Building a "electromagnet string organ"

[knutolai]

So I have this idea for a string type instrument and need some guidance with constructing the electronic part of the instrument.
I have got a bunch of 40106 schmitt triggers and 8 ohm magnetic buzzers (removing the disc removes the sound, making it a silent electromaget)

The basic flowchart for each string would be:
Oscillator (40106) - Electromagnet (buzzer) - String

the specs are of the buzzers are:
Type                               YMD0905B-1
Rated Voltage                   1.5v AC
Operating Voltage       1v-3v AC
Rated Current(MAX)       30mA
Min Sound Output at 10cm  85db
Coil Resistance                   8 ohm
Resonant Frequency        2048Hz
Operating Temperature       -20℃ to 70℃
Storage Temperature       -30℃ to 105℃
Housing Materials       ABS, PPO
Weight                               1.0g

I need to amplify the oscillator signal from the 40106 so that the signal is strong enough to resonate the string. However from looking at the specs the buzzers seem a little fragile and Im uncertain what type of amplifier would fit for this application. I have also thought about the option of using two or more electromagnets in parallel to resonate each string. Any suggestions? I tried with simple voltage followers to amplify the current. Didnt seem to work.

are there any better electromagnets than magnetics buzzers that are not too pricey?

[R.G.]

Not a bad idea. I think you need to change the concept in one respect, then it should work.

A stretched string has an almost insanely high resonance and frequency selectivity. Mechanical resonances are often much, much higher than electronic resonances. If you try to drive the string with a frequency from the 40106 oscillator that is not very close to the frequency the string wants to vibrate at anyway, the string will do almost nothing. Think about plucking one string on a guitar while leaving the others open. The one string vibrates at its frequency, and there is some vibration transmitted through the body and neck to the other strings. But the other strings make almost no vibration.

What is really useful is to pick up the string's own output, amplify it, and drive the string electromagnetically with its own vibration, just as an e-bow does. This drives the string at exactly the frequency it's most sensitive at, so it is happy vibrating there.

Make your keys be UN-mute keys to let the amplifiers drive the strings.

[deadastronaut]

as in a 'sustainer/e-bow' you mean' but on multiple strings?..

i made an ebow a while back, it worked. but very cumbersome (quick n dirty design)..but i thought i'd try it out...curiosity etc..

http://www.youtube.com/watch?v=CO0dsElQ8Mk&list=UUGP0eO8ADt0H9FY5UcHTf8A&index=6&feature=plcp

it wasn't the ''specified 50+8 ohms...''

its a common ebow circuit thats floating around the web..

then there is the ''eternally unfinished and somewhat spurious''  diy sustainer too.. .

http://www.youtube.com/watch?v=fYKXd1Tz6WA&list=UUGP0eO8ADt0H9FY5UcHTf8A&index=1&feature=plcp

http://www.youtube.com/watch?v=2GY62teeInU&list=UUGP0eO8ADt0H9FY5UcHTf8A&index=8&feature=plcp

pretty much the same circuit but with a fet input .....lots of trial and error to get it this far, still a work in progress that i'll come back to one day!.......erm.......one day!....

the sustainer pretty much worked apart from 'noise' affecting the bypassed signal though, .....but all good fun anyway.

[knutolai]

Nice projects. I especially like the last one. Does all the string receive the same feedback signal? Thats smart.

The (longterm) idea is to have a zither/harp with electromagnets (or ebows) for every string. These would be controlled by a homemade keyboard 
Alternatively every string could be controlled with a switch and a volume pot. More appropriate for ambient type stuff

I found a "Square wave oscillator" circuit using the lm386 (http://www.ti.com/lit/ds/symlink/lm386.pdf page 5)
Since this is a oscillator using a power amp, maybe the signals come "readily amplified"?

I think it would be interesting to deviate from the normal feedback circuit and use a multi-turn potentiometer for each string. This would be impractical should the instrument go out of tune, it would however make it possible to resonate the string at different overtones.

[Gurner]

I found a "Square wave oscillator" circuit using the lm386 (http://www.ti.com/lit/ds/symlink/lm386.pdf page 5)
Since this is a oscillator using a power amp, maybe the signals come "readily amplified"?

