RE: Pulse width

[markn]

Hi Guys,
Thanks for the interest in my project. I have the 555 timer set up with the following resistors; 10k from 5 vdc to pin 7, 100k variable from 5vdc to pin 6, jumper from pin 2 to pin 6, and a 15uf capacitor form ground to pin 2. I am able to control the speed of the sequencing with this setup, however, the pnuematic valves which i am controlling will stay open to long with this circuit. I was hoping to find a way to change the square wave pattern from the 555 to something more assymetrical---- off 1-2 secs. on .5 second.

 

[David Bridgen]

It doesn't sound to me as if you have the 555 wired corrrectly, but never mind.

You may know that the normal way of wiring the 555 in astable mode doesn't allow a duty cycle less than 50%. One way round this is to put a diode across the lower of the two timing resistors as shown in the diagram.

Keeping your value of 15uF for the C, if you make R1 a 22k resistor in series with a 47k variable, and R2 a 100k resistor in series with a 100k variable you will be able to adjust the on and off times independently around the figures you want.