Society of Robots - Robot Forum

Electronics => Electronics => Topic started by: BANE on May 08, 2009, 05:41:11 PM

Title: pwm w/ 555
Post by: BANE on May 08, 2009, 05:41:11 PM

Hello,

I'm trying to make a circuit for my robot fan system.  I basically want the two micro fans to run at two speeds; 100% and 50%.   Running at 100% I will just apply 5v to a N channel mosfet from one pin (B) from my microcontroller.  At 50% i want to have a 555 timer provide PWM to the same N channel mosfet from another pin (A).  I think this will be simple.  Here is a circuit for the 555 w/ mosfet (http://www.dprg.org/tutorials/2005-11a/2005-10a-004.gif)
I want the 555 to run on 5v and the fans to run on 9.6.  Also I don't want to have a pot in it, just a straight resistor for the end product.  The question I have is at what resistance do i need to have on the trigger to produce a 50% duty cycle?  Is there a formula?

being that the pot is 100k can i assume 50k will work?

thanks,

Bane

Title: Re: pwm w/ 555
Post by: Soeren on May 09, 2009, 03:32:53 AM

Hi,

Since you only want full speed and 50%, your circuit is quite overkill.
You can do this with one transistor for 100% and another transistor for 50% where you connect a resistor or a zenerdiode of a suitable value in series with the fan.

Title: Re: pwm w/ 555
Post by: BANE on May 09, 2009, 09:41:13 AM

Gosh, I'm always overdoing things :P
I'll draw something up hopefully more simpler

Bane