Geophone Sensors - A How To...

"Geophone" Vibration Sensor A "How To..." by Chuck Thurston

What is a Geophone?

Well, its an electronic sensor designed to pick up seismic or low frequency vibrations. It consists of a wire coil on a spring suspended weight, inside a round permanent magnet. The small movements in the coil, due to vibration, produce a signal to a circuit that simply lights an LED display based on the signal's intensity.

Here's where to order the basic kit for one of these:

http://www.bgmicro.com/GeophoneKit.aspx

The Geophone Kit from BGmicro sells for $39.95. All you have to do is connect a 9 volt battery and the sensor.

There's only a couple of problems with this design. First, the LEDs are mounted into the board face up and with short leads. So it's nearly impossible to mount the board so the LEDs go through a plastic project box. If you don't feel like replacing them with your own LEDs, with longer leads, than just mount the whole thing in a clear plastic box and you're good to go. The other issue is the 50kohm potentiometer isn't going to be accessible from the outside a project box without a well placed hole in the box and a very small screwdriver. You'll have to tune the circuit to a setting that seems to work for all situations and leave it that way, which also isn't going to be the most sensitive setting for all situations and thermal changes. My solution was to replace it with a larger potentiometer mounted on the box itself with a nice knob for tuning.

On the circuit board itself, you'll also want to use the provided jumper to select the LED display for "bar" mode instead of "dot" mode. The "bar" function is a lot easier to see changes in the dark and from a distance. This just means making sure the provided jumper in in place.

	Here's what I used to make this device. The kit from BGmicro, more red LEDs, a 10 Kohm potentiometer with an on/off switch function, a control knob and a plastic project box. Total cost was about $55.
	Ok, first we have to deal with the circuit board. I hate the look of this thing just being mounted in a clear plastic box. It's ugly. And I want mine to be easily tuned to allow for the varied conditions it might be used. So I unsolder the LEDs and the existing potentiometer from the board.
	That's the LEDs gone.
	And the potentiometer gone.
	Then I solder ten new LEDs into place, being sure to watch the orientation. It's pretty easy. Each LED has a flat spot on the plastic head and just match it up with the direction of the flat spot shown on the printed circuit board. All the flat spots go towards the IC chip, incase the boards printing isn't so good.
	You want the height of the LEDs leads to place the base right around the height of the blue wire connectors. This will let the LEDs extend through the plastic project box's top.
	And add three wires, which will connect to the new potentiometer.
	OK. Now would be a really good time to connect a battery and the sensor itself to try the thing out. If something doesn't work, or you have the LEDs in backwards, you want to know before you assemble the thing further.
	Use some fast setting two part epoxy to glue the sensor in place. A good solid hold on the bottom of the box is needed or the gaps will dampen the sensor's response to shock. Another thing to point out here is the sensor is directional, so you want it placed with the contacts to the sensor upwards when the sensor will be in use. Carefully measure and drill all the holes for the ten LEDs and the potentiometer in the top of the box.
	Place the LEDs through the box cover.
	And hot glue the thing in place.
	Hot glue all the other wires into place. This will help the thing stay together when it's opened at some point to replace the battery.

	Connect up the sensor. Be careful to make sure you get the polarity right from the board to the sensor, + to + and - to -.
	Add some foam filling just to keep the battery from moving around.
	And assemble the box.
	That's it. DONE! Once the sensor is turned on, you adjust the potentiometer so the LEDs just start to turn on under no vibration. Then back it off just to the point the last LED goes out. That's the most sensitive setting and it's ready.

If you are able to read electronic schematics, build things from them, and want to build a much more versatile Geophone, here's a version I've made from scratch that uses the same geophone sensor, from BGmicro. This version, however, uses jumbo blue (and brighter) LEDs and has the geophone sensor on an eight foot cable. The benefit of this is the sensor can be placed on a stairwell, chair, table, or even embedded inside a doll or toy as a trigger object. The display can be placed where it is in view of a monitoring camera. Here's what this one looks like:

And here's the schematic