Tuning fork oscillator, any ideas?

[kcin]

Dear all,
I'm a visual artist (never studied electronics but can read a circuit diagram and do build stuff) and I work also lot with sound. I'm making a work that uses 2 tuning forks with mirrors on the tines. As the forks vibrate a laser beam reflects off the mirrors and projects lisajous patterns. I have been having problems driving the forks in a reliable way. I used a simple buzzer type circuit before but the contacts burn out over time, then I came across the idea of doing it with 2 coils: a drive and pick up type arrangement using a driver and an amp so the electronics put the forks into self-oscillation. I'm struggling to find an overall solution and a simple circuit that will do this and would welcome any suggestions. The forks are about 20cm tall.

Thanks in advance for any advice.
Best Nick

 

[MikeMl]

To turn the fork into an oscillator, you need five things: the fork itself, a sensor to convert the instantaneous position of one of the tines into an electrical signal, a power supply, an amplifier, and finally a method of using an electrical signal to impart movement back into the tines...

Seems like you can take these one at a time.

1. Is the fork made of a ferrous material which will interact with a magnetic field?

If yes, then reluctance based sensing would work. So would a solenoid drive system.

If no, then the sensing could be optical (use the laser directly), acoustic (use an electret mic), capacitive (make the tine one plate of a parallel plate capacitor)...
Driving the tine could be acoustic, ...

 

[Mikebits]

If you want to make cool patterns with lasers. You should use motors and silver backed mirrors. I did one such project. See images

 

[user_88]

alternate method ... without tuning forks

Another method of achieving the same optical effect is to get a tin or metal can of some sort, open at both ends, and stretch a party balloon, or possibly some other elastic material, over one end. Next, attach a small mirror, or a piece of a mirror, on the external side of the elastic material. Any adhesive or glue would serve this purpose. A coffee can would work well here.

If you direct sound waves internally into the open end of the can, and an optical beam is reflected off the mirror, the desired Lissajous figure will be apparent at the reflected beam image point.

The advantage of this method is that you could simply direct the output of an audio speaker into the open end of the can. Then reflect the laser beam externally off the closed end of the can.

 

[Mikebits]

Another method of achieving the same optical effect is to get a tin or metal can of some sort, open at both ends, and stretch a party balloon, or possibly some other elastic material, over one end. Next, attach a small mirror, or a piece of a mirror, on the external side of the elastic material. Any adhesive or glue would serve this purpose. A coffee can would work well here.

If you direct sound waves internally into the open end of the can, and an optical beam is reflected off the mirror, the desired Lissajous figure will be apparent at the reflected beam image point.

The advantage of this method is that you could simply direct the output of an audio speaker into the open end of the can. Then reflect the laser beam externally off the closed end of the can.

Standard mirrors are no good for reflecting lasers as they distort the beam too much, you end up with a blur. Sound into one reflex device will not produce desired effect as you need x and y to produce Lissajous.

The system you suggest will produce a random pattern. I have experimented with this. Had some cool results but no Lissajous.

 

[user_88]

Standard mirrors are no good for reflecting lasers as they distort the beam too much, you end up with a blur. Sound into one reflex device will not produce desired effect as you need x and y to produce Lissajous.

The system you suggest will produce a random pattern. I have experimented with this. Had some cool results but no Lissajous.

... Concede the point about a precise Lissajous figure ..... there should be an interesting optical effect however.

 

[Mr RB]

Maybe you could glue the bottom of the tuning fork to a piezo element either directly or though a stirrup linkage etc.

Then drive it with a 555 timer to find the resonant freq. You could go to the extra complexity of adding a sensor and making it part of the feedback loop so the whole thing oscillates about the fork mechanical resosnce, but I think a cooler effect would be to use an open-loop osc (like a 555 timer) and then modulate that freq around the centre freq with a second osc. Then you would get all the nice beat freqeuncies and modulation instead of just a pure tone. And it would be easier to make.

 

[Mikebits]

Maybe you could glue the bottom of the tuning fork to a piezo element either directly or though a stirrup linkage etc.

