring. The head of the screw passes through the center of a metal ring. When the sensor starts to shake, the spring and screw start to swing back and forth like a pendulum. As long as the shaking continues, the swing of the screw will keep increasing as the spring keeps absorbing mechanical energy. At some point the screw head will touch the metal ring, completing a circuit between the spring and the ring. Since the actual contact time of the vibration sensor is very short, a retriggerable timer circuit is used to create a usable control signal. As long as the vibration sensor keeps
opening and closing its contact, the timer will keep resetting itself to the beginning of its timing cycle, and the output of the timer will remain active. Once the vibrations stop and the energy in the spring is dissipated, the timer will remain active until the end of its timing cycle; only then will it turn the output off. If the sensor is damaged and is closed continuously, the timer circuit simply times out, turning the output off. The output of the timer circuit controls a relay, That lets the unit be very versatile when controlling other devices. The switched load is limited only by the rating of the contacts in the relay itself. When the circuit is activated, a flip-flop turns on an LED, which will remain on after the circuit times out. Only pressing a reset switch will turn the LED off. That will show if the circuit was set off at any time since the reset button was last pressed. The earthquake sensor uses all CMOS ICs, giving the unit a very low current draw when the detector is not triggered, To be more specific, the standby current load Is less than one microamp, rising to 40 mA while the relay is energized and dropping to 5 mA when just the LED is on Circuit Description. The schematlc diagram for the Earthquake Detector is shown in Fig. 1. When power is first applled through J1, Cl is charged through R1. During constant shaking, S3 (the vibration sensor) closes, letting Cl dump its charge to the timer circuit. As S3
opens, Cl is quickly...