# Generating a frequency without changing duty cycle

Thread Starter

#### Zotto

Joined Mar 3, 2016
45
Hey I am trying to design a frequency generator ranging from .25hz to 25hz approximately, while having a constant duty cycle that is as small as possible, preferably ~5%. The design I am currently using includes a 555 timer, but I am not sure if it can produce what I am looking for. Would really appreciate your advice. Thanks.

Here is the circuit I am using: http://www.instructables.com/files/orig/FJJ/V35I/GBF5KOPV/FJJV35IGBF5KOPV.jpg

#### GopherT

Joined Nov 23, 2012
8,009
while having a constant duty cycle that is as small as possible,
Do you want the duty cycle to remain constant while frequency is changed AND as small (low) as possible?

Or,
Do you want the duty cycle to remain constant while frequency is changed AND the circuit to be as small as possible?

#### crutschow

Joined Mar 14, 2008
34,063
If you just want the duty-cycle to remain low (short pulse) that is fairly easy.
If you want the duty-cycle to stay at a constant value, that's more difficult but doable with a counter.

#### GopherT

Joined Nov 23, 2012
8,009
@Zotto,

Also, when you mention duty cycle, do you mean the ON time (e.g. 10 mSec) or do you mean the percent of time it is on vs. time of a full on/off cycle 40%).

Thread Starter

#### Zotto

Joined