[sdiy] twilight zone question

[Jason Proctor]

i'm starting to think that mike is right with his mad tl072 theory. 
consider the following -

+ input = 5v
- input = 7.5v
output = 0v (correct)

+ input = 0v
- input = 7.5v
output = 15v (????)

seems like the 072 spazzes if the input is < 1v. but i'm not 
understanding why i'm not seeing this everywhere. it's only when the 
0v comes off the protoboard, and i know the two grounds are fine. 
weeeird.



At 11:16 PM -0700 10/16/06, Mike I wrote:
>Hi Jason,
>About an hour ago I looked at Ken's site and the schemo for his clock
>divider module and some other divider module - both circuits use LM358 in
>the same way at the inputs..... I forget which CGS circuit you have, but
>either one should be able to be connected directly to the 0 to 5V outputs of
>the PIC processor (caveat: I don't know anything about this Blacet module,
>and I assume it puts out TTL-compatible logic levels). There should be no
>need for an intervening 0 to +15V level translation, the LM358 in the
>existing CGS board will do a 0/+5 to 0/+15 translation directly. I would
>advise against replacing the LM358 in the CGS board with TL072 though. If
>you look at the TL072 specsheets, the inputs should not be brought any
>closer than 3 to 4 volts of the most negative supply rail  (in your case,
>ground). The LM358 inputs can be brought right down to ground and it will
>still behave properly.
>Good luck,
>Mike