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Evan
10-24-2007, 03:02 AM
In search of the lost chord...

That really describes it well. This is where machining meets natural science and music.

I have built an Aeolian Harp. It's something I have had in the back of my mind for many years and now I have finally found the time and motivation to do it.

An aeolian harp is played by the wind. It can be extremely simple as mine is, or very complex. They can be very large, or small enough to hold in your hand. In all cases the sounds they produce are otherworldly and very unexpected.

There is a major drawback, an aeolian harp is a very quiet instrument. I have addressed that issue in my design in a couple of ways. It has an efficient mechanical audio amplifier coupled to an efficient electronic audio amplifier and FM transmitter. This makes it possible to listen to the sounds of the weather and celestial spheres in the comfort of our living room.


This is it:

It consists of two steel poles designed to be just placed on the ground and anchored by concrete blocks One pole is tall in order to catch the wind better and the other is short to make for easy access and adjustment. Between the poles is stretched a wire, in this case a multi stranded 12 gauge copper wire.

http://vts.bc.ca/pics3/harpa.jpg

The low end is where the sound is amplified and captured. The mechanical amplifier is a carefully constructed (un)tuned cavity with many resonance modes. The front and back plates are essentialy flat and parallel but the inner side walls are sloped steeply at about 45 degrees from back to front. The outside diameter is 8". The inside diameter changes from about five inches to seven inches over a distance of 2 inches describing a conic volume. This prevents the cavity from developing a standing wave and resonating at only a single frequency. The front plate is aluminum flashing mounted so that the resonating chamber is airtight. A good quality condenser microphone is installed in the rear plate. The harp string (wire) is stretched from the front plate to the tall pole top end.

http://vts.bc.ca/pics3/harpb.jpg

It was a windy day today which is what decided me to complete (nearly) this project. I have been working on parts of it for several weeks and have been testing various wire types during that time. I have not referred to any plans or other similar information as I have always had a good idea of what I wanted to build. I'm not sure there are any plans or information on something such as this harp.

Here are some more views.

http://vts.bc.ca/pics3/harpd.jpg

The harp is a success. It works better than I expected. The sound is very hard to describe. It is very rich and complex with a base chord consisting of many harmonics and higher chords that may fade in and out with changes in the wind. There is also an underlying deep bass rumble that on one occasion rattled the windows via my surround sound system. It sounds almost like an electronic therimin implementation of a harp. It isn't disharmonious either with the chords being harmonically related.

I am recording the sounds and will edit together a sequence that I will post here sometime tomorrow most likely.

Technical notes:

The resonator is made from old surface grinder diamond/CBN wheel hubs. The transmitter uses a stable FM transmitter circuit that has a separate LM386 IC op amp set for audio gain of 200. This frequency modulates the oscillator stage. The oscillator stage is well isolated from the final amplifier stage and is only lightly coupled to the final amp by a 2pf capacitor. It's a very stable design and draws about 70 milliamps at 8 volts to produce about 200 milliwatts of final output power. It has been tuned to operate on 106.90 mhz by using my frequency counter. I used surface mount techniques for the transmitter and the PCB was milled using the method that I have previously shown. I developed the layout for the board so that only the separations between conductor islands need be milled with the entire back side of the board serving as a ground plane. Many of the components are surface mount parts.

http://vts.bc.ca/pics3/harpc.jpg

In this area we have no FM stations that operate above 100 mhz. The nearest one is hundreds of miles distant so interference is not a concern.

The power for the transmitter is supplied by an industrial quality set of six 4 amp hour nicads. They are maintained by a solar panel capable of supplying 1/3 amp at 12 volts in full sun. I still have to mount the solar panel and finish a few details on the transmitter, mainly installing the weather proof housing. The housing was originally for a ku band block downconverter.

I expect the harp to do more than just respond to the wind. The audio is very sensitive and so far I have identifed the sound of falling fir needles hitting the wire. I am sure that rain will also have a signature and won't be surprised to hear snowflakes when they strike the wire. Temperature also plays a big part in the nature of the sound. Simply blowing some shop air on the front of the resonator causes an almost immediate change in the resonance patterns and sound quality. From only the small amount of cooling produced by the compressed air the basic background chord goes up in pitch by about half an octave.

