A geophone is a sensor that picks up small low frequency movements. This can be used by seismologists to study the structure of the earth or by ghost hunters to detect the footsteps of a poltergeist.
Here is a simple geophone display circuit...
The advantage this circuit has is that it's simple and cheap. It's widely used in inexpensive ghost hunting equipment and can be purchased from BGMicro Electronics The problem with this circuit it's low sensitivity; it only detects very heavy ghosts.
So we're going to have to amplify the signal, by a few thousand times, in order to detect Casper when he's tiptoeing. And here's a circuit that can do it...
This circuit can detect light footsteps from across the room on a concrete floor. There's an additional "zero" control that allows the device to be set so it lights up at the slightest activity.
The project has been tested with a couple of different geophone sensor models. A low cost geophone sensor is the GS30CT (SM-7 equivalent), available at BGMicro Electronics. A more sensitive geophone sensor is the SM-24, which is available at Sparkfun Electronics.
The circuit works by amplifying the geophone signal 140 to 400 times in the first stage (U1). The useful output frequency range from the geophone sensor is from 10 Hz to a few hundred Hz, so the first stage also filters out frequencies above a few hundred Hz. Then the signal is averaged and amplified further by U2. The resulting signal is fed to the display chip (U4) that drives the LEDs. Since this is a single-supply design, a voltage reference of 1/2 the supply voltage is needed and it is provided by U3.
This circuit also features automatic dimming of the LEDs based on the ambient light level. The LED brightness is set pretty high, so this device consumes a lot of power (10 mA idle to 200 mA maximum).
Because this is a high gain amplifier, construction technique is important. The connections to the geophone sensor and to the sensitivity control potentiometer should include a grounded shield. It also helps to have a ground plane or ground pour on the printed circuit board. The biggest source of noise is going to be 60 Hz and the harmonic at 120 Hz; since these frequencies are right in the middle of the frequencies we're interested in, they can't be filtered out. So the pickup of 60/120 Hz will be the limiting factor for how much gain can be used.
The power supply should be between 3.6 and 5.0 VDC. This could be accomplished using three alkaline, NiMh, or NiCd batteries or a regulated 5V wall power adapter. If the power supply voltage is too low, the LEDs won't light. If it's too high (> 5.5V), the ICs will be damaged.
Here is the finished project...
That's an acrylic box from The Container Store. This form of construction is good for entertainment purposes, but it will pick up low frequency sounds as well as ground movement. For a true seismic sensor you'll probably want to mount the sensor in some sort of spike that can be jammed into the ground.