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Audio circuit to drive solenoid

Hello all,

I am looking for some design help on a circuit to control a rotary solenoid using an audio signal. First, some context:

This is for a musician/artist friend who is building a prototype of a new musical instrument. The instrument consists of a series of nitinol (MuscleWire) strings that are plucked by a wooden stick attached to a rotary solenoid. He currently has eight strings+solenoid combinations going. When it's played, the instrument sounds like a chorus of detuned cats...

To drive a solenoid under his current setup, my friend simply connects it across the positive and negative terminals of one channel of a car stereo amp. He uses either an analog synthesizer or a digital music application on his computer to generate pulse trains of low-frequency signals. I guess you could call it a primitive form of PWM. He manually tweaks the signal shape, period and volume until he gets the solenoids to activate and pluck the strings at the rates he wants.

All this works (remarkably) well except for two problems: 1) The solenoids get really hot after a short time. No wonder, as they spend a good part of the time uselessly activated. I haven't actually hooked up a scope, but I suspect that only the first 10-20 milliseconds of the audio pulses are going to actually activate the solenoid; the rest of the energy is going to heat his apartment. 2) Each solenoid takes up a precious channel of an amp that is rated to 50 watts while actual power needs are probably only a watt or two.

So what I'd like to do is design a circuit that would take the line- level audio output from his synth or soundcard (1 V rms on the Macbook he uses) and use it to trigger a pulse that is just long enough to drive the solenoid. I don't have the specs on the solenoid, but I believe it needs about 10 milliseconds for activation, then another 60

- 100 ms to run its course to 45 degrees. The solenoid is rated at 12 volts, but I'm not sure whether that is the 100% duty cycle rating or something less. In any case, my friend uses 13.8 V regulated power supplies for his setups, so I'd like to use that to power the circuit.

My first idea for this was to half-wave rectify the audio signal and use it to drive a power transistor, but I don't see how to limit the resulting pulse length, then shut off, regardless of how long the audio note plays. RC circuit maybe?

I welcome all ideas, great and small. Thanks!

doug beeson montreal,canada

Reply to
Doug B.
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So you're looking for a circuit which produces a one-shot pulse, of specific (and presumably adjustable) amplitude(s) and timing(s)?

If he's truely PWMing the solenoid, it could be getting hot from the AC. Solenoids are often big hunks of steel, and big hunks of steel get hot when you put AC into them.

Since this is a musical instrument, I'm going to bet it would be much more convienient to implement a MIDI interface. Not something I've done, but you can probably find a chip to do the hard part.

Tim

-- Deep Friar: a very philosophical monk. Website:

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Reply to
Tim Williams

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Good idea. You can get some inspiration at the following web page that describes a MIDI-controlled trumpet interface.

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Regarding the overheating: for continuous sting-plucking your solenoids should be rated for continuous duty.

-- Joe

Reply to
J.A. Legris

Yes, I think that sums it up. I know of the classic 555 for one-shot pulses. Is there an alternative?

I have thought of a MIDI interface. Problem is, my friend is really not interested in using anything new than his standard analog interface so...not this time.

Reply to
Doug B.

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Maybe he needs some convincing. For example, it's a simple matter to extract the velocity parameter from each MIDI note-on message, which can be used to vary the intensity or repetition rate of your string pluckers, i.e. dynamic modulation. Come to think of it, the present technique of driving solenoids with the audio amps is probably already providing dynamic response. Your friend may be disappointed by the flat response of one-shots.

-- Joe

Reply to
J.A. Legris

I think using a rectifier bridge to create a DC solenoid bias is the first order solution, then return the solenoid to ground through an FET modulated by a 555 timer or other circuit to lower the duty cycle.

--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what\'s the reason?
For if it prosper, none dare call it treason."

"Follow The Money"  ;-P
Reply to
RFI-EMI-GUY

Thank you for your suggestion.

When you say "a DC solenoid bias", do you mean using the audio signal to switch on the solenoid? If so, what would you recommend to do that? FET, Darlington, something else? Also, I presume that I could simultaneously start the charge cycle on the 555 so that it will shut down the solenoid at the end of the RC period, correct?

doug beeson

Reply to
Doug B.

I am going to do a similar thing but with a sliding mechanism, using a gearmotor. I will send a control TTL pulse through a TA7288P then using a 555 to cutout the power to the motor using a Mosfet. You could use a comparator driven with the rectified audio to drive a Mosfet for the solenoid. You might be able to slip another "and" control for the timer.

greg

Reply to
GregS

On the face of it, i recommend switching the interface to the instrument to MIDI. It just seems so much more natural for the interface to me.

Reply to
JosephKK

Crikey. Point out your friend that the sound card already has a duplex midi interface.

Reply to
JosephKK

Yes; rectify the AC and apply the DC component to the positive side of the solenoid.

You can use a full wave diode bridge rectifier.

Also, I presume that I could

If the use of DC bias alone does not solve your heating problem, you could use a free running 555 timer to produce a 50% duty cycle square wave at some multiple of the highest modulating frequency. This timer could drive a Mosfet switch on the negative end of the solenoid.

Make sure all of this is filtered well on the supply lines so that nothing feeds back into the ammplifiers.

--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what\'s the reason?
For if it prosper, none dare call it treason."

"Follow The Money"  ;-P
Reply to
RFI-EMI-GUY

I might have the solution you're looking for. Please contact me.

Reply to
RobPileckis

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