555 timer problem

[robertbarron]

The 555 monostable requires a low going edge to "trigger" it. If this signal remains low it will affect the timing since the "threshold" signal is ignored whilst the trigger signal remains low.
I need any simple way to take a low going trigger signal and convert it to a quick down up signal irrespective of how long the initial signal remains low. I have been playing around with simple RC circuits but without an oscilloscope I cannot get one to work.

 

[Alec_t]

Use a capacitor on the Trig input, like this:

 

[AnalogKid]

The internal trip point for the Trigger input is Vcc/3. The time for the capacitor to charge up to this is approx. 0.4 x R x C. As long as this is shorter than the output pulse (I'd say 25% for margin), the output timing will not be affected.

ak

 

[robertbarron]

The internal trip point for the Trigger input is Vcc/3. The time for the capacitor to charge up to this is approx. 0.4 x R x C. As long as this is shorter than the output pulse (I'd say 25% for margin), the output timing will not be affected.

ak

 

[robertbarron]

Thanks for trying to sort this out - but, as I said in the question, my trigger pulse IS longer than the monostable timed period and I was wanting the high to low transition to change to a high-low-high pulse so the monostable works properly even though the "switch" used kept a low output.

 

[robertbarron]

Use a capacitor on the Trig input, like this:
View attachment 26149

Thanks a lot Alec. I have tried all versions of your circuit but this one got by me. Again many thanks for setting me straight.

 

[AnalogKid]

Although my post is brilliant, it is too succinct for its own good.

Alec's post shows a differentiating network capacitor C1 and resistor R2, to shorten a long input pulse into a short triggering pulse. My post describes how to calculate C1 and R2 based on the output pulse width you want to be shorter than.

ak