Re: How inaccurate is a 555 or 7555 REALLY?


WildIrish wrote:
.002% would be good enough for me.... but i don't know where to find
examples
on making a tiny quartz crystal divide down to 420hz in a tiny package
(I don't have more than about 3/4" box to fit things into.) or any
other component
oscillators that can reach 420hz with the least number of components.

Any basic circuit diagrams online that someone can point me to would
be greatly appreciated! Sine wave output preferred, but I think square
will do
if that ads complexity (I bet it would). Again please remember I'm a
novice
with minor experience :)

32768Hz divided by 78 gives you 420.103Hz, which is 0.0244%.

That is a seven bit divider - which you can set up with a single
74HCT40104

http://www.ortodoxism.ro/datasheets/philips/74HCT40103D.pdf

which is availalble in a 10mm by 6.2mm surface mount package - rather
smaller than your 19mm square target space

10MHz divided by 23810 gives you 419.992Hz which is rather better.

If you poke around the standard crystals available off the shelf from a
broadline supplier like
Farnell you should be able to find something gets you closer to 420Hz -
10MHz is pretty much the the worst possible place to start.

Dividing by 23810 calls for a 15-bit divider - two 74HCT40103 parts in
series, or a single programmable logic part which you would be able to
find in a 44-pin square package - the first example I came up with
occupied a 12mm square., which is about the same as a pair of surface
mount 40103 parts. Phil Alison should be able to show me up by finding
something smaller.

--
Bill Sloman, Nijmegen (but in Sydney at the moment).

.