SIMPLE PITCH LFO                     By Scott R. Gravenhorst 
                                     chordman@ix.netcom.com
                                     06-23-95
                                     Updated 05-25-96

This FatMan modification adds a simple LFO function to modulate 
the pitch of a VCO.  Two of these circuits may be built to add 
LFO pitch modulation to both VCOs.  Modulation is implemented 
using the 555 timer's unique modulation input on pin 5.  This 
method completely isolates the modulating signal from the main 
pitch control voltage.  

No trace cutting is required.

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CAVEAT: Due to the use of a simple, but brute force filter for
wave shaping in this circuit, there is interaction between the
LFO rate control and the effect level.  That is, as the rate
is increased, the level control may have to be increased to
compensate.  Also the filter cap switch will act similarly.
I am currently working on a voltage controlled (or manual) LFO
that will not have a filter, but rather will provide either
square or triangle outputs and will not exhibit the phenomenon
described above.  The new circuit, will of course, be more
complex.
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NOTE: VCOLFO1.BMP contains the circuit diagram.

The Effect:
Assuming that one VCO is producing a constant pitch, when mixed 
with a VCO modulated by an LFO, the phase relationship between 
the VCOs changes constantly.  A little lead, then a little lag, 
etc.  This causes a nice 'rotating' flange effect that can 
thicken the sound produced.  At higher LFO frequencies, a 
warbling effect is produced.  A level pot is provided to control 
the 'swing' and a frequency pot is provided to control the rate.
Two switches control the waveshape from a square wave to a near
sine wave.  These these controls and the switches in combination
with the pitch and pitch offset controls determine the particular
effect.  There is also an LFO cut-out switch.

How It Works:
The LFO is a 555 timer IC wired as an astable multivibrator 
(oscillator).  The square wave output on pin 3 is then fed to a 
very low frequency lowpass filter to provide a near sine wave 
output.  This 'sledge hammer' technique was used to simplify the 
circuit as well as attenuate the output of the LFO 555 timer.  
Varying the component values will produce other waveshapes.  The 
filtered signal is converted to AC by passing through a large in-
line capacitor and applied to the modulation input of the VCO 555 
timer IC.  The changing voltage on pin 5 of the VCO 555 timer
causes the trip point to change and thereby changes the period of
the oscillator and thus it's pitch.