That duty cycle is going to be hard to achieve with a single 555=20 timer. You are far better off using a pair of 555 timers or the=20 smallest PIC that you have laying around. If you want to use a pair of 555 timers, set the first one as an=20 astable oscillator with a period that matches your desired repetition=20 rate (1 second?). Configure the 2nd 555 as a monostable timer with=20 your desired pulse width. dwayne At 09:28 AM 12/11/2013, Lindy Mayfield wrote: >I wanted to try to create an analogue circuit that would trigger a=20 >sonic range finder (SR04) so that I could check things out with a=20 >scope. It requires a 10us pulse to start it. (Then the output line=20 >goes high relative to the distance.) I don't think I have the=20 >ability (yet) to do much more with it though. The intent is to=20 >connect it to a Pi. > >What I wanted was a 10us pulse every second or so, and I found a=20 >handy 555 laying around. However, using the nice calculator I found=20 >below, I couldn't manage to get anywhere near the results I wanted=20 >no matter what I put in. (The largest capacitor I have is 100=20 >uF.) Is this possible? I know it should be. > >Thank you! >Lindy --=20 Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax www.trinity-electronics.com Custom Electronics Design and Manufacturing --=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 .