T&L Publications

I'm trying to switch telephone voice signals using a 4066 switch and would like to know how to implement this safely without burning the ic.

Simple:

Find the IC manufacturer's web site and download the datasheet for the IC. Look at the "absolute maximum ratings" table. Do not exceed the maximum ratings for the IC.

Bob

Hi there,

I had to do something similar once but instead
of having to protect an ic from over voltage i
had to protect the input to an audio amplifier
from the same.

First off, since i was only interested in the
voice signal i capacitively coupled the signal
into the amp's input section. This eliminated
the sometimes high dc level present on the line.
Second, since the actual voice levels are so low
i was able to clip with two pairs of si
diodes: two pairs of two diodes in series and
the two pairs connected in antiparallel. This
meant the level was clipped to about +/- 1.4 volts
across the input to the amp (would be your ic).
This stopped the relativly high 'ring' signal
from overdriving the amp's input section.
Of course you also need a current limiting
resistor. If i rem right, i used 10k ohms, but
this was probably 10 years ago. Maybe 1k will
do it.


Take care,
Al

The 4066 (like most CMOS IC's) don't like signals that are outside their power suply's voltages.

To protect the IC from this "out of range" signals its usual to clamp the input to +V and ground (VDD & VSS) with a pair of diodes. You can use 1N914 / 1N4148 signal diodes or 1N400x (4001 to 4007) rectifiers).

The IC has internal protection diodes, but if you use external ones your circuit will work better in "dangerous" enviroments like phone lines, automoviles, etc.

Let's see if I can draw it in ASCII: