My daughter, Chelsea needed to come up with a project for her fifth grade science fair. In typical "Dad wishes he were still a kid" fashion I suggested she build a seismograph on the hope that she would develop a continuing interest which survived the project. (Too much TV) Since she has always had great interest in the earth sciences, the suggestion was well received. When I was about her age I built a wooden "garden gate" type seismometer from junk which actually worked (many of my projects at that period were well constructed but did NOT work that came later). Unfortunately that project came to an abrupt end under the sledge hammer when my father (who should have been more encouraging since he was a Bell Labs engineer), discovered I had sounded the basement slab to find the contact point with bedrock and sawed a five by five slab to "isolate" my instruments. The basement had a one-foot split-level because of bedrock encountered during excavation and blasting was too expensive. But I digress.
Hoping to neutralize the long term effects this childhood trauma had on me, I decided to be supportive to the extreme with my daughter, and set out immediately to bone up on what has happened over the last thirty years (not that much). I landed, almost immediately, at your web site and related links, which have been most informative. After taking inventory of both my own and the local junk-yard(s) I came up with a design which was too sophisticated for children to construct without a higher than normal amount of parental participation. Chelsea felt that if it was worth doing, it was worth doing right. She did not want to go to all the trouble and NOT catch an earthquake, and was willing to limit her participation to assembly of parts fabricated by me, and not using any of the "loud" tools. The teacher had no problem since she was thrilled that ANY father would take the time to help out and encourage such an ambitious project. (She also is grateful for the classroom use she enjoys of the very nice barometer we built last year but she ain’t getting the "seismo" as my daughter has named "her")
In an effort to keep the costs from spiraling out of control (as usual) I made a special effort to build the system from "inventory". The choice of coil/magnet I found to be an interesting sub-project and came up with an approach which others certainly must have used, but not to my knowledge. I feel there are two primary problems with the usual horseshoe/relay coil amateur design:
I felt that a lower impedance, higher quality and larger diameter coil was worth trying, and chose a GE current meter coil from the junkyard which had a 2 inch ID and was only a couple hundred turns of 22 gauge and about 10 ohms. But a peek at the windings showed them to be of high quality with "padding" to achieve excellent uniformity.
I also believe that the "solenoid" type detector arrangement is superior not only because the circular field interaction is additive, but also because it virtually cancels out interference caused by vertical or twisting moments on a cable suspended boom such as in a Lehman.
I just happened to have a speaker from my boat hailer that was "old" and needed to be replaced with a new model (later this spring) and could be "sacrificed" to scientific experimentation. After unscrewing the horn from the transducer, I removed the cover plate and voice coil assemblies. The magnet assembly is the usual "wrap around case" design which results in a circular gap for the
voice coil with extremely high field strength in the gap between the inner pole plate and the outer pole ring. There was virtually no fringe field, but once my "test" block came in contact with the center pole it was almost impossible to remove. The magnet weighs five pounds and the gap is about 1.5 inches in diameter.
I spun down an old chunk of iron rod to the same diameter as the inner pole plate and adjusted the length (thus weight) down to the point where it would balance the whole assembly at the face of the outer ring of the magnet when placed on edge. A bead of epoxy is all that is required to keep the iron bar from slipping once you have it perfectly centered in the pole plate. The epoxy also seals the gap from foreign bodies (and domestic, for that matter). The outer pole ring conveniently has a ring of 8-32 pem-nut fasteners included at no extra charge, so a simple aluminum plate can be fashioned to attach the whole mess to the end of your boom. No additional mass is necessary as the entire assembly weighs in at about 8 pounds.
The output of this design is excellent and more than ample to use resistive damping and still only run the pre-amplifier gain at around 200, meaning that the plain old TLO-82 runs very stable and very much "on center". In fact I just use a dual TLO-82 for pre-amp and 1HZ filter in the little box you can see in the photo. The damping resistor is a whopping 33 ohms. One interesting note; while playing around with the gain and matching circuitry of the pre-amp, I found I could use some of the amplifier feedback current available (due to the lower gain and impedance of the design) to actually induce feedback current into the coil and extend the period by a factor of two.
