More work with the voltage controlled resistor module. But 
first I'd like to introduce another circuit I've been 
working on for a little while. 





I call it the Pitch Interplay Phototheremin, a fancy name
for a very simple little circuit. Essentially, it consists
of six squarewave oscillators of different frequency all
mixed together to produce a complex waveform. The sound
becomes interesting when you can change the pitch of all 
these oscillators at the same time. To do this, I simply 
starve the oscillator IC of power. In the Phototheremin
circuit, a photocell is used. Since the cell has a bit
of lag whenever the light level changes, it acts as a very
simple decay envelope.

I've also tried my voltage controlled resistor module in
place of the photocell and the results are interesting.
In this manner, nay voltage can control the speed of the
six oscillators. R1-6 determine the frequency of each
oscillator, however they could also be replaced with pots
so the frequency is adjustable. I should note that C1-6
also control the frequency. Use whatever values you like.
I plan to build a module that includes this circuit.

Inspiration for this circuit came from Ken Stone's Psycho
LFO circuit, the CD40106 datasheet, and Megaohm's
Six Squares module.

Listen to the circuit running with a photocell. It can
be played with a flashlight. The clip ends with the
circuit modulating another oscillator.

Listen to the circuit under voltage control. Sample and
hold sounds very cool controlling this circuit.

I've been experimenting with voltage controlled resistors
as a way to convert some easily constructable circuits to 
voltage control. In theory, it should be possible to make
almost anything voltage controlled by adding one or more of
these voltage controlled resistors in the circuit.

The circuit I am using comes from the LM13700 datasheet. It
is a floating voltage controlled resistor, using both halves
of the IC. Here is the schematic, taken from the datasheet:





In practice I've had to change a few resistor values, mainly 
the 15K resistor on the voltage control input. I tried a few 
values and found that a 3.3K resistor works for the circuit
I am trying to control.

I've made a breadboard layout to make building this circuit 
very easy. I used Ray Wilson's Virtual Breadboard, check it 
out if you have a minute, it's very useful.





The inspriation for needing such a circuit arose when I 
bought a cheap BBD chorus pedal and wanted to control the 
speed with my synthesizer. Cracking the pedal open, I 
quickly found where to connect the two point of the VCR 
circuit. The range is not as good as I would like, but 
having the speed voltage controlled makes up for the lose. 
Look for a synth module in the future based on this 
chorus pedal.

Listen to my Soundlab controlling the speed of this chorus
pedal. Two saw oscillators into the VC-chorus, with an
envelope, then square wave, then triangle wave controlling 
the chorus speed.