I think it would be interesting to deviate from the normal feedback circuit and use a multi-turn potentiometer for each string. This would be impractical should the instrument go out of tune, it would however make it possible to resonate the string at different overtones.

As RG has already mentioned, the oscillator needs to run ludicrously close to the string's resonant frequency (or interval thereof) to get any life out of the string beiung driven whatsoever (think tuning in a weak AM radio station.......if the tuning dial is a touch either way = no workee). In practice this means that even if you set your oscillators to be perfect tuned for each string for one session (& by perfect I'm meaning less than .1% frequency deviation), then next time you came to use it, the string will likely have deviated (atmospherics...humidity etc) or your oscillator might have deviated....therefore it won't work.

You might have some fun with a manual rotary control controlling the frequency of oscillator, which in turn drives a string via an electromagnet for manual  sweeps  ...but I doubt that you'll be able to set & forget on a string by string basis.

[jaapie]

I had the same idea a year or two ago and managed to get a 1 string prototype up and running based off the method R.G. suggested. I wound a coil to about 8 ohms and drove it with a Ruby 386 amp, using a second pickup for audio output and to feed the signal back to the coil. It actually got the string vibrating pretty hard, but the tone left quite a bit to be desired-- very dull and bland. Of course, with a full bank of strings things might start sounding more interesting

[knutolai]

so Im looking for pickups for the project. They are supposed to be around 50 ohm impedance. Does anyone know if these will suit my purpose?
http://www.ebay.com/itm/50pcs-Buzzer-42-ohm-42R-1-5V-Passive-12085-/150665839197?pt=LH_DefaultDomain_0&hash=item231462425d

Ill cut out the metal disc inside like shown in this cheesy video:
http://www.youtube.com/watch?v=UkQhRFS6sRM

[R.G.]

As a side note, I have accumulated the parts for a similar device, but using the tines of a garden rake instead of taut strings. It's to be a kind of electronic mbira, with soft attack. Some day I'll actually have time to put it together. I also built one note worth, so I know it works, too. It misses the "poink!" attack of a real thumb-plucked mbira, but has a nice soft bowed kind of attack.

[PRR]

> tines of a garden rake

These are real handy for harvesting rocks, and sing lower than a stone-rake:




> Rated Voltage                   1.5v AC
> Rated Current(MAX)       30mA
> Coil Resistance                   8 ohm

These do not add-up the simple way. (1.5V/30mA is 50 ohms.) Therefore the operating impedance is highly affected by the vibrating clapper; also the fact that it turns itself off most of the time.

Without the clapper/contacts it is probably close to 8 ohms.

The pedal-world way to drive an 8 ohm load is a LM386. Since '386 and 9V can give 3V, this is on the high side of the 1V-3V spec. Actually you will get less. Also I suspect the buzzer is fairly robust, AND is cheap enough that you won't have to skip lunch if you toast one. However 10 ohm resistor in series will get power down and also isolate the '386 from coil hijinks.

As said, you have to hit _A_ to ring a string tuned to A. The potato-hoe might tolerate many cents of mistuning, but a guitar (piano, violin) string has to be within a few cents. Odds are that a Schmitt's R-C network is too fussy and drifty to work well. Obviously you feed-back from the string's own ring, but I tried this once and I hope you do better than I did.

For pickups, I'd just use guitar pickups. Why re-invent wheels?

[knutolai]

Nice one R.G. Id like to see that when its finished

The reason I can think of for not using regular guitar pick ups is that, from the way I understand this, each string has its own feedback loop. If I used guitar pickups these would mix the signals coming form the strings. I dont quite understand how deadastronaut got this to work in his 6 string ebow. Does the string "choose" which partials of the complex wave they are presented with its going to resonate with? Hope Im making myself understood here 

My project is inspired by a sound installation by a guy called Peter Flemming. He uses a lot of 555 timers (oscillators) to vibrate different materials. Here is a video of him vibrating strings:
http://peterflemming.ca/details/vib_wav.html

pretty cool how he is able to trigger so many harmonics in the string.