The velocity/output is virtually flat over excursions of +/- .75 inch. While I am optimizing for tele-seismic events here in Connecticut, this design might be useful for you nuts who live on the edge and want earthquakes that you can "feel" to record faithfully without hitting the stops.
You will also note a slight modification the Lehman design in that I use a small c-clamp to hold a needle-point for the top pivot. The c-clamp must be large enough to "wrap around" your top pipe support. Simply remove the c-clamp screw and replace it with a pointed set screw (see below), then drill a hole through the foot of the clamp for your support wire. I used stainless trolling line (again from the boat) which is strong and cheap.
Both pivots are made from a set-screw and drill stock. I annealed both, then the set-screws were ground to a point in the lathe and the "female" piece of ¼ inch drill stock is "dimpled" in the lathe with a round headed mill. I then case-hardened them by heating cherry red and dropping into oil.
The top hinge has a hole drilled in the pipe at the proper angle (you can adjust with a pipe-wrench) at the back side of the pipe and a "seat" for the drill stock drilled by way of that first hole into the inner wall of the front of the pipe. The boom is aluminum tube with a plug in the end (actually it is a piece of a Signal Corps 6 meter yagi antenna element) and the usual 24" in length.
The plug is drilled and threaded in the lathe to accept the ¼ - 20 set-screw. The lower hinge cross-member is a piece of tool steel with a "seat" for the small drill stock "female".
The natural period is easily adjusted to 40 seconds, but until I add some supports to the vertical stand I am holding it down to 20 while the particle board base groans. With the pre-amp energized the period is bumped back up to forty seconds. The whole thing results in an attractive, elegant and high performance design suitable for the average ten-year-old.
For choice of recorder I was forced to go a bit non-linear. There were some old E/A recorders at the junk-yard which brought back fond (and painful) memories, but they were gold plated and I felt too delicate and fussy for children. I also have plenty of old PCs and a Metrabyte DAS-8, but I thought the PC route would not be appropriate for kids since most of the literature they read about quakes shows antique Milne recorders, drum recorders and other "strip" type recordings.
I wanted something where she could just check before she went to school and tear off anything she wanted to show in class that was cheap and easy for HER to maintain. Naturally, such a device does not exist so I had to make it.
Since I had a broken old Code-A-Phone answering machine, a timer motor from a clothes dryer, some fishing line, a couple of 7" x 19" rack mount blanks and assorted metal stock, a handful of electronics, a machine shop in my garage, and a fine-point roller pen I thought, "Why not make a strip chart recorder that uses disposable pens and adding machine paper?". A 128-foot roll of this paper costs a dollar when you buy packs of ten, and you can re-wind them and use both sides if you are really (unbelievably) cheap. The modern roller pens give great performance at feather-weight and slow speeds and there is no mess. Most importantly, the "old fashioned" strip requires routine observation and time track maintenance which requires Chelsea to take measurements and calculate arrival times without the "instant gratification" of a computer telling her "no events".
I will take some Macro shots of the recorder later and submit construction details if anybody is crazy enough to want to follow this path, but a short description might be interesting. The recorder has three basic assemblies; the base plate (no description required other than it is one of the rack blanks with mounting holes for the other assemblies and stick-on rubber feet)), the transport frame and the pen-servo.
The transport frame encloses the roll of three-inch wide adding machine paper between the side plates, and the roll is replaced by lifting off the top plate that supports the paper under the pen. The roll simply sits underneath and rolls against a bulkhead as paper is drawn from the bottom of it, with additional drag being induced by dropping in a 3" x ½" steel roller which nestles against the roll as the paper is drawn up to the rear "turn-around" roller under the pen. From there the paper is drawn forward over the top access plate, around the front drive roller and under the pressure roller and on down into a box on the floor.
I made the side plates by dividing the remaining rack blank into two 3.5" x 19" sheets with a jig saw, bolting them together and squaring them up with the milling machine. Then, while still in the Bridgeport, I cut their length down to 12", drilled all of the bulkhead mounting holes, and drilled and reamed the bearing holes for the rear idler and front drive rollers. It is absolutely essential this assembly be square and rigid, or you will have problems with "creep" in the roll transport. The three bulkhead pieces (front, back and middle) are 2" x ¼" aluminum flat cut to the roll width plus .1 inch in length. Cash register and adding machine paper comes in a variety of sizes, so I chose between sizes that a Staples or Office Max would always have in-stock, which is 3 inch or 2¼ inch. I chose three inch, but you can do any you want and just change the bulkhead and roller widths.
The idler and pressure rollers were black anodized aluminum rollers from the junk-yard (probably from a copier or film processor) of about .6 inch diameter. I cut them down to the proper length and inserted my favorite ¼" drill stock stubs (polished) in each end for bearing in the side plates. The front pressure roller actually is supported by slots in the front edge of the side plates and held against the drive roller by a pair of retractable spring activated arms. When you pull the arms back, the pressure roller drops out of the top slots and down into a shallower set of slots which hold it away from the drive roller for threading. There is some distortion in these paper rolls probably due to fluctuations in tension and humidity during manufacture as well as if an event causes a lot of ink to be deposited. I found that by cutting six grooves across the pressure roller and installing small o-rings to actually make contact with the paper and drive roller, tracking problems with distorted paper were eliminated.
I chose a recording speed of 8 inches/hour, which gives good enough resolution for my teleseismic events while allowing a single roll to last 8 days (in case you forget the weekly change).
The timer motor turns the drive roller at 2 RPH. You can adjust the diameter of the drive roller to get the speed you want, but I chose 1.27323954473516268615107010698011 inches which gives exactly four inches of paper transport per revolution at mean sea level under standard atmospheric conditions and zero gravity assuming the power grid is still on frequency.
A simple dimple (?) on the edge of the drive roller activates a roller arm micro-switch every revolution which gives a time mark (with +/- 1 second repeat accuracy) every half hour of about 1.5 minutes duration which actually allows for excellent time calculations even for an adult. The top of the recorder supports the past hour of strip before being swallowed by the drive rollers, allowing Chelsea to stop by periodically and note the time and date of the most recent time marker on the strip with a pencil. She learned very quickly that taking a minute several times a day would save tedious marker counting later if an event arrived.
The pen servo is mounted behind the transport assembly and houses all the filter/amplifier/driver circuitry as well as the drive motor and feedback potentiometer and additional filter circuitry for the preamplifier power feed. +/-15 volt power for all circuits is provided by an external "Power One" type supply, although any old PC power supply or switching supply would work just fine.
The signal from the preamp comes in to another two-pole 1HZ low-pass filter followed by an adjustable gain amplifier with gain adjustable between ten and forty. From there it passes though a pair of 47MFD caps in non-polarized series (to eliminate any slight offset changes due to gain adjustment) to a summing amplifier where pen offset, time markers and whatever else can be added. The output from the summing amplifier drives the pen amplifier which sums an RC feedback loop (for response tailoring) with output from a 20K voltage divider pot attached to the shaft of the pen arm drive wheel. The pen amplifier drives a complementary pair of NPN/PNP transistors in the usual push-pull configuration which source or sink current through the drive motor to ground.
The drive motor is a nice smooth little multi-pole Sanyo motor from the deceased answering machine, and it drives (via an 0-ring belt) the same capstan flywheel and shaft it used to since those organs were ripped from the same carcass as well. I spun a little V-groove pulley to go on the capstan shaft next to the flywheel. This pulley drives the 4" pen arm drive wheel via fishing line, once again showing that once you have spent $125,000 on your Tiara fishing boat you should be able to build a relatively inexpensive seismograph. The pen arm drive wheel was made by cutting a disk from 1/8 inch aluminum sheet with a hole saw, pressing in a hub with ¼" shaft hole and set-screw, and spinning a v-groove on the edge with the lathe.