Dawai
10-24-2007, 04:56 AM
OKAY, what was the "organ" that you stood in front of and waved your arms to create a rf disturbance?

I think the "fish" sounds of a local creek was my favorite.

(actually I think you are setting up a snare wire for thieves) A clothes line at neck level works best.

Evan
10-24-2007, 05:42 AM
The "organ" is the Theremin I mentioned, invented by Leon Theremin.

aostling
10-24-2007, 06:06 AM
Can't wait to hear it. I miss the sound of the wind, down here in the quiet of the desert. Perhaps someday a live feed, to a website? I would like to tap into that.

Your Old Dog
10-24-2007, 06:17 AM
I think it's pretty neat! Can't wait to hear what kinds of sounds it can generate.

I can't imagine why multi strand wire of either steel or copper would be better than single strand tempered wire such as hard drawn mig wire or staple wire as used by some ham's for their long-wires. I would expect the multi strand to pickup more noise from the strands interacting and if anything, to null sound waves rather than transmit them unblemished. I have some wire I bought at a yard sale. They guy didn't know what it was but it looks like music wire only too much of it to be so. I think it's stainless as some used in the garden is still pristine. If you needs some, pm me a address and I'll get you off what ever you need. This coil weighs about 30 pounds.

BTW, did you ground this wire or does it float above ground?

(Oh to live in a community where the streets are paved in aluminum to feed my small machine shop :D )

A.K. Boomer
10-24-2007, 09:12 AM
I think Iv heard them sounds before, Its almost like a high frequency elk call, Iv heard them from the telephone/electrical line wires when the wind gets going at a steady pace, Personally I like the sound of the ocean and of wind in the pines, the sound pine needles make in the wind have a remarkably soothing effect on me.

Evan
10-24-2007, 09:30 AM
It certainly doesn't sound like an elk call. I'm trying to clean up the recording I made yesterday but I was using a very mickey mouse setup and the quality sucks. Of course, the wind died down this am as soon as I hooked up a direct input to my computer instead of recording from the speaker.

A.K. Boomer
10-24-2007, 10:21 AM
Of course, the wind died down this am as soon as I hooked up a direct input to my computer instead of recording from the speaker.


Thats Murphys Law Evan, it states that when it really matters it wont happen!

Not to change the subject but You and many others here are outdoorsy kinda Guys and there is something thats had me curious for many years as im not a hunter and heard many hunters describe that an Elk gets its "wistle/bugle" from its rear teeth -------- Iv even seen these magical teeth and cant see how this happens, being of the curious type the thing that reminds me most of an elk wistle/bugle/whatever is blowing threw flex gas line, furthermore, because I could not except the fact that these little teeth are doing this I asked a hunter friend what an elk esophagus looks like and he discribes a series of ridges of high low in the I.D. , I then stated to him that this is where the elk gets its distinct sound and he said he didnt think so because of course he said it was the teeth, to which I reply'ed -- have you ever blew through a flex gas line, and he said thats what they use to call an elk --- to which I reply'ed -------------------- Bingo,,,,,,,, Then he itched his head,,, what do you catdz think?

tony ennis
10-24-2007, 10:34 AM
Wow, very cool. I would very much like to hear the clips.

-----------------
If you change the transistor on the right of your circuit with a Jurgonsenn TL2665A cross-harmonic modulator, it will double as a death ray. Hope this helps.

andy_b
10-24-2007, 10:56 AM
Evan,

interesting project. i can't wait to hear the sounds!
i've seen instances where strips of fabric tied between two points will start fluttering in the wind. is this basically what happens with the wire for the harp?

andy b.

Dawai
10-24-2007, 11:22 AM
I can remember this guitar player at a party, a bath tub full of 190 grain and grape juice and citrus, and god know's what else people dropped in.. He had his head laying on his guitar talking to it..

In a little while he turned it up, it was picking up truckers talking, a poor linear amplifier kicker with a loose antennae connection makes a broadband broadcast.

He had thought he was talking to god.

EVAN: are you in the grape juice? HA.. just a old brain cell that connected for a moment.

