# 555 timer

1. Sep 30, 2008

### Winzer

I need to build a timer(using a 555) with a frequency of about 1KHz and a duty cycle of about 25% to provide my clock signal. So the 555 is obliviously in astable mode. From my engineering data sheet:
$$D=\frac{R_B}{R_A+2R_B}$$
$$f=\frac{1.44}{\left(R_A+2R_B\right)C}$$
where C is my capacitance value and the two R's my resistance values. This should be a strait forward calculation right?
then from the above equations
$$R_B=\frac{D 1.44}{C f}$$
and$$R_A=\frac{D 1.44}{C f}\left[\frac{1}{D}-2\right]$$
right?
I get $$R_B=36k\Omega$$ and $$R_A=72k\Omega$$
This is a capacitance value of $$0.01\mu F$$
Does this look correct?
Thanks

2. Sep 30, 2008

### Redbelly98

Staff Emeritus
Looks good.

72 kΩ isn't a standard resistor value, but you could just use two 36k's in series.

3. Sep 30, 2008

### dlgoff

Or a potentiometer. You would be able to adjust out the tolerances from the other resistor and capacitor.