I am looking for ways to use my electric guitar to control a color
organ or other lights & similar devices. Can someone point me to a
schematic for a circuit that takes the signal from an electric
guitar's magnetic pickup and translates that to a resistance which
varies depending on how hard the guitar's strings are picked? Are
there any circuits that translate the signal coming from the guitar
into a resistance or capacitance that varies based on the note being
played (the pitch / frequency of the string being plucked)? Any
pointers or advice would be most appreciated...
Start with small AGC preamp with a bandpass filter to exclude pitches
outside the range you want. These might be found in kit form, I used one
that Maplin used to sell. Then feed the output to a Scmitt trigger based on
a CA3140E op-amp with a high value (megohms) resistor for positive
feedback, to act as a squelch control and to sharpen the pulse edges.
Choose a value that gives the right rejection of low level noise, but is
still sensitive enough to get all your signal.Send the output of that to a
PLL IC, 4046, using the XOR inputs. This works for whistling and other
sources of clearly defined pitch. If you find if won't lock on to a guitar,
first take care to use a fine monophonic playing style, and if you still
get trouble, use the other input. Either way it should be possible to get
good tracking. The resulting clean square wave can feed a freq/volt
converter, or you might be able to tap the PLL's VCO control voltage.
Like all circuits for this task, immunity to loud noise is poor, the
question is what happens if it hears loud noise, and what can you do about
it? In this case, it will make the PLL go to the highest frequency you've
set it to be able to do. (You can limit it). With a pitch to MIDI control,
that can be made to go silent by blocking the message data for that top
note, but in analog, you'd probably have to set the limit to one note
higher than you want, and set a comparator to detect that and mute the
output.
This circuit will be very small, very cheap, and very agile, but it doesn't
do Volts/Octave output. You might want that, and if you can get by with
just three octaves you might do it with a log converter based on a current
mirror. There's a Moog ciruit published online but I can't remember where
it is. Big Briar (firm run by Robert Moog) still sell the stabilising
thermistor needed for accuracy for that design.
If you want five octaves or better, best write to Doepfer and politely urge
them onwards on their pitch to MIDI project, as that will almost certainly
include analog control output for their A100 modular synthesizer. This
project is in slow development, and the speed might be proportional to
interest in it.