Make Electronics

Chips, Ahoy! 191

Experiment 19: Learning Logic

BAckground

The confusing world of TTL and CMOS (continued)

CMOS family:

40xx
The old original generation, now obsolete.

40xxB
The 4000B series was improved but still susceptible to
damage from static electricity. Many hobby circuits still
use these chips because they will run from relatively
high voltages and can power LEDs and even small
relays directly.

74HCxx
Higher-speed CMOS, with part numbers matching the
TTL family, and pinouts matching the TTL family, but
input and output voltages not quite the same as the
TTL family. I’ve used this generation extensively in this
book, because it’s widely available, and the circuits
here have no need for greater speed or power.

74HCTxx
Like the HC series but matching the TTL voltages.

74ACxx
Advanced version of HC series. Faster, with higher
output capacity.

74ACTxx
Like the AC series but with the same pin functions and
voltages as TTL.

74AHCxx
Advanced higher-speed CMOS.

74AHCTxx
Like the AHC series but with the same pin functions
and voltages as TTL.

74LVxx
Lower voltage (3.3v) versions, including LV, LVC, LVT,
and ALVC series.

As you can see, these days you have to interpret part num-
bers very carefully. But which family and generation of chips
should you use? Well, that depends! Following are some
guidelines.

What you don’t need:

1. Speed differences are irrelevant from our point of view,
   as we’re not going to be building circuits running at
   millions of cycles per second.


2. Price differences are so small as to be inconsequential.


3. Lower-voltage (LV) CMOS chips are not very interesting
   for our small experimental circuits.


4. Try to avoid mixing different families, and different
   generations of the same family, in the same circuit.
   They may not be compatible.


5. Some modern chip varieties may be only available in
   the surface-mount package format. Because they’re so
   much more difficult to deal with, and their only major
   advantage is miniaturization, I don’t recommend them.


6. In the TTL family, the LS and ALS series cannot handle
   as much output current as the S series and the F series.
   You don’t need them.
   What you should use:


7. The old 74LSxx series of TTL chips was so popular,
   you’ll still find schematics that specify these chips. You
   should still be able to buy them from sources online,
   but if not, you can substitute the 74HCTxx chips, which
   are designed to function identically.


8. The old 4000B series of CMOS chips are still used by
   hobbyists because their willingness to tolerate high
   voltages is convenient. While TTL or TTL-compatible
   chips require a carefully regulated 5 volts, the 4000B
   chips would handle 15 volts—and also delivered
   enough power to energize LEDs or even very small re-
   lays. Some hobbyists also have a nostalgic affection for
   the 74Cxx series of chips, which had the same pin con-
   nections as the TTL chips but could still tolerate higher
   voltages and higher output current. The trouble is,
   some of the 74Cxx chips are almost extinct, and while
   the 4000B chips are still available, they are considered
   almost obsolete.