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