555 timer question: how to calculate required resistors and capacitor to achieve desired frequency?

Monostable means you get out one pulse when the circuit is triggered. I think you want an astable circuit. Try R1 = 1k, R2 = 50k, C1 = .1 uf or pick your own at,

Well, you can guess the values using this calculator. Just enter the 2 resistors and cap in the upper 3 boxes, and the time intervals and frequency will be displayed in the lower 3 boxes.

-Bill

The resistors obey the equation:

Ra + 2Rb = 1 / (frequency x C x 0.693)

Here is a procedure you can use to get those resistors. Like others, I'm assuming you mean astable operation.

I'm using the Texas Instruments data sheet, where: Ra is the resistor between pins 4 & 7 (reset & disch) Rb is the resistor between pins 6 & 7 (thresh & disch) C is the capacitor between pin 6 and ground

You'll need to:

1. pick a capacitor

1. calculate the value of (frequency x C x 0.693) (Remember to divide by 1,000,000 if the capacitance is in microFarads) Take the RECIPROCAL of that number ("1/x" key on a calculator), and call this number S2

2. decide on what duty cycle (%DC) you want. It must be more than 50.

1. calculate the sum Ra + Rb = S2 x %DC / 100 and call this number S1

2. Now that you have values for S1 and S2, we can calculate Ra and Rb: Rb = S2 - S1 Ra = S1 - Rb

If you find you want to use larger resistors, choose a smaller capacitor and repeat the process. Or use a larger cap if you want to use smaller resistors.

Hope this helps,

Mark

You sure you aren't wanting an "astable" at 140Hz?

Using the 555 tool at

You will still need to "select" a component to start with. Cap Ct is easiest and experience suggests that for 140Hz a good starting value is 470nF. Plug in the supply voltage and then experiment with Ra and Rb values making sure they are equal if you want 50:50 duty cycle. Just so you get a bit of practice start of with Ra = Rb = 8.2K and go from there....

You can select another Ct value and experiment with Ra, Rb again.

Uh, that page seems to give erroneous results. According to the Texas Instruments' 555 datasheet,

T1 (or T_high) = 0.693 * (Ra+Rb) * C

The ucl.ac.uk website has dropped the "Rb" term in their equation.

Mark

You're right, I didn't pay much attention to the text alongside the circuit. I know it has been like that for years so I thought it would have all the bugs out of it by now.

Tony Van Roon's 555 tutorial

looks to be a pretty comprehensive teaching aid so maybe the OP can get more info from it.

Your can use this program to calculate both monostable and astable circuits for the 555 timer:

Funny how tough it is sometimes to be 100% error-free, whether it's program code or just writing some text. It's mostly luck that I bothered to check at all, but somehow the reference to a "50:50 duty cycle" in your earlier post rang a warning bell in my head, because I vaguely remembered that this was impossible with nonzero resistor values.

Regards,

Mark

Yes, you can't get 50% duty with that configuration, but it can be close. Ra needs to be a small fraction of Rb for near 50% DC which means most of the resistance is in Rb. The values can be closely approximated from Rb=0.693 / FC or C=0.693 / FRb and then just use 5 or

10% of Rb for Ra. But the error is big enough that .693 can be rounded to 0.7. Also, Ra should be 1K or larger.

-Bill

None of you seems to have noticed the diode across Rb. That takes it out of the circuit for the charge half-cycle. The cap charges through Ra and D1, and discharges through Rb.

Oh, by the way: a link might be useful:

Were you guys looking at a different page? How did I get to that one? ?:-/

I felt so clever some decades ago when I came up with this on my own that I almost dislocated my shoulder patting myself on the back. :-)

Cheers! Rich

Yes, you are right. I forgot to look at the drawing. But actually the diode isn't needed for 50% DC if you put Rb in series with pin 7, and tie the cap from junction of Ra and Rb to ground. It's on page 10 of the data sheet, but the ratio of Ra:Rb has to be 51:22 which makes selecting the parts a little difficult. The diode is the easy way.

-Bill

You're right. I had also emailed them to tell them about their "error", and they replied pointing out the diode just as you did.

Yep, that's the page.

Cheers,

Mark

...

There's another one that I learned of quite recently - from one of the posters in one of the electronics groups - it's where the output is fed back to the timing cap, and that node goes to both threshold and trigger, but it only works with a CMOS unit. It's the "less accurate" one here:

Cheers! Rich

I can give you the formula that you need but you'll have todo the math. formula to find the frequency of an rc network is Fsub c =1 over 2 x pie x RxC