Hi all

I am tying to design a 555 circuit that can control the duty cycle by an input voltage. Is this possible?

I have looked into many 555 circuits in the pass two days, but non of them are voltage control.

Thanks in advance

 

Set up a standard astable with two resistors and one cap.

Give an AC voltage input on pin 5 (CV) and the freq will change within a range of 20-50%(or so) from the capacitor set astable freq. Duty cycle changes some as well.

This is a Freq modulation scheme and has much less affect over the output than other control methods

 

Hello,

In post 6 of this thread by Bill_Marsden you will see a PWM circuit:
LEDs, 555s, Flashers, and Light Chasers

The control voltage will be about 1/3 Vcc to 2/3 Vcc at the comparator input.

Bertus

 

Set up the standard 5-95% variable PWM circuit that uses two diodes and a pot.

Substitute a digital pot for the variable resistance one and you would have a voltage controlled duty cycle 555

 

Does this help??

http://www.dprg.org/tutorials/2005-11a/index.html

 

As Bertus said, LEDs, 555s, Flashers, and Light Chasers has it in chapter 5.

The frequency remains stable, and the duty cycle can be adjusted from 0% to 100% with tweaking of the resistors. It is fairly linear to boot, but if it isn't linear enough it can be made more so with a transistor/resistor addition.

The rest of this article also has many examples how to use this configuration in other circuits.

 

Thanks for all the reply~

I think The circuit Bill Marsden posted is what i am looking for.. Thanks!!

 

A LM393 is a better comparator, since it is an 8 pin dual.

 

some more questions.

I was just looking at the comparators , and they can not sink too much current. Compare to the 555, it can sink at least 100mA.
I can easily throw a BJT or MOSFET into the circuit and make this work, but i am trying to make this circuit as cheap as possible.

is there any other ways that will make this possible? I don't really care about remaining the frequency stable. I just need the duty cycle change over voltage.

John Lau

 

You did notice the transistor driver? It will go to 100 ma easily, or more, even though it is a PNP device to Vcc. Just remove the LED and resistor on the emitter. Are you needing to ground?