# How to build an astable (or other) 555 circuit with a mark/space output that is "space > mark"?

#### redeyedjim

Joined Jul 14, 2018
33
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?

Joined Jan 24, 2017
176
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?

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.

#### redeyedjim

Joined Jul 14, 2018
33
Thanks, @hexreader - I think that will do the trick!

#### ericgibbs

Joined Jan 29, 2010
12,226
hi jim,
This is a simple way, add an inverting transistor.

E

#### Attachments

• 115.3 KB Views: 10

Joined Jan 24, 2017
176
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...

#### redeyedjim

Joined Jul 14, 2018
33
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.

#### Daniel Sala

Joined May 28, 2015
65
Hi,

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

#### AnalogKid

Joined Aug 1, 2013
9,043
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

#### djsfantasi

Joined Apr 11, 2010
7,552
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

#### Wendy

Joined Mar 24, 2008
22,328
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?

Last edited:

#### redeyedjim

Joined Jul 14, 2018
33
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.

#### Wendy

Joined Mar 24, 2008
22,328
Upload the PCB layout? With a proper write-up it could be a completed project, I could help with that too.