I am building a standard 555 astable pulse generation circuit:



I am aware that the mark and space qualities of the wave form are set by the relationship of R1, R2, and C1, as per these formulae:

Mark Time = 0.7 × (R1 + R2) × C
Space Time = 0.7 ×(R2) × C

Because "space time" includes "mark time", it isn't possible to use the above circuit to create an output that is space > mark.

My question: is it possible to create a space > mark output from a 555 circuit, and if so, how to do it? If it isn't possible, is there a similar circuit that will permit this sort of output?

Thanks in advance!

 

I am building a standard 555 astable pulse generation circuit:

View attachment 216326

I am aware that the mark and space qualities of the wave form are set by the relationship of R1, R2, and C1, as per these formulae:

Mark Time = 0.7 × (R1 + R2) × C
Space Time = 0.7 ×(R2) × C

Because "space time" includes "mark time", it isn't possible to use the above circuit to create an output that is space > mark.

My question: is it possible to create a space > mark output from a 555 circuit, and if so, how to do it? If it isn't possible, is there a similar circuit that will permit this sort of output?

Thanks in advance!

It is important to note that Tm must be greater than Ts since R1 cannot be 0

(the minimum is 1k

).
Many circuits require Tm and Ts to be about equal. This is achieved if R2 is much larger than R1.

 

Google PWM 555

https://circuitdigest.com/electronic-circuits/555-timer-pwm-generator-circuit

 

Thanks, @hexreader - I think that will do the trick!

 

hi jim,
This is a simple way, add an inverting transistor.

E

 

hi jim,
This is a simple way, add an inverting transistor.

E

Oh Eric...how come I didn't think of that..come on Bayo...think...

 

Google PWM 555

https://circuitdigest.com/electronic-circuits/555-timer-pwm-generator-circuit

Thanks! Building a PWM generator was a great suggestion, as it does exactly what I want and is going to be an excellent solution for me. Thank you for suggesting it! Thanks, too, for linking to that particular PWM schematic, which explained how to set frequency as well as duty cycle.

hi jim,
This is a simple way, add an inverting transistor.

E

Thanks! It seems I won't need this at the moment, but that's very clever and I will remember this for the future.

 

Hi,

Another way to vary mark to space ratio is a diode (or two diodes) around R2.

 

Here is a version of the diode method mentioned in #8 above. Pots P1 nd P2 can be combined into one pot for a version that varies the duty cycle while holding the frequency approximately constant.

Someone on this forum has a large 555 circuits collection.

ak

 

Here is a version of the diode method mentioned in #8 above. Pots P1 nd P2 can be combined into one pot for a version that varies the duty cycle while holding the frequency approximately constant.

Someone on this forum has a large 555 circuits collection.

ak
View attachment 216709

@Wendy

 

I felt a disturbance in the farce. You bellowed?

If you wouldn't object to using a comparator I have a 555 circuit the can do between o% to 100% duty cycle with an absolutely rock solid frequency, Interested?

 

Oh, what the heck.

LEDs, 555s, Flashers, and Light Chasers
Chapter 5
The 555 and PWM

Good enough?
I like to design as a hobby, and will do custom circuits if asked nicely.

Wendy's Index

This circuit is a voltage controlled PWM generator.

 

Thank you, @Wendy ! For my current needs, the compact 555 PWM circuit shared upthread is perfect! I made a version of it that fits on a 15mm square PCB, including the pot, and it’s great for my needs. Thanks for these suggestions, though - I’ll keep them in mind for future projects.

 

Upload the PCB layout? With a proper write-up it could be a completed project, I could help with that too.