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# Running a 555 timer? Answered

So, I have this program that I got online that simulates a 555-timer. Basically, if you want it to put out X Hz in Astable mode, it'll tell you which capacitors to use and stuff. It'll also show you how you can modify the duty cycle.
I'm wondering if anyone can explain something to me: When I run the program in astable mode, the duty cycle must be above 50%. I want to power an LED that flashes very briefly, but has a relatively large interval of being off until if flashes again. Something more along the lines of a duty cycle of maybe 5%-20%.
First, could you explain why the program constrains the value like this? Is it because the 555 can't run at a duty cycle that isn't greater than 50%? If that is the case, why can't it?
Second, if I can't run it like that, what would you propose I do to power the LED in the manner I described?
btw, if you want the program follow this link: http://clarkson-uk.com/555-timer/
It's actually a pretty cool program! :D

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## Discussions

To quote Wikipedia,

"To achieve a duty cycle of less than 50% a diode can be added in parallel with Rb towards the capacitor. This bypasses Rb during the high part of the cycle so that the high interval depends only on Ra and C."

This does work, but you have to account for the how the ~0.7V drop across the diode affects the duty cycle. Other than that, the program works well for the standard setup, so good luck adding the diode.

555 duty cycle can be ~50% to very high, and frequency can be very low to very high.

The trick is, you have to invert the output from the 555 because the basic astable oscillator sources and drains out the same components, hence the 50% limitation. Just use a transistor to make the high/low inverted and design the circuit to be 95% on.