Shackleford-Gundersen Seismometer Construction Notes

Last Updated: 2/19/00

The Pendulum:

The length of the pendulum should be 10 to 12 inches (~25 to 30 cm) long. I use a piece of 1/16 inch thick aluminum channel material that is in the shape of a U (expect that the bottom is flat). The channel sides are 1/2 inch tall and 25/64 inch wide.

The following components are mounted on the pendulum:

Magnet: The magnet is used for damping the pendulum, and should be mounted somewhere between 1/2 and 2/3 of the way down the pendulum. Its location is not that critical.

Oscillator and Transmitting Antenna: I use a standard 2Mhz (or 2.4576Mhz) TTL oscillator in my SG sensors. J6, on the SG Electronics board, has two output signals to power the oscillator. Pin 3 is the +5 volts DC output, and pin 4 is ground. The +5 volts should go to pin 14 of the oscillator and the ground should go to pin 7 of the oscillator. If you ordered the oscillator from me I have soldered some bypass capacitors, a .1uf mono and a 2.2uf tantalum, across the power pins of the oscillator, and, I have marked the bottom of the oscillator can with a "+" indicating pin 14, and a "-" (or "G") indicating the ground pin 7. The power to the oscillator needs to go through the hinge that holds the pendulum. You can use some very small wires to pass the power to the pendulum holding the oscillator, or, use the power pass through hinge (see the photos of my SG sensors) that I use.

The oscillator should be mounted (I use super glue) near the bottom of the pendulum, just a little bit above the transmitting antenna. The transmitting antenna is mount at the end of the pendulum. You will need to solder some very small wires to the output of the oscillator, pin 8 (I have marked an "O" on the oscillators can that I supply), and the transmitting antenna. If you are using double sided PC board material for your transmitter antenna, you must solder wires to both sides of the board. I have been using receiving / transmitting antenna plates that are 1/1/2 inches (~40mm) square.

Pendulum Weight: I add some lead weight to the bottom of the pendulum. The total pendulum weight should be around 60 to 80 grams.

Damping Coil:

Location: The damping coil should be mounted on the frame and located so that the magnet, mounted on the pendulum, is sticking in about 1/2 the length of the coil. You need to make sure that the magnet is not touching the coil as you swing the pendulum back and forth a little.

Wiring and Polarity: Two pins, of the 4 pin connector J6 located on the SG Electronics board, supply the signal to the damping coil. Pin 1 of J6 should go to one of the coil wires and pin 2 should go to the other coil wire. You have a 50 / 50 chance of getting the polarity right. If the pendulum, during setup of your sensor, oscillates then you will need to reverse the wiring to the coil.

You may also find that the Damping Adj trim pot can not reduce the signal enough for proper dampening, or, may not be able to supply enough signal. The amount of signal need for proper damping depends on the weight of the pendulum, the strength of the magnet, and the number of turns of wire (I use 1000 turns of #34 magnet wire to make the coils I supply), used to make the coil. If the Damping Adj pot can not supply enough signal then you should reduce the weight of the pendulum or add more turns of wire to the coil. If the trim pot can not be turned down enough, then you should add a resistor in series (not across the coil) with one the of the wires (it doesn't matter which one) feeding the coil. A 4.7k 1/4 watt 5% resistor is a good starting point. The resistor should be located near the coil. For more information on damping please see this page

Receiving Antenna Plates:

Spacing: The receiver plates should be spaced about 3/16 of an inch (~5 mm) on each side of the transmitting plate. After hanging the pendulum you should level out the base plate and then secure the plates (I use super glue, but screws are probably better) to the base plate so that they are the proper distance from each side of the transmitter plate.

Receiver Plate Tuning: If you ordered a set of antennas from me then you will not have to tune the receiver plates to the oscillator frequency. I have already done this for you. If you are making your own, or want to test the ones I sent you, then please do the following:

Mount a transmitting plate the same size as the receiving plate on a fixture. Then connect the transmitting plate to the oscillator. Locate the receiving antenna plate about 3/16 on an inch (~5 mm) away from the transmitting plate. Using a DVM (Digital Volt Meter) measure the output voltage and tune the coil (or capacitor) for maximum output. Each antenna should produce over 2 volts, and produce about the same voltage, but one should produce a positive voltage and the other a negative voltage. Note:This procedure should be done with the receiving antennas disconnected from the SG Electronics board.

For more construction notes and setup information please see the original Scientific American article and the photos of my sensor here. I also have more construction notes and photos here.

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Larry Cochrane -