555 timer design

Let's see - according to the data sheet:

f_osc = 1.44/(R₁ + 2R₂)*C

Duty Cycle = R₂/(R₁ + 2R₂)

Does that do it for you?

If you solve the equations you'll see that R2 = 14400Ω, and R1 = let's see = 0Ω!

D'oh!

yes, that's right, for 50% duty cycle there is only one resistor. The connection is different than that for the more general case. Consult your data sheet for specifics.

Due to the fact that the high time is dependent on R1+R2 and the low time is dependent on R2 only, there is no way to achieve 50% duty cycle by using the standard circuit.

I don't know if the 555 bug book is still available but it had two or three circuits that can achieve the 50% duty cycle. On method used diodes (to select different resistors during the charge and another set during the discharge of the capacitor), one method used a different way of interconnecting the components (I'd have to go look for that book, it's still in a box from when we moved residence...about 2 years ago); the third method used a flip-flop (the 555 drove the flip-flop or divide-by-two counter). The third method produced the best results.

I'll have to go book hunting if you can wait. It must be in one of three boxes, and they're all full of books (no place for a shelf, unfortunately, my computer just took the last space).

This is one of the circuits. No, I haven't found the book. This is from memory but I think it's correct. You'll have to buffer the output (pin 3) since a significant load will affect the output voltage and, hence, the charge and discharge times.

When pin 3 output is high, the capacitor will charge through the resistor. When the voltage at pin 6/2 reaches 2/3 of V+, the output will go low. The capacitor then discharges and when the voltage at pin 6/2 reaches 1/3 V+, the output will swing back high and the cycle repeats.

Just remember, the first high-pulse will be slightly longer since the capacitor has to charge from 0V to 2/3 V+. For the rest of the time, the capacitor charges from 1/3 to 2/3 V+ so the frequency remains constant. Usual necessities are stable power supply, constant temperature, use of good components, etc.

Good luck.

There's a simple one resistor circuit in the (National Semi) data sheet - I would have posted it, but I think designing in a chip without reading the data sheet first is bad engineering practice. And we want him to learn the right way, right?

Bhankiii (are you related to wii? just curious. :-)), would you go ahead and post a link to that data sheet? You've piqued my curiosity. Thanks! -e

See figure 12.

Clever. Thanks.

Bhankiii

It's the same circuit that I posted, except that I put in the pin numbers instead of the pin names.

I always used a 555 operating at twice frequency, then a divide by 2 flip flop to get a square wave. I have heard of the other methods, but have never seen them.

Bill

Google NE555N and it should put you right.

The circuit shown below allows for duty cycles from nearly 0% to 100%. For a duty cycle of exactly 50%, T₁ = T₂, which means that R_a and R_b are equal. In this case, you can substitute 'R' for both R_a and R_b in the equation below.

Given: f = 50e3 Hz, C = 1e-9 F,

Let R_a = R_b = R,

R_a + R_b = 2R

Inserting in the equation for f, below, and rearranging terms we have

R = 1.49 / 2fC

R = 1.49 / 2 * 50e3 * 1e-9

R = 14.9e3 ohms, or 14.9 k

The standard 5%-tolerance resistor value nearest 14.9 k is 15 k. The standard 1%-tolerance resistor value nearest 14.9 k is 15.0 k.

Assuming C is exactly 1 nF and that both R_a and R_b are exactly 15 k, and substituting these values back into the original equation, your frequency will be:

f = 1.49 / 2RC,

f = 1.49 / 2 * 15e3 * 1e-9

f = 49.7 kHz. Only 0.6% less than your desired frequency.

Chances are your capacitor will have a much wider tolerance than this. For your diode, try using a 1N914 or similar. If you're in the U.S., these can be had at your local Radio Shaft store.

Hope this helps.
-e

Gentleman thank you very very much and I hope that whit a super like you the future engineer will make things happen.

thanks excellent

An additional note, just for fun: If you'd like to be able to freely vary the duty cycle while keeping the frequency constant, you can substitute a 30 k pot in place of Ra and Rb. Here the wiper of the pot takes the place of the node where Ra and Rb connect, and your diode would be connected between the wiper and one leg of the pot in the same manner as shown in the circuit diagram. If you cannot locate a 30 k pot (not a common value) you can replace the capacitor with a value that, in combination with the actual value of the pot you use, will produce your desired frequency of 50 kHz. Once you know the value of your pot you can rearrange the terms in the equations shown to solve for C. You may need to iterate this process several times before arriving at a practical combination of values. Note that if you try this approach, it would be advisable to insert a resistor in the Ra leg to prevent a direct connection between Vcc and the 'discharge' pin of your 555 should you happen to crank the pot all the way in this direction. Look on the 555 data sheet for info on a reasonable max current this pin will tolerate and size your resistor accordingly and with a bit of safety margin. Note also that the presence of this resistor in your circuit will alter your operating frequency, so it will be necessary to adjust the other component values accordingly.

-e

there are some software available for calculating the Capacitor and resistor values

try searching it in google

I'd like to design a 555 timer that can give me 50 percent duty cycle at 50Khz frequency with a capacitor of 1nF.(designing means that we have to get those R1 and R2 resistor) please can any one help me to get those resistor's values?

Regards

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Texas Electrical Wroks

Pakistan

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Best Regards,

Younis Sabir

Texas Electrical Works

Pakistan

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Yessss super of the supers I don't know the words to qualify this program and from there I really know how you are then I hope to write you any time I'll need your help.

Please his excellence can you do the same for a 16F84A PIC programmer.

Also amongst others,can you perhaps tell me how to make a file like a program bcoz I done my programming on C language;I can design some programs but I don't know How to make it work like this program you sent me bcoz we just use a C software to make this run and user can see all informations but I want him only to run a program like this one you sent me with out see how a program was made.

Thanks for everything and more hopefull from you.