This got me into confusion before, and it's still not clear, what duty cycl= e is. Meaning that I don't understand your answer. Yet. :-) The frequency is the time between pulses. The duty cycle is the percentage= of time the line is high. So I guess that a perfect square wave would hav= e a duty cycle of 50%? PWM is simply changing the duty cycle from 1 - 99% =20 Normally the frequency is 20 ms? (I've written PWM programs before, but of= course I just used the PWM functionality. Never "bit banging." So when you say "high duty cycle" do you mean the time the line is high com= pared to low? So like 75% would be high? And my other question, why do you need to invert a square wave? It doesn't= seem logical to me at the moment. =20 And also, I am going to try to put together your schematic on a breadboard.= (That's usually as far as I get with "projects" at the moment.) I think= I have all the parts, except for U2, I have to look up what that is. Thanks! Lindy ________________________________________ From: piclist-bounces@mit.edu [piclist-bounces@mit.edu] on behalf of Bob Bl= ick [bobblick@ftml.net] Sent: 17 December 2013 22:57 To: Microcontroller discussion list - Public. Subject: Re: [EE] Help configuring a 555 On Tue, Dec 17, 2013, at 11:26 AM, Peter Johansson wrote: > On Tue, Dec 17, 2013 at 11:39 AM, Bob Blick wrote: > > > Feel free to substitute any NPN transistor. > > Is the transistor used just to invert the waveform? Yes, since the 555 naturally generates high-duty cycles it's the easiest way to get short positive pulses. Cheerful regards, Bob -- http://www.fastmail.fm - The professional email service -- http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .