555 Astable with diode "Delay On"

Thread Starter


Joined Aug 11, 2009
To obtain a "Delay On" the most frequent recommendation is to use a PNP transistor. Easy. But if controlling a higher voltage circuit, the 555 does not cannot raise the base of the PNP transistor high enough to turn it off.

Looking for solutions, the simplest I've discovered is here: http://clarkson-uk.com/555-timer/operation/frames3.html.

It works perfectly on a breadboard.

What I don't understand, though, is what is the purpose of the diode ?

Hoping for help (and understanding),
Mike Hains


Joined Mar 24, 2008
I think it is to discharge the capacitor when the power is turned off. Other than that I don't think it is needed.