AW: 2-comparator sawtooth generators?

Rick Jansen sscprick at
Wed Jul 5 22:13:17 CEST 1995

In message <2FF86A07 at MSMGATE.M30X.NBG.SCN.DE> you write:

> From practice: It works; I've built it. Of course there will be a slight
> overshot, that depends on the different delay times and the
> current limitation of the discharging device.

> BTW: There are millions of circuits manufactured & sold, that work on that
> principle - most of them are labeled "555" on their package. But the 555
> has other problems that make its use for musical VCO's difficult. Ask PAIA 

The overshot is not slight in this case. An LM311 will *never* be able
to stop the discharge of the cap in time (2.2nF, 3V, 50ohm). A 2-comparator
circuit is not useable for a sawtooth generator where the goal is an as
short as possible discharge time, I think.

A 555 as an astable multivibrator is an entirely different situation,
as its used to generate squarish waves, where both for the up and for the
down state the RC-function and a comparator determine the time. There is
no 'zero' reset time necessary anywhere here.

I had not thought about the current through the switch until now,
but I guess it exceeds the 4066's specs. 12.5 ohms and 5 volts gets
you 400mA initial current, which is rather large. Or do you get away
with this for short periods as with leds? (Anyone know what the current 
limit is for HC4066 switches?)

Rick Jansen
rick at
S&H's a module and s&h's looking good

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