Electronics > Beginners
555 PWM Adjust Duty Without Changing Frequency *Closed*
Kippre7790:
This thread has been closed as I have reopened a new thread dealing with the entire project. I will not be looking here after this point. head to the link below to follow the thread:
https://www.eevblog.com/forum/projects/diy-variable-multi-output-power-supply-project/
Hello, I am trying to use a 555 timer to create a pulsed output. the problem is that I want to change the width of the pulse based in an analog voltage input without changing the frequency it's set for. From my slim experience with 555's pin 5 is a control pin, and when I put a resistor of some value to ground it does lower the duty (all the way to 0%) but also lowers the frequency and can slow down to almost half the target frequency.
I came across this circuit online that is really simple in its construction and works almost perfectly. current flows at one resistance to charge the battery and flows through another to discharge. But it uses a POT to adjust the width, Problem is I need it to adjust with a voltage and not with a turn of a knob which would be easy normally but it uses all three pins which just made me go nuts to try and figure out how to do this with say transistors or something. (spent hours working on it and got nowhere)
So I need a simple circuit that based on an analog voltage will gradually "turn" the resistance/flow up on one line and down on the other. I tried using mosfets and it worked well in simulation but I don't know if it would work in real life and I don't know much if anything about fets and have tried to stay away because of lack of explanation or understanding on how exactly they work. (yes they're voltage controlled and all... but a transistor is easy, it's just the current on the base multiplied by its beta/gain, which is a simple stat to find on any transistor.
If there is a completely different way to make the whole circuit to get what I want I'm game for that too!
paulca:
I think you can use two 555s. One set to create pulses at the right frequency that trigger the second to generate the pulse of the correct width. As the second does not care about the off time, only the control voltage is needed to set the pulse length.
It might have been Ben Eater's clock I seen this in.
Ian.M:
Yes. Use two 555 chips (or one 556), one 555 as a fixed frequency oscillator producing a narrow low-going pulse, the other as a monostable with an on time adjusted by its CV input, triggered from the other's pulse output.
Kippre7790:
well crap, never thought to use a monostable 555... |O
should work just fine, will let you know if it works for me.
brucehoult:
--- Quote from: Kippre7790 on February 21, 2020, 01:09:01 pm ---If there is a completely different way to make the whole circuit to get what I want I'm game for that too!
--- End quote ---
That completely different way would be to use a small microcontroller such as the ATtiny25/45/85 in an 8 pin DIP package. There are 6 I/O pins which can be configured as all digital input or output (including PWM), and up to 4 analog inputs with 1024 different levels and up to 15k samples per second. Runs by default at 1 MHz, or up to 20 MHz at higher voltage/current. They have 2, 4, or 8 KB of program space, and 128, 256, or 512 bytes each of RAM and EEPROM.
Here is 10 ATtiny25 for $14.76 on Amazon. https://www.amazon.com/ATMEL-ATTINY25-20PU-ATTINY-20MHZ-pieces/dp/B00MMYSK86
Maybe you can find cheaper, but to be honest I just go for the 85 because they cost basically the same for four times the capacity.
Your task will require one analog input, one PWM output, and just a few lines of program code.
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