On 2006-03-31 00:04:24 -0800, said:


yhan wrote:
I have here 7805 regulator ready for use. I just dont understand if a
12 volt circuit design can be operated on 5 volts.

I'm very sorry, I was quite careless. Like someones already mentioned,
it won't work (and actually the chip will burn) if the signal levels
are too high, even though u used proper dropping at the Vcc. You may
never use higher signal level than specificated...

Well, actually you can interface 12V CMOS to 5V TTL if you use current
limiting resistors between the two familys. For example you can have a
12V high/low feeding into a TTL input if you put a 1.2K resistor in
series so the input protection diodes will not be blown, you can output
TTL to 12V by using open collector buffers - these (7416s for example)
can handle up to 15V with proper design.

Back in the early days MOS (17V) circuits would be directly interfaced
to TTL by some clever power supply design - split the 17 supply to
+5/-12 with the TTL running on the +5 section, then use resistors to
interface the two families. Look for Gottlieb System 1 MPU schematics
for some ideas.

Don Lancaster's CMOS Cookbook covers this TTL/CMOS as well, and I think
the TTL Cookbook mentions interfacing as well.

John :-#)#
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