I have seen a number of different circuits of seismic sensors on the net, however most of these have utilized a piezo transducer as the sensor element, God knows how a piezo would detect earthquake tremors.
It simply appears absurd because a piezo transducer would be solely able to sense high frequency vibrations and never a swaying action.
An earthquake will never generate a shrill noise rather it produces a gentle swaying pattern, when it hits.
Therefore using a piezo element is a flop idea, according to me.
Of course a piezo would detect tremors only if it were used in the form of a load-cell, by integrating some kind of load, assembled for implementing an oscillating action during tremors.
In the present earthquake sensor circuit, I have used water as the detecting agent.
After some experimentation I found that water is an excellent sensor of vibrations as well as a swaying kind of motions.
You can test waters behavior by keeping a bowl of water on the table and giving a gentle knock on the table.
Even the slightest of legit vibration is enough to create a nice ripples on the surface of the water.
I could have used an LED/LDR arrangement for detecting these ripples, however since we are not interested in sensing vibrations rather only swaying actions, I made a little out of the way approach.
Through a few of my previous posts I have already discussed water level sensor circuits where the water's conducting property is well exploited for the purpose.
The same property has been harnessed for getting the intended results.
Looking at the circuit diagram we see that the configuration in fact has nothing serious.
The electronic part consists of a couple of transistors rigged into a latch circuit.
The input to this circuit is achieved from a small half round container filled with water.
The positive supply from the circuit is dipped inside the water while the hot end of the input is placed in such a way that it stands just a mm above the water.
During a possible earthquake (God Forbid) the water responds to the tremor and starts generating a swaying movement.
The moment the water moves, its level gets disturbed and connects the hot end of the circuit with the positive terminal, dipped inside the water..
The positive of the supply immersed in the water instantly makes contact with the HOT end of the circuit via the water, the circuit gets triggered and immediately latches.
The connected buzzer sounds, sending an alarm signal.
The container can be made by cutting a small children plastic ball into half.
After doing the required settings inside this half cut ball, it may be filled with water as shown in the diagram and sealed.
The container then should be fixed somewhere such that the level off water inside the ball retains perfectly horizontal position without any deviations.