AVR-Chat

Go for the LM324 (it has 4 opamps in a dil 14 and can go down to something  
like 200mV BELOW negative rail at the input ;-) ).
There are also single and dual amp per package versions (see datasheet)  
but it is so cheap most people simply use the quad.

You could make a negative voltage with a 555 or 7660, yes.
Look at the ICL7106/07 datasheet for how to make negative voltages  
relatively easily (you do not need the 7106/7 ic but it is a neat version  
for negative supply in the d-sheet).
I think the datasheets of all suppliers show the trick, but intersil and  
maxim sure do. Basically you make AC and rectify it (or you charge a cap  
and reconnect it, just a point of view difference), but look at the  
D-sheet.

As another solution you could maybe "lift" the input voltage with a biased  
diode or something, or maybe add a DC voltage to it, but all three options  
are really stupid compared to the lm324. Maybe you have some on a scrap  
board, yesterday i took this satellite receiver apart and there were like  
5 LM324 in there.

ST


On Thu, 31 Mar 2005 11:49:07 -0600, Chuck Hackett  
<egroupscdh@WhiteTrout.net> wrote:

> I'm using a 741 op-amp to condition an input signal.  It's setup like a
> threshold detector to detect a voltage level between .1 and .6 volts
> (adjustable).  It's my understanding (and experience) that the 741 can  
> not
> operate that close to one of its rail voltages so I need to supply it  
> with a
> negative rail voltage (I'm thinking -9v).  I have 12v supplied to the  
> rest of
> the circuitry and need to avoid having a second supply (remote  
> location).  I was
> thinking that I could generate -9v using a 555 timer, caps, R's and  
> diode to
> rectify a negative voltage.  Can someone point me to a web site circuit  
> for this
> using a minimum of components?  I assume I only need a very small  
> current.
> In the short term (this weekend) I need to stick with the 741 which I  
> have on
> hand but can someone recommend an inexpensive "rail to rail" op-amp that  
> would
> be better for my final configuration which would eliminate the need for  
> the
> second supply?
> Thanks in advance for any guidance you can give.  I'm mostly listening  
> on this
> list but I'm learning a lot.  Thanks to all contributors for the  
> resource.
> Someone on the list recommended "Embedded C Programming and the Atmel  
> AVR" to
> someone else.  I ordered the book and read it in short order.  I found  
> it very
> helpful in my "getting up to speed" <g>.
> Regards,
> Chuck Hackett