555 Timer Circuit

[dirtyhanfri]

Hi

I'm building a cooling system for my CNC and I need a circuit to trigger a relay regularly.

I would like to adjust both timings, frequency and duration duty cycle.

Looking at different 555 circuits I found the attached one, Varying R1 sets the freq. and R2 the duration of the pulse, but I'm not sure about C value or where to start with R's.

Any ideas? I would like to move around 30 secs-2min for frequency of the pulse and 1-5 seconds for the duty cycle.

Thanks

EDIT: I found the formulas, quite simple calculations.

 

[synthiaks]

Im not sure if its not a combination of both that sets duty cycle and frequency.  A different value of one of the resistors will change both values it think.

I could be wrong though

 

[TheJames]

Arduino's are pretty cheap and would do this with precision and ease.

 

[synthiaks]

Arduino's are pretty cheap and would do this with precision and ease.

Sure or a microchip 8bit PIC but then you need to ASM or C and sometimes solder is the prefered programming language

 

[JohnRoberts]

A couple minutes seems long for an old school (bipolar) 555. The caps would need to be large and low leakage. Perhaps they make newer CMOS versions that could support larger value resistors and cheaper low leakage caps.

Back in the day I would have used something like a 555 at a reasonable shorter period and a digital divider (like a ripple counter) to get useful (minutes? long) periods with accuracy.

One timer+divider to generate the long period, and perhaps another for the shorter active cooling.

Of course this could be nicely done with a cheap micro but the overhead to learn how to program it up might be excessive for a single application.

I am an old analog dog, that only started programming micro's this century... I can't imaging not using them now, but a simple timer is a good project to learn on either way.

JR