Re: How inaccurate is a 555 or 7555 REALLY?

WildIrish wrote:
Two questions. I'm a novice with 555's, some minor experience.
I need to develop a very small 555 timer circuit that runs on a 1.5v AA
battery.

1. Which 555 is out there that runs that voltage and small package?
(smd would
be fine as long as It has solder pads i can solder to under microscope)

2. I plan to calibrate it up to run at 420hz. I'm still unsure if 555
is accurate enough,
I have read some Maxim and National spec sheets and I'm unclear on
accuracy.

How inaccurate is it truely in percentage at 420hz? I really can't
afford more than .1%
inaccuracy, is there a way to reach these levels of accuracy using 555?
special
R and C to make that happen?

You could hand match R and C to potentially get that, but then it'll
change with temp and other parameters. So practically you won't
achiveve 0.1%

If it's just a fixed frequency you want then use a crystal oscillator
and divider. That can be made as small as or smaller than a 555
solution.

Alternatively use a tiny microcontroller in an SO-8 or maybe SOT-23
package. Internal oscillators are only 1% accurate or so, but if you
use an external crystal you'll get what you want. Use software timing
or a PWM to give you the frequency you want.

Dave :)

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