Help with 555, Mark/Space

deepak_3112 · Nov 3, 2011

sir, i want to generate 1khz frequency using 555 timer from 0% to 100 % duty cycle.
When im using the circuit with diode in parallel with R2 i am able to get duty cycle, but the frequency also is getting change.
so, pls can u tel me the solution for it.

ChristianCMcKay · Nov 4, 2011

With all due respect, I feel like you're slightly confused about what duty cycle means. Maybe I'm wrong, but on astable multivibrators, the duty cycle is usually fixed unless there's a potentiometer in place, but even still with a diode in parallel with R2, you will only get a duty cycle less than 50%.

Also, to ask a question in a new thread, you can click here. I've only been dealing with electronics for about 2 years now. The other people on this forum are much more helpful.

duffy · Nov 4, 2011

Deepak - unfortunately, the 555 will not do this alone, it is only good for about 50% to 100% duty cycle, and the frequency will vary.

To get what you want, you need to add a comparator. Here are 2 circuits that show this:
https://dc166.4shared.com/img/vw5P9d2-/s7/VARIED_REP_RATE_DUTY_CYCLE_W_5.JPG
https://www.kennethkuhn.com/students/ee431/lm555_squarewave.png

jpanhalt · Nov 5, 2011

An obsolete application note AN170 describes how to get duty cycles between 5% and 95%.

Source: AN170
...it becomes necessary to insert a diode in parallel with RB, cathode toward the timing capacitor. Another diode is desirable, but not mandatory (this one in series with RB), cathode away from the timing capacitor. Now the charge path becomes RA, through the parallel diode into C. Discharge is through the series diode and RB to the discharge transistor. This scheme will afford a duty cycle range from less than 5% to greater than 95%. It should be noted that for reliable operation a minimum value of 3kW for RB is recommended to assure that oscillation begins.

That circuit is quite common. The circuit without a diode, but using the output at pin 3 is susceptible to changes in frequency and duty cycle with changes in voltage. It is a tiny bit simpler circuit than the conventional circuit with a diode, but I prefer not to use it because of its poor stability.

Please post your circuit, if you have continued problems.

John

Edit: Since I already downloaded AN170 to ETO for a previous post, here is the link:
https://www.electro-tech-online.com/attachments/555an-pdf.52952/

duffy · Nov 5, 2011

^ That one still changes the frequency with the duty cycle.

DerStrom8 · Nov 7, 2011

Hi.

I have used the following circuit from RMCybernetics.com on many occasions, and have had great success with it. It uses a 555 timer and a 393 op-amp to adjust the frequency and duty cycle independently of each other. I believe this is what you're looking for:

**broken link removed**

Parts List:
IC1 LM555
IC2 LM393
R1 10k
R2 10k
R3 2.2k
R4 10k
VR1 1M
VR2 10k
C1 47nF
T1 IRF740

It works great as a light dimmer, motor speed controller, or transformer driver. There are a lot of other potential uses, as well. What do you plan to use yours for?
I hope this helps!
Regards,
Der Strom

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Help with 555, Mark/Space

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