AINTME said:
I think this will put me on the right track - it is a retriggerable
oneshot I'm after - I did not know what it was called - having a name
for it will help me with my search and also the fact that you say
that I need extra components other than the timer.
Thank you for your help!
By combining your posts, I understand it as this:
You want a (+) output as long as the input is (+)
and it must stay (+) for 45 seconds after the
input drops. There must be no gap, no matter how
small, in the (+) output when the input signal
drops. You might be able to get away with oring
the 555 output with the input, but the brief
transition time of the 555 from non-triggered to
triggered when the input drops could create a gap.
My suggestion is that you add 3 transistors and support
components to the 45 second 555, like this:
PNP
+12 ---------------------+----------------- ----+
| e\ /c |
| --- Q2 |
| | |
[10K] [1K] |
| | |
+-------------------+ +---> Out
| | |
/c |c |
+----+-------+---| NPN / |
| | | \e Q1 +------| NPN |
[.1uF] [100K] [D1] | | \e Q3 |
| | |a | [3.3K] | [1K]
| | | | | | |
| | Input-+---- | --[555]--+ | |
| | | | |
Gnd --+----+-------------+-------------------+------+
(+) on the input will turn Q1 on through D1. That will turn
Q2 on, so you get a (+) out. When the input goes (-), the .1 uF
in parallel with the 100K will hold the base of Q1 (+) briefly,
keeping Q1 (and therefore Q2) on while the 555 triggers. That way,
there will be no dropout of the (+) output during transition. D1
prevents the (+) on the base of Q1 from interfereing with the
(-) trigger on the 555, so the 555 will trigger when the input
goes (-). The 555 output goes (+), which turns Q3 on. Q3 turns Q2
on, so the output stays (+). When the 555 times out 45 seconds
later, Q3 (and therefore Q2) turns off, and the output goes (-).
Ed