[sdiy] 74xx series - what to do with them?

[Dave Manley]

On 11/17/2011 2:34 PM, Terje Winther wrote:
> A neigbour gave me some parts the other day, including some 4xxx series chips that I know how to use, but also some 74xx series chips that I am a bit unsure of how to make good use of.
> Anyone know any interesting synth circuits using 74xx series chips?

Hi Terje,

Most 74xx are just good for generic digital logic.  There are a few oddball
parts that go beyond simple logic functions.

You don't mention which family of 74' - there are bipolar and cmos versions.
Guessing these are the older bipolar version (74xx, 74LS, 74ALS, 74F) then they
will have 'TTL' compatible IO, and the main issue is that the output high
voltage (Voh min) is only guaranteed to be 2.0v.  The supply rail is also limited
to 5V.  Since a lot of synth circuits run on +/-12 or +/-15 volts the logic
levels are incompatible.  The low Voh mean you'll have problems with 74xx
driving a cmos input which typicaly requires Vih > 0.6*Vdd.

You can find appnotes online for interfacing between different logic families.

HTH,
Dave