Yes it was a therimin noise generator.
http://www.rachelhamilton.com/listen.html A hippies toy? Too many drugs? Okay, I listen to pink floyd myself.. and Roger Waters is still the king. Gimme a slice of white bread..

Evan
10-24-2007, 12:04 PM
OK, since it doesn't look like the wind will pick up I will post a piece. It will give you some idea anyway even if the quality sucks. Give me a few minutes.

Your Old Dog
10-24-2007, 12:31 PM
Not to change the subject but You and many others here are outdoorsy kinda Guys and there is something thats had me curious for many years as im not a hunter and heard many hunters describe that an Elk gets its "wistle/bugle" from its rear teeth -------- Iv even seen these magical teeth and cant see how this happens, being of the curious type the thing that reminds me most of an elk wistle/bugle/whatever is blowing threw flex gas line, furthermore, because I could not except the fact that these little teeth are doing this I asked a hunter friend what an elk esophagus looks like and he discribes a series of ridges of high low in the I.D.

Not to stray too far from the thread but a big Negative on that. Elk make their unique whistling sound as they purse their lips and the air passes between the two sides of their hoof :D

We now resume the thread in progress :D

Spin Doctor
10-24-2007, 12:52 PM
Somehow I thought this was a Moody Blues thread

http://en.wikipedia.org/wiki/In_Search_of_the_Lost_Chord

By the way Evan, nice job of mixing interests to produce something totally useless but pleasing to the builder.

kendall
10-24-2007, 02:26 PM
Somehow I thought this was a Moody Blues thread

http://en.wikipedia.org/wiki/In_Search_of_the_Lost_Chord

By the way Evan, nice job of mixing interests to produce something totally useless but pleasing to the builder.


That was my first thought! 'Hey, I have that album!'

now I'm going to hum Timothy leary's dead all day......

ken.

Evan
10-24-2007, 02:38 PM
OK, finally. It's the first time I've used Nero 7 wave editor to edit something. I'm not impressed. It must have crashed about ten times.

Anyway, it's here, 1 megabyte

http://vts.bc.ca/misc/aeolianharp1.mp3

Note that there are a few artifacts from the editing. I removed the noise floor and normalized the volume as best as I could. Also note that I DID NOT add any special effects, tremolo, vibrato or echo. Even the sounds that sound like hum are natural.

lenord
10-24-2007, 02:40 PM
Evan,

You never cease to amaze !
I can't wait to hear what sounds that contraption picks up. Looks like something most of us can build. It would be good to have those sounds as a background in the shop. It gets kinda windy here in Texas, so it might actually work.
I do recall the sound of the electric wires when a tornado was coming. I imagine this is better though. You can hear a needle drop on it ? KEWL ! I wonder what a dust storm sounds like on it or if it picks up other sounds, like from the freeway that is nearby.

Just downloaded and listened. Way cool !

Lenord

Evan
10-24-2007, 03:08 PM
It picks up other sounds just fine. Fortunately it is very quiet here. I think the mic may be overloading so I am going to try putting some baffling material inside the resonator. It also may be overmodulating but that is easy to fix as I have a gain control on the transmitter board. This is all new ground here so it will take some time to igure out the best setting and materials, especially for the wire. I tried smooth wire but it didn't seem to respond as well. I may try it again as I didn't have the resonator built then.

For best results you need to have decent speakers or headphones.

topct
10-24-2007, 07:45 PM
If you try and switch a Hammond organs mechanical tone generator to run before it comes up to speed it can sound like that. Or if someone put 30 weight in it. :D

The difference is YOU can play it.

jkilroy
10-24-2007, 08:12 PM
Very nice Evan, wonderful sound. The generated wave forms are very natural if you get my drift, very nice.

Evan
10-24-2007, 08:42 PM
They are entirely natural with the exception of some aliasing that can be heard as minor background artifacts which sound like little squeaks. That is what happens when you process to remove noise. That and normalizing the volume is the only processing I did. The noise removed was simple pink noise from several sources, the transmitter, the receiver and the sum of all the background sound on a windy day.