The pen arm is driven by a vertical pin attached to the wheel (at .75" radius) which passes through a slot milled in the pen arm shaft. There are two locations from which you can drive the arm with the pin, the outer location (pen side) gives the closest to linear response, while the inner location gives enhanced small signal response with geometric roll-off on higher amplitudes.
I have not yet decided which position I, excuse me, Chelsea is going to like better. "She" is going with the linear model for now.
The pen arm is rather simple. In addition to the aforementioned groove, it pivots vertically in a fulcrum block at the back end, which rotates upon another vertical pin. The block is milled out such that the arm along with its offset counterweight can be rotated up to, and stay in, a vertical position for roll changes. It can also be simply lifted off the fulcrum pin. The counter weight is a copper pipe cap with lead melted into it, followed by a small layer of plumbing solder (which floats to the top) for a shiny, less poisonous surface after cleaning up on the lathe.
A vertical V-groove in the end of the pen arm aligns the pen and a strong rubber band loops from around the pen above the shaft, back through the milled slot and forward again to around the pen below the shaft to hold things in place. I have experimented with several different fine-point micro roller pens and they all seem to work fine. A rough rule of thumb for adjusting the counter-weight is to use about twice the weight of the pen. If the pressure is too high and the pen starts bleeding it will cut through the paper and form an ice-breaker slot from which it cannot escape. Too light and you may not record large excursions well. Bleeding is usually caused by contaminants that build up on the tip, so lifting and wiping is part of the time check routine as a preventive measure.
It took almost two weeks and it has been a fun project. So where’s the beef? The day after we got things going (02/05/98) I was traveling on business when a got an urgent call from Chelsea. She said, "We got an earthquake. It doesn’t look that big because I think it is probably far away". "How do you know that", I queried, thinking of all the possible failures in our circuitry or seismometer placement which could have caused this obvious artifact. "Because I can clearly see the P and S waves, but they are so far apart it just has to be far away, which means it might also be pretty big". I told her to log on to the PSN site and check the recent events and call me back. Ten minutes later she called. "It was in the Santa Cruz Islands". "How do you know?" "Because we got it a little later than some guy in Texas, and if he could get it in Texas than we should definitely get it here." I haven’t explained shadow zones to her yet.
When I got home I simply fed the strip into my PaperPort scanner and here you are. Not bad for a pair of ten-year-olds, huh?
Other than some non-linearity in the pen-driver amp servo feedback (since corrected) the trace looks pretty good, especially the long period stuff, though I think there is some resonance artifact at the current period which will require either increasing the damping, lengthening the period or checking the phasing of my coil feedback currents. The original strip sans the digitization is a site to behold good old fashioned analog recording at its finest.
You would think that a solidly built 70 year-old home with three foot thick stone foundation would minimize tilt artifact. What a surprise! Some of the locations we tried in the basement could probably have picked up the cat on the second floor, but the current location doesn’t "see" me until I am about 8 feet away, which is still pretty bad considering the reduced period I am running. There is a root cellar with a dirt floor and exposed bolder in a corner of the basement (the root cellar was obviously not planned) and I intend to cast a pier and make a vault since we have no garden and I am not especially panicked about Y2K.
So where do we go from here? I have some interesting ideas for a long period vertical, which might be the next logical instrument for "Chelsea" to build. I am also thinking about some "smart" AGC circuits to maximize the dynamic range of captures. I also have an old Metrabyte DAS-8 that I would like to throw into a PC so Chelsea can report "in style", and she wants to have an official "station identifier".
The disease has clearly taken a firm grip.
Very Truly Yours
Thomas W. (Tom) Leiper