Building a 555 PWM Circuit for a Three-Channel Function Generator

[Jdo300]

Hello All,

I am working on making my own custom three-channel function generator with duty cycle adjust. I bought three of these electronics kit function generators (http://www.electronickits.com/kit/complete/meas/fg500k.htm) .

I want to build a project box with three 555 timers wired up as PWMs to vary the duty cycle of the output from the function generators. I looked around online for some simple circuits to wire the 555 timers and found lots of setups. All I know is that it needs to be in one-shot mode so that it only pulses once for every incoming square wave from the function generator. Also, I want to have the widest possible range for the pulse width as possible. The target range I'm shooting for is between 1 ms and the lowest pulse width that the 555 can handle. Could someone point me to a good reference circuit to use?

Thanks,
Jason O

 

[Panda]

Down load the 555 datasheet, IIRC they have a full set of application notes

 

[Jdo300]

Hi Panda,

I got a copy of the 555 data sheet here:

http://www.national.com/ds/LM/LM555JAN.pdf

It does have a simple diagram to wire up a PWM using the 555 (see screenshot) but I am not sure what size components to use to get the pulse width range that I am looking for. I did notice that there were some equations on the page (pg. 9) where I copied the picture from but I'm not sure which one applies specifically to my application.

Is there an online calculator out there to determine the components one needs? I have seen calculators for the astable mode but not the PWM mode.

Thank you,
Jason O

 

[Jdo300]

Hello All,

After looking over the diagram for the 555 PWM circuit in the datasheet, I'm beginning to think that I was using the wrong terminology in saying that I want to make a pulse width modulator. What I am really wanting is an adjustable duty cycle setup using a pot. I have a 200KOhm pot and I can plug in whatever cap value I need to get the desired range but could some one let me know if the diagram I posted from the datasheet is the one I need to use in this case?

I also found a website that has a calculator for determining the one time of the 555 in monostable mode:

http://www.electronics-lab.com/articles/basics/components/555calcs.htm

I am wondering if the C and R values in the calculator correspond to the same C and R values on the diagram I posted before. If so, then what do I do with pin 5, the modulation input? Would I just tie it to V+ or ground since I want to use my POT to vary the duty cycle manually?

Thanks,
Jason O

 

[Panda]

If I understand you... you want to have a variable duty cycle squarewave at a set of switched frequencies?

Using an Astable circuit the two equations are:

Duty Cycle = Rb/(Ra+2Rb)
Frequency = 1.44/(Ra+2Rb)C

What these two equations show is that by changing the duty cycle you will also change the frequency.

What you could try is to make an Astable running at a fixed frequency and Duty Cycle as high as possible.
Then Build a Monostable with a variable pulse width from slightly longer than the OFF period of the Astable up to the full period of the Astable triggered by the Astable output.
The Monostable output should then have a fixed frequency determined by the Astable with Duty Cycle determined by the Monostable Period.
Here is the worlds worst state diagram...

-----1_1---------1_1-------1_1-------- - Astable O/P
...!...!.....!... - Monostable Trigger Point
____1---1______1---1_____1---1______ - Monostable O/P

Not sure what Duty cycle range could be achieved using this method.
You could build this using a single 556.

 

[Jdo300]

Hi Panda,

I already have the supplies to externally trigger the 555 timers. I bought the frequency generator to give me better control of the frequency tuning (which would not be fixed since I'm doing experiments). I was just hoping that I could set the 555 up in such a way that it only pulses once for every input pulse of the function generator. The duty cycle would always be much less than 50% for the experiments that I am working on. I actually built a single 555 timer that I can tune the frequency and duty cycle with but, as you said, changing the duty cycle changes the frequency.

 

[Panda]

If you have a stable frequency input then just build an astable with variable pulse width between not a lot and the trigger period and trigger it from a square wave input.
The 555 Monostable should trigger off the negative edge. If it triggers off a logic 0 then you may need to sharpen your pulse through a high pass filter and inverter to create a negative glitch.

 

[Jdo300]

Thanks for the info. I was reading somewhere that the minimum pulse width for the 555 in monostable mode is 5 uS? This may work but ideally, I would like to get it smaller than that. Would you happen to know of any simple IC's that can take an input signal and adjust the duty cycle of the output? Ultimitely, the output would be driving some MOSFETs.