Then drive it with a 555 timer to find the resonant freq. You could go to the extra complexity of adding a sensor and making it part of the feedback loop so the whole thing oscillates about the fork mechanical resosnce, but I think a cooler effect would be to use an open-loop osc (like a 555 timer) and then modulate that freq around the centre freq with a second osc. Then you would get all the nice beat freqeuncies and modulation instead of just a pure tone. And it would be easier to make.

Your theory is somewhat sound, but seems over complicated. Motors work nicely.

 

[kcin]

Thanks for the replies. Great feedback!
In past experiments I used two 64 hz forks and silver plated mirror and got a nice big laser projection of a circle. I was looking at Lissajous' original experiements with tuning forks and attempting replication. I used a solenoid and a wire contact for the self oscillation switching - great to experiment with but not reliable enough to last for a few weeks in a gallery. I like Mr RBs 555 idea, seems the most stable and I like the idea of some variation in the self oscillatiion by having an open loop. I've never worked with 555s before. I'll look for some schematics. What would the system require?

Thanks again.
Nick

 

[Mikebits]

Does it have to be tuning forks?

 

[kcin]

Maybe you could glue the bottom of the tuning fork to a piezo element either directly or though a stirrup linkage etc.

Then drive it with a 555 timer to find the resonant freq. You could go to the extra complexity of adding a sensor and making it part of the feedback loop so the whole thing oscillates about the fork mechanical resosnce, but I think a cooler effect would be to use an open-loop osc (like a 555 timer) and then modulate that freq around the centre freq with a second osc. Then you would get all the nice beat freqeuncies and modulation instead of just a pure tone. And it would be easier to make.

Hi Mr RB,
I really like the idea of the open loop. Sorry for my ignorance but can you be a bit more specific in terms of the set up - schematics etc? Would like to try this out. Would be great if you could point me in the direction of any circuits that would do this.
Thanks a lot.
Nick

 

[kcin]

Hi Mike thanks for your reply and your suggestions.
What to you think you be the most elegant solution? I'd like the visual emphasis to be on the forks rather than the driving system. But I also need the driver to be fairly simple and reliable.
Thanks, Nick

 

[kcin]

BTY the forks are ferrous metal.

 

[Mikebits]

Tuning forks have a limited span of vibration time, will you constantly be around to re vibrate them? Can you tell why you want to use tuning forks?

 

[kcin]

I'm interested in these early sound experiments by lissajous and Helmholtz where they used electrically vibrating tuning forks and want to look at ways to replicate them using more up to date electronics. They'll probably be used as a teaching aid in teaching about sound and vibration. And they'll be on display in a gallery.
Thanks for the interest.

 

[Mikebits]

Okay, well with two spinning mirrors you can make lissajous, with three or more you can make a myriad of patterns, this can all be controlled via computer if your savy enough. Anyways, I am done with my input. good luck with the tuning fork thing.

 

[MikeMl]

BTY the forks are ferrous metal.

Ok, something like this? Pick up coil biased to create a magnetic field. Could be one of those old suction-cup magnetic pickups used to record telephone conversations, or a sensitive relay with the clapper removed. Driver coil= old speaker driver, headphone driver, relay/solenoid coil with clapper/plunger removed. Amp inverting, reverse polarity of either driver coil or pick-up coil to get positive feedback. Gain TBD, depends on air gaps, number of turns in the coils, etc.

Q of the fork is likely to be several hundred to even a thousand. This means that if you are trying to excite it with a free-running oscillator like the 555 that was suggested, then the freq of the oscillator would have to be very close to the natural frequency of the fork. Likely, a 555 wouldn't be stable enough unless there is some feedback to the timing network to create an "injection-locked-oscillator". In that case, you might just as well make an oscillator out of it so that it runs exactly at the resonant peak of the fork...

 

[Mikebits]

Nice drawing Mike, how did you digitize your etch a sketch? hehe nice idea bye the way.

 

[MikeMl]

Nice drawing Mike, how did you digitize your etch a sketch? ...

Virtual White Board

 

[user_88]

If you could rigidly affix a short iron rod to the top of the tuning fork .... you would have a larger coupling coefficient between the vibration of the fork end and the coil ..... That is, making the coil an air core with the oscillating iron core internal to it .... Do the same for both pick-up coil and drive coil.