"Franc Zabkar" wrote in message
...
On Sat, 22 Nov 2008 10:35:28 -0000, "Arfa Daily"
put finger to keyboard and composed:


"Franc Zabkar" wrote in message
. ..
On Sat, 22 Nov 2008 02:28:57 -0000, "Arfa Daily"
put finger to keyboard and composed:

Just a suggestion, but wouldn't it be a whole hell of a lot simpler to
just
use a 555 timer IC ? Just the chip, two Rs and one C (two Cs if you want
to
be pedantic and decouple the control pin) to make a monostable which
will
give utterly predictable results over a wide supply voltage and
temperature
range, and has a very simple timing formula of approx 0.7CR.

Arfa

Another option would be a 74LS123. Perhaps the OP's application
doesn't require precision, and perhaps he has a spare gate.

- Franc Zabkar

Granted. But as you say, the actual thresholds on TTL can vary widely from
family to family, which may give a huge discrepancy betwen the figure you
think you should get, and what you really do get. OK I suppose if it's
just
something for yourself, but could cause problems if you were going to do a
few of them.

I was thinking also, that the circuit the OP is describing is in fact just
a
(power on ??) delay with a buffer gate on the end, as there doesn't seem
to
be any mechanism to reset the timing network after the initial pulse.
Unless
of course, the OP's "common starting point" is in fact switched between
rail
and ground. You sometimes used to see tricks like this on older logic
boards, to provide a delayed reset to other bits of circuitry such as a
CPU
IC. These days, they tend to use dedicated reset ICs, which again, give
predictable and repeatable results.

Arfa

Wouldn't a power-on reset pulse of less than 1us be much too short?

For example, Micrel's MIC2774 Dual Micro-Power Low Voltage Supervisor
"generates a 140ms (minimum) power-on reset pulse".

- Franc Zabkar

Yes Franc, I think it would. But then the value of the OP's cap at 0.0022nF
seems unrealistically low, as well. I was thinking more in terms of 10k by
0.1uF when likening this to the old reset circuits ...

Arfa