Most earthquake detectors use a heavy weight attached to a coil moving inside a magnetic field. Earth movement will cause the ground and attached magnet to move. The suspended weight and coil tend to stay stationary and hence a signal is generated. Elaborate electronic filters and damping needs to be employed to get a meaningful signal.

My project avoids some of these problems by the use of ready made detectors with a low impedance and their own built in damping. Here is a block diagram of the system:

Below is the entire apparatus. A and B are two old and now dysfunctional hard drives. Removing the discs gives access to the pickup arm which in turn is driven by what is known as a 'voice coil'. This coil is suspended between two very powerful magnets. Energising the coil will react with the magnetic field and drive the pickup across the disc surface. Like all such arrangements it can be used in reverse, ie. if you move the pickup arm, a current is produced in the voice coil, which when amplified, can be interpreted by a computer program.

C is a very heavy weight made from roofing lead. This is suspended from the ceiling with a length of picture wire. D is an arrangement of activators attached to the weight. They are in close contact with the drive's pickup arm, which is pressed against the activators by the springy cable which connects the pickup assembly to the rest of the hard drive. One sensor will detect horizontal - the other lateral movement. When the ground moves, the suspended lead weight tends to remain stationary and so the two moving pickup arms will push against the activators, thereby generating a small current in the voice coils.

E and F are two lead weights that keep the apparatus steady. G is a switch arrangement allowing selection between the two coils. Either A or B, or A and B in series. The latter is the most effective configuration.

As far as I know no-one has ever used hard drives for this purpose and I like to think that the 'Lueg earth quake detector' is an original invention.

The arrangement is very effective because the two voice coils were originally designed to drive the pickup arms at high speed, which means that they have a very low resistance. This in turn means that they do not suffer from electro magnetic interference. I found no need to filter out mains hum even though four computers and a monitor were operating just a metre away.

This is a fairly standard design using two 741 operational amplifiers. The circuit is powered by a dual 15V power supply.

The incoming signal from L1 and L2 is connected via the selector switches and R1 or R2 to the inverting input of a 741 op-amp. This in conjunction with R3 gives a gain of either -1000 or -300, depending on which resistor you select with L1. The high gain is possible because we are dealing with very low frequencies. An earthquake typically has a frequency of 1Hz or lower. VR1 is a 10k trimmer connected to the offset null pins. This is necessary because the ac output of the circuit needs to be shifted positive to match the A to D converter. Adjusting VR1 shifts the output signal until it is in the centre of the trace display. The output of the all this is fed into a unity gain buffer which produces enough current to drive any 8 bit a/d converter. R5 and the LED give a power ON indication. The value of R5 needs to be adjusted to suit your power supply.

If you live in an area shaking with frequent earthquakes then you do not need this high sensitivity and you can choose lower values for R1 and/or R2 to reduce the gain. But remember - R4 should be close in value to R1 and R2. The gain of the circuit is -(R3/R1 or R2), so if you want say a gain of -100, you would make one of the resistors 10k in value.

Below is a suggested circuit layout. I can supply the draw file on request. This is only of use to you if you have a RiscPC.

If you etch your own circuit board, the ink-side of the negative should be on the copper side of the board.

The complete electronic setup is on the right. A is a ready built A to D converter module made by Unilab, together with B, a small cable adaptor. The converter board uses a ZN449 8-bit converter and is quite versatile - my life wouldn't be the same without it. Any other eight bit A/D converter will do just as well of course, as long as it is compatible with your computer. C is the amplifier described above. 

Visitors using a PCs  may be interested  in the Windows version of this project:

Just click here

I have written a rather nice little share ware program which records the output of the setup. !Quaker is fully multi-tasking and will record about five minute of activity. A trigger level can be set to initiate a five minute recording and successive graphs can be automatically saved to disc. If you use RiscOS 4 you can save several days' worth of recording into one directory. The front-end and five minutes' worth of signal is shown below.

The first two traces at the top are the result of me tapping against the ceiling and then closing the door of my play-room. The rest is background noise. It should be apparent that the setup is hyper-sensitive. A consequence of this is of course that should you ever happen to use the project during a real earthquake, you are likely to detect your desperate predicament mainly because of the violent bouncing of your monitor rather than by observing the off-the-scale waveform on its screen.

You can download the program from the share ware section of my site. Click on the link and scroll down to 'Interfacing'. As always, if you have any problems of a technical nature, please don't hesitate to contact me.