[sdiy] Low Frequency Square to Sine Waveshaper

[Magnus Danielson]

From: Kyle Stephens <lightburnx at yahoo.com>
Subject: [sdiy] Low Frequency Square to Sine Waveshaper
Date: Sun, 25 May 2008 20:33:44 -0700 (PDT)
Message-ID: <402247.99465.qm at web33904.mail.mud.yahoo.com>

> Using a 555 timer and a 4024 ripple counter, I want to
> make an LFO with several outputs at 1/2n of the master
> clock frequency. At lowest, this'll go to about 0.1 Hz
> (not the clock freq but at the lowest divisor, 1/6 or
> 1/8, depending on how many outs I ultimately use).
> 
> Square waves are fine and dandy, but I'd like some
> sine wave action too. For getting consistent shaping
> (not absolutely necessarily accurate, but cool if
> that's doable), what would work best? Would RC lowpass
> filters alone do the job?

No, but you could do it another way around. Run the 555 at a higher frequency
than you wish, say 32 times higher and then use the ripple counter outputs as
both sources to the square and sine-waves. However, to create a sine-wave you
weight together 5 squares (the fundamental and four overtones) with resistors
and then put a capacitor to ground (or over the negative feedback of a
buffering op-amp) on the output for final rounding off effect. To create a sine
of half the frequence, just add the same output network.

The resistor values can be calculated. More resistors can be used to achieve
better initial shape.

Cheers,
Magnus