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Last Updated on: Tuesday, June 01, 2021 03:06 PM

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More  Alarm Circuits

Earthquake Alarm Circuit
designed by David Johnson, P.E.

Geophones are dandy motion sensors.  They are metal cans containing a powerful magnet, suspended with springs inside a coil of wire.  The magnet functions as a floating mass.  The slightest vertical motion of the device causes the coil to move relative to the magnet. 

This induces a small voltage in the wire coil.  These devices have been used for decades by oil and gas companies to conduct surveys of underground rock and strata, using induced seismic shocks. Although millions of them have been manufactured over the years they are often hard to find.  You can make your own geophone by suspending a strong magnet with a rubber band, so it hangs inside a coil of wire containing several hundred turns. Any vertical movement will produce a detectable voltage in the coil.  To reduce false signals produced by air movements, I suggest that the assembly be housed inside a pipe or some other container. 

The hobby circuit below shows how this do-it-yourself geophone can be used as an early warning earthquake alarm.  The electronic circuit uses a low power dual op Amp from National Semiconductor.  The weak voltage from the coil is first Amplified by the left side circuit.  That signal is fed to voltage comparator.  The output of the comparator is routed through a filter network then to a n-channel FET switch.  The switch can be used directly to turn on a beeper or it could be connected to a computer system.  I included a LED light as a visual indicator.  The LED will turn on whenever the circuit detects motion.  With the components shown, the overall sensitivity is less than one millivolt peak to peak at the coil.  By decreasing the value of R5, you can decrease the sensitivity so any desired level.


Commercial Geophone

Click on Circuit Below to view PDF of Schematic

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