Jon said:
I have a 5 volt supply application where I want to have an output signal go
high and stay there after 5 or so seconds.
Every circuit I have seen using the ubiquitous NE555 timer recycles
eventually.
Is there a simple solution to the problem, or should I look for another
circuit for a time-delay?
Thanks in advance.
Use a monostable mode config..
Since a low signal is needed to
start the trigger, Use a PNP transistor
to pull down the timing cap to common while
the input trigger signal is being held down.
this keeps the cap from charging and holds the
timer from expiring.
when you release the input from the trigger so
won't the PNP transistor you put in there to keep
the cap from charging. when this happens, the
cap will then start to charge and force the 555 output
off when it reaches 2/3 of the Vcc voltage.
http://www.uoguelph.ca/~antoon/gadgets/555/555.html
Look at the monstable example there and add the
pnp transistor.. Collector to common , emitter to the
cap (pin 7/6),. use something like a 1k ohm resistor
to bias the transistor from the same signal used to
trigger the input.
P.S.
this is a retriggerable configuration, meaning, if
the input signal gets seen before the timer expires, the
pnp transistor will discharge the cap and start the time
off delay again with out any notice on the output..
if that is ok then you're all set.
I think for the most part, this is what you want.