Change 555 frequency with amplitude?

[nexekho]

Hey!  I'm designing a piezo preamp/mixer for an acoustic guitar.  I'm using a pair of noninverting op-amps (in a 5532) to boost each piezo individually (with one of them wired backwards to make it inverting without putting any resistors in its path), and then running them both into a differential amplifier based on another op-amp.  That's the first thing; it works great in a simulator, but I can't test a piezo in it and I hear they can be a bit funny about doing certain things.

The second thing, though, is a bit more fun.  I have a spare op-amp IC if I buy two duals, which I can use.  What I'm after is for the average amplitude of the final signal to control the rate of a 555 timer which will then control a 4017 linked up to ten LEDs in circle around the sound hole, so whenever the guitar is strummed the light spins faster and when there is silence it barely moves.  I'm finding it tricky to do this in the simulator though; all of the methods I've tried to smooth the wave out into an input for the astable either kill the voltage until it's far below the 555 threshold or it doesn't quite smooth enough and the waveform trips the comparators in the 555 making the 4017 go ballistic.

I have tried to find examples of other people doing the same thing, and have discovered a VU meter IC meant for driving LEDs that might be used to make it work at ten levels, but I'd like to avoid this route if possible.  Anyone got any ideas?  Thanks.

[CynicalMan]

Have you tried something like this on a lower frequency?
http://ecelab.com/circuit-vco-555.htm

[nexekho]

Aha!  Thanks.  Didn't know they were called that, looked up VCOs and got it working, thanks!

[deadastronaut]

@james...sounds cool, i'm dying to see that....i love an led or ten..

[nexekho]

I'm not sure if I'm actually going to build it or not, largely just learning the ropes in Falstad.

EDIT:  Oh, also, I need to figure out some way of inverting the signal cleanly because at the moment it spins faster when you DON'T strum.

[nexekho]

Still having a bit of trouble with this.  This is the backend of my preamp - using a dual 5532 to give the signal a bit of a post-boost (want to use this with headphones) and to also split off/buffer the signal.  A diode is used so we're only getting the top half of the wave, before going through a low-pass filter.  After a bit of tuning, I've got the signal/noise ratio quite high.  It's the wrong way around, however - if I ran this into the CTL input of a 555, the oscillation slows down whenever the signal line is hot when the desired result is the other way around.  The CTL input seems to disrupt the filter a bit too.  Sure I'll get it working eventually.

[nexekho]

Wait!  I have it!  Using a LED + LDR I should be able to control the 555 the way I want.  In this example, full volume is about 20 spins per second while no volume is 1.3, though that can obviously be changed using the 100nf cap attached to the 555.  Kind of a reverse tiny tremolo - in that instance, the 555 flashes a LED which lights a LDR to change the signal volume.  Here, the signal volume lights a LED which lights a LDR which controls a 555!

(potentiometer is 1M to simulate 1M darkness resistance of the LDR)

[EATyourGuitar]

Wait!  I have it!  Using a LED + LDR I should be able to control the 555 the way I want.  In this example, full volume is about 20 spins per second while no volume is 1.3, though that can obviously be changed using the 100nf cap attached to the 555.  Kind of a reverse tiny tremolo - in that instance, the 555 flashes a LED which lights a LDR to change the signal volume.  Here, the signal volume lights a LED which lights a LDR which controls a 555!

(potentiometer is 1M to simulate 1M darkness resistance of the LDR)

thats exactly what I was going to suggest. someone gave me a schematic on this forum that uses a 386 to drive one or more LED's. the 555 oscillator circuit I use likes a 500k or 1M pot. you can further tweak the frequency range by changing one resistor and cap in the 555 oscillator circuit. as you already know. the pot set at 1M is usually the lowest frequency. about 1hz in my builds using the getlofi kit and schematic. so full dark is about 1hz. is it possible to get a fade in fade out on each LED? will a cap across each LED do it?

[nexekho]

You'd need a low-pass filter; a cap to ground and a resistor before the LED IIRC.  Is it ok to have a single resistor on all the LEDs like that?

[nexekho]

Still need to have a good thorough look-over of this to make sure there's no silly mistakes but DAMN did this get complex fast.
http://pdfcast.org/pdf/velociraptor-schematic

Stereo preamp->differential amplifier for mixing->three-band EQ courtesy of Aron->post-EQ gain->diode clipping courtesy of GeneralGuitarGadgets->output amplifier->killswitch->output
(from clipping)->amp with low pass->555 timer->4017 LEDs.

NOTE: The resistor/etc. values are incorrect at the moment because I was focusing on layout.