Your Old Dog
10-24-2007, 10:16 PM
Evan I just listened to your recording. Thank gawd I now know what that noise was coming from SWMBO's sewing room when the bright light was stabbing out from the keyhole in the door :D Even since that day I've been telling "her" that "I" have a headache!

darryl
10-24-2007, 10:58 PM
Very cool! I like it!

Hey, being wind derived sound it deserves to also be powered by the wind. You could use a hard drive motor as the generator, gear it up to a multi-bladed fan ( a neat looking one of course) and run the three diodes from the three phase motor coils into a super capacitor. Limit the voltage using a transistor amplified zener diode, and let the resistance of the motor windings take care of current limiting for when the wind speed picks up a lot.

There are so many ways to arrange to pick up the sound from the string (or strings, if you were to add others). You could use super magnets bonded to the wire or the diaphram, and a coil of wire to act as the pickup device. That would get rid of the mechanical overloading of the electret mic that is caused by the wind itself.

Possibly a more elegant pickup scheme would be to use some piezoelectric plastic film- I'm sure you would have played with that at some time. You'll have no problem figuring out the details.

Thanks for showing us that, and posting the sound clip. This has to be about the strongest impetus I've had lately to get back to doing some purely fun and amusing project. Thanks, Evan.

Carld
10-25-2007, 12:14 AM
That's interesting Evan. I saved the sound file. I think I will build one but I don't think I'll use an amp. I want it to be all acoustic. We have a set of very large wind chimes and love to hear them ring.

Evan
10-25-2007, 12:16 AM
I was thinking of using an old playback head from a tape recorder. Stick it close to a magnet etc. I seem to recall that they need a steep precompensation curve.

The wind down in our trees isn't reliable or strong enough to run a windmill. I wish it was except then it would be windy a lot more. (?)

Solar power is iffy too. That's why I am using such a high ratio of panel output to load draw. It only needs an average of three sun hours per day to keep going 24/7 and has about 4 days reserve capacity.

I have been thinking of adding some more enviromental monitors to modify the sound. A sun sensor would be a good start. Perhaps a geomagnetometer to sense the swings in the geomagnetic field when the aurora are playing. They generate excursions of up to +-2 degrees in the field with a time constant of about 30 seconds to a minute.

Carl,

One of the reasons for the electronics and the 200mw transmitter is so some of our neighbours can listen too.

dp
10-25-2007, 12:18 AM
Not to put too fine a point on it, but I believe what you have there is a read-only can and string telephone: http://www.town4kids.com/town4kids/kids/science/explore/can.htm

:)

I was wondering if it is technically an aeolian harp - the sound from them is derived from the soundbox - yours appears on first blush to be a transducer in that it conducts vibration to the diaphragm where it is converted to sound like any good ol' fashion piezo-electric beeper.

I wonder what it would sound like if you put a record player pickup against the diaphram rather than audio coupling it to the amplification. Tipping the pickup at an angle immediately provides stereo output. I'd kicked around building a seismometer on that principle.

Someone somewhere did that with utility poles and recorded the sounds. Interesting way to spend a day. As always, nice work!

S_J_H
10-25-2007, 12:38 AM
Evan, I wish I had neighbors like you! I'm serious.

me- hey Evan what's up?

Evan- ahh not much, just building a Aeolian Harp.

Me- nice...


But no... with my neighbors I get talk about how the weed and feed is working.
Steve

darryl
10-25-2007, 02:37 AM
One somewhat interesting (to me anyway) way of picking off a signal from a diaphram is to use a capacitive sensor. What I was experimenting with was using the fm transmitter, but modulating it with the moveable diaphram. I had a ground area on the circuit board of about 1 sq inch, then next to that another sq in of area that connected to the tuned circuit. That part of the board was placed adjacent to the diaphram so that vibrations would translate directly into fm modulation. I had to glue an area of aluminum foil to the diaphram (which was just a speaker) in order to complete this 'variable capacitor' which tuned the transmitter.

If your diaphram is metalized or metal, it would be connected to a ground point on the pc board and the ground area on the board wouldn't be required. It worked fairly well, and I found it was easy to overmodulate, but not that easy to keep the center frequency right on. Digitally tuned receivers had a problem with it, but manually tuned ones were ok. I could talk into the speaker and be heard on the radio.

