CGS03

Psycho LFO

Courtesy/Thanks to: Ken Stone

Description: PCB's for all Ken Stones products can be bought on his website:
CGS.
from cgs site:
This is a bizarre little LFO that produces a wide range of pseudo random effects. It is
essentially a reworked version of my 1984 "fluorescent simulator" for model railways, and
came about when I was wondering what I could do with a left-over PCB from this project.
The effect was so good, I've designed a PCB specifically for it.

The circuit consists of several distinct blocks. The first is the collection of square
wave oscillators, three of which run at fixed though different frequencies. The fourth is
variable between much slower than the others to much faster than the others. These
oscillators are all based on single schmitt trigger stages of a CMOS 74C14, 40106 or
similar.

The second block is a traditional op amp inverting mixer stage. Signals from the four
oscillators are mixed through 560k resistors, the overall gain controlled by a 100k pot
in the feedback path of the op amp.

What is interesting here is that in order to keep the output signal of the mixer positive
without the need for an additional inverting stage, the chip used to build the four
oscillators is powered from the negative rail, it's positive power pin connected to 0V
and its earth pin connected to -15V. Take special note of this, because it is an unusual
way to power a digital chip, and inadvertent poking with a logic probe powered from the
positive rail could cause you grief!

The third stage is a simple glide circuit consisting of a potentiometer, a capacitor and
unity gain voltage follower. The glide range is quite adequate with a 100k pot, though
some might like to experiment by putting an larger value here. The switch is to allow
easy switching in and out of a preadjusted glide setting. It can be omitted, and a link
soldered in its place on the circuit board. My prototype had the switch, though the
modules I have made since do not.

The power supply is decoupled using a pair of 22 ohm resistors and 10 mfd capacitors. The
two unused sections of the chip are tied as per good CMOS practice.

Parts list
This is a guide only. Parts needed will vary with individual constructor's needs.
If anyone is interested in buying one of these boards, the price is US$5. Check the PCBs
for Sale page to see if I have any in stock.

Notes:
A different effect can be achieved if the four 560K resistors are replaced with differing
values. Try a 1R, 2R, 4R, 8R set, where R is at least 100k. You may need to reduce the
value of the level pot if you do this.
Want it to run slower? Try increasing the 2.2mfd capacitors in the oscillators to 4.7 mfd
or even 10 mfd.
If you really wanted to, speed controls could be added to the other oscillators.
Ferite beads can be used instead of the 22 ohm decoupling resistors if desired.
Remember, the digital chip (40106) is powered from the NEGATIVE rail in this project.
Take care if connecting a logic probe or if adding external logic for whatever reason.




Part Quantity
Capacitors
100n 1
2u2 25V 5
10uF 25V 2
Resistors
22R 2
1k 1
18k 1
100k 1
150k 1
220k 1
560k 4
Semi's
40106 1
LM358 1
Misc
wire
100k Lin pot 1
100k Log pot 1
1M Lin pot 1
Knobs 3
SPST switch 1
PCB 1
Jack 1