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The best i know is the one i used in my high school science fair project..
Is just a nail attached one end to metal spring ,as the one used in toggle pens.
then a metal 'O' ring ,close about 1 mm ,and you have a good and sensible
switch ...when it detects a minor movement or vibration nail touch the
ring and close the switch..
I remember an oscillator based on 555 Ic and an output to a buzzer.
( | ) < "O" ring or a little metal hook.______ sw pin 1
|
| < nail or metal wire (uncovered)
0
0 < spring
0__________________________________ sw out 2
Another type is a pendulum that should be two meters long with a heavy weight on the end. You make a capacitor from the metal pendulum and a metal plate very close to it. Use this capacitor in a wheatstone bridge circuit. Null the circuit when there is no earthquake. When the earth moves you will get a low frequency AC signal out.
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