I don't think it would be difficult to rig up a modern version of this variable capacitor that could be mechanically coupled to the wire or the diaphram in order to achieve fm modulation. Just an idea.

Evan
10-25-2007, 04:17 AM
The diaphragm is just a piece of polished aluminum roof flashing. It could be easily insulated from the rest of the cavity with a gasket making the entire resonator a variable capacitor. This could then be coupled to the oscillator with a short length of coax cable. I may try that as it eliminates the audio stage and reduces the current draw. The degree of modulation can be controlled by putting a trimmer cap in parallel with the resonator.

One thing I want to try is to replace the diaphragm with something less massive, perhaps a sheet of mylar. Currently there is practically no energy above 4000 hz. I am sure there are overtones present that are buried. Bringing them out would result in much better crispness.

We have a very good set of neighbours. We all know each other well and get together on a regular basis to swap gossip about other people in the valley. Well, at least the women do. :D The men talk about tractors, the weather, my latest project and all the usual stuff. They show up for the pie. The nice thing about our neighbours is that everyone is instantly ready to help out without question whatever comes up. I also don't get buried in requests to do some welding or similar since most of my neighbours can weld and fix their own stuff.

Your Old Dog
10-25-2007, 06:03 AM
We have a very good set of neighbours. We all know each other well and get together on a regular basis to swap gossip about other people in the valley. Well, at least the women do. :D The men talk about tractors, the weather, my latest project and all the usual stuff. They show up for the pie. The nice thing about our neighbours is that everyone is instantly ready to help out without question whatever comes up. I also don't get buried in requests to do some welding or similar since most of my neighbours can weld and fix their own stuff.

You can add about a million bucks to the value of your home with good neighbors. I envy you that.

Evan
10-25-2007, 06:44 AM
I was wondering if it is technically an aeolian harp - the sound from them is derived from the soundbox - yours appears on first blush to be a transducer in that it conducts vibration to the diaphragm where it is converted to sound like any good ol' fashion piezo-electric beeper.

I think the important difference is that all the piezo beepers I have seen are not wind powered. Also, a transducer converts one sort of energy to another. The harp does not. You can stand next to it and hear it. The resonator IS a sound box. There isn't any rule it has to be made of wood.

However, there is an idea in your comment. If I glue a piezo beeper to the diaphragm it will flex as the diaphragm does and produce a corresponding electrical signal with the same impedance characteristics as an electret. That's worth a try.

andy_b
10-25-2007, 12:43 PM
can one of these harps be made loud enough that you could hear it sitting on your porch if it was about 50' or 100' away? or would that require a 1000' harp string and a 5' resonator chamber?

i love the sounds from it and i would like to build one, but i don't really have an interest in building one if the sounds need to be electronically amplified and transmitted to me.

andy b.

dp
10-25-2007, 01:03 PM
There are lots of outdoor harps - some quite large:

http://www.sonicarchitecture.com/catalogue/Aeolian.html

Swarf&Sparks
10-25-2007, 01:40 PM
Oh yeah, and if you just had a wind-powered Lesley speaker.... :D

Nice toy Evan :)

Evan
10-25-2007, 02:26 PM
can one of these harps be made loud enough that you could hear it sitting on your porch if it was about 50' or 100' away? or would that require a 1000' harp string and a 5' resonator chamber?
I don't know for sure. I expect it depends on the wind velocity. The energy contained in the wind varies as the cube of the velocity so doubling the wind speed would impart 8 times the energy to the harp. I have mine set down in the middle of the forest where the wind is broken up and slowed by the trees. The up side of that is the sounds it makes are more varied and interesting ( I assume).

Carld
10-25-2007, 02:38 PM
Evan, you can get a piezo buzzer from Radio Shack and take it apart to get the diaphram out and epoxy that directly to the resonator. We use them on guitars, banjos, and resonator guitars. You get better response with just the diaphram. They make good pickups.

Swarf&Sparks
10-25-2007, 03:21 PM
or, if you have a surplus of cash, you could get a
B....B....
Which will do the same but comes with much higher bragging rights:rolleyes:

snowman
10-25-2007, 04:42 PM
Dear Evan,

You are a freak.

Sincerely,

Snowman
PS :)

Evan
10-25-2007, 08:14 PM
Carl,

Thanks for that info. I have plenty of piezo elements in my junk stash. I will try that out this evening. I was pretty sure it would work.


Dear Evan,

You are a freak.

Sincerely,

Snowman

What, are you just now figuring that out? :D

darryl
10-25-2007, 08:29 PM
Maybe this site should be called FSM. The Freak Shop Machinist :)

Evan
10-25-2007, 08:49 PM
I have a few more oddball projects up my sleeve. One I want to build is an aurora detector. Try looking up jam jar magnetometer. I think I will use a flux gate though.

Here, I looked it up.

http://www.britastro.org/aurora/jamjar.htm

dp
10-25-2007, 09:05 PM
I have a diagram of a very simple saturable core magnetometer I was working on in the late 1980's. It turns out to be an extremely sensitive geomagnetic detector. I was easily able to measure the dip angle here in the Seattle area. It uses simple 555 timers and quad op-amps.

The reactor is about 1 1/4" dia. and is driven with low-power circuitry. The toroid core saturates at very low current and when driven with a square wave, the deflection of the geomagnetic field is instantly drawn through the center of the toroid past a sense coil wound over the outside of the toroid.

It was part of a wing leveler I was developing for light planes. In combination with Holzman coils driven by a rate gyro to compensate for bank angle of the plane it was a pretty decent home-made compass, but also acted as a heading holder feature for the wing leveler.

Evan
10-25-2007, 09:29 PM
I have a diagram of a very simple saturable core magnetometer I was working on in the late 1980's.

OK, cough it up please. :D

darryl
10-26-2007, 12:16 AM
I built a device once called an audio frequency receiver. Basically a large diameter coil of wire with an audio amp and an active filter between the two. The filter excluded frequencies outside of the audio range and notched out 60 hz. It would pick up various radiations in the audio frequency range, some of which were various tone patterns, and much of which it was said was created by lightning. One of the more interesting sounds that came out of it was called the dawn chorus, a phenomenom of signal production by lightning and round-the-world travel. Kind of an echo thing- I have heard it but not at its best.

I experimented with taut wires and pickups in my late teens. I don't mean guitars- I was looking to make a unique guitar effect, kind of like a spring reverb but different. Like it but totally different :). Anyway, I had razor blades mixed in there with transducers to drive it, and pickups to detect the results. It was pretty neat being able to turn up a volume control and set the wire and razor blade contraption to 'singing'. Adjustment was by stressing the razor blades in different ways.

Evan
10-26-2007, 01:06 AM
It was part of a wing leveler I was developing for light planes.
Ever hear of an electrostatic field autopilot? There is a continually present charge difference between the ground and the ionosphere. In the bottom 50,000 feet or so where the weather happens it amounts to several hundred volts per vertical meter everywhere. In an electrical storm that can go up to thousands of volts and just prior to a lightning strike it can be thousands of volts per centimeter.

In clear air the charge is undetectable using any sort of ordinary voltmeter since the air is an effective insulator. However, if the air becomes ionized it will conduct sufficiently for the electrostatic charge to flow to a nearby conductor. This can be accomplished by using a weak alpha emitter source such as americium 241 in conjunction with a charge collector plate. This allows the air charge to be measured by a FET input voltmeter. The charge may vary in strength but it does so consistently over a local area of free air.

The circuit needed to detect this is almost trivial, just a FET op amp capable of operating with feedback resistors in multi megohms range. If three sensors are used , one on each wingtip and one on the tail you have a reasonably good wing and pitch leveler. It has been used in model aircraft with the control output mixed with the remote command signals. If the pilot of the model stops actively controlling the aircraft it simply continues in straight and level flight. It senses the change in voltage between wingtips when a wing goes up or down and uses that to control the roll axis. It sums the wingtip sensors and compares that to the tail sensor and uses that to control the pitch axis.

It gets better though. Any structure or vegetation such as trees conduct well enough that they are at ground potential over the entire structure or tree. They create a region in the vicinity of lower charge that extends quite a distance from the object. If the aircraft with the electrostatic autopilot approaches such an obstruction at an oblique angle it will produce a charge difference between the wingtips as one wing is closer to the object than the other. This will cause the autopilot to command a turn away from the obstruction. If it flies directly toward the obstruction it will command the aircraft to climb since the wingtips will be closer than the tail sensor.

I started to build one back in the early 80s or late 70s but never got around to actually testing it. There was an article about it published in Model Airplane News back then with plans to build it.

dp
10-26-2007, 01:29 AM
I did see that and in fact have a huge amount of data and same circuits regarding that. It is another idea that is fraught with adventure when you are at altitude. There are phenomena such as St. Elmo's Fire that play havoc with sensitive systems like that and the last thing you need to experience in an airplane is a sudden destabilizing influence on your flight control system. The amazing thing is the amount by which the voltage changes by rocking the wings even on a small model glider. It takes a lot of filtering to take out the noise from those systems. It has been done, though. That voltage at ground level is continually modulated, btw, and makes an interesting driving source for a theremin type instrument. I attached one of the sensors I was working on to a voltage controlled audio oscillator so I could hear the changes as well as watch them on the chart recorder. Cool stuff right up there with the aeolian harps.

I was up air testing the wing leveler (the superset of the circuit I sent you) with another flyer who had installed all the hardware. We were doing fine right up until he turned on the wing leveler without first disabling the course holding feature and holy crap we snapped into a standard rate turn at a non-standard rate and headed 160 degrees from our previous heading. The SRT was a feature I'd put into the circuitry else we'd have barrel rolled right into the world. As it was, being a wing leveler and not an altitude hold system, it was attempting to put us into a death spiral. To counter that I designed in an all-zeros button so that all changes of attitude were immediately reduced to null and all we had to do at that point was pull up to level flight. After that I never let him fly PIC again when I was in the plane.

Evan
10-26-2007, 03:48 AM
Thanks for sending the article. I was beginning to read it when I fell asleep. Now that I have had a good couple of hours of sleep I will be having a good look. I happen to have one of those old Radio Shack flux gate compasses so I think I will see about using it as a detector.

I also found this page on the net.

http://beale.best.vwh.net/measure/fluxgate/index.html

dp
10-26-2007, 04:00 AM
That is pretty much the heart of the device. Magnetics, Inc still makes permalloy tape wound cores and I took a quick look and saw several that look like they'd work fine for the job. That circuit I sent has the demodulator in there that will provide +- polarity normalized at 1/2 Vcc to give you a differential output around magnetic north.

Evan
10-26-2007, 04:36 AM
For an aurora detector all you need is to know how much the field is swinging. Absolute direction doesn't matter. All it is is an activity indicator.

andy_b
10-26-2007, 12:36 PM
I was up air testing the wing leveler (the superset of the circuit I sent you) with another flyer who had installed all the hardware. We were doing fine right up until he turned on the wing leveler without first disabling the course holding feature and holy crap we snapped into a standard rate turn at a non-standard rate and headed 160 degrees from our previous heading. The SRT was a feature I'd put into the circuitry else we'd have barrel rolled right into the world. As it was, being a wing leveler and not an altitude hold system, it was attempting to put us into a death spiral. To counter that I designed in an all-zeros button so that all changes of attitude were immediately reduced to null and all we had to do at that point was pull up to level flight. After that I never let him fly PIC again when I was in the plane.

i was following this story and thinking that was a pretty cool project for a remote control plane. until i hit this part, that is:

"After that I never let him fly PIC again when I was in the plane."

YIKES!!!!!!

andy b.

dp
10-26-2007, 10:03 PM
i was following this story and thinking that was a pretty cool project for a remote control plane. until i hit this part, that is:

"After that I never let him fly PIC again when I was in the plane."

YIKES!!!!!!

andy b.

Here's my plane (and a young me) I was going to install it in - I decided to put off doing so as I was not convinced the wing leveler sensor (a fluidic rate sensor) was adequate.

http://thevirtualbarandgrill.com/gallery/flying01

I now return the thread to its naturally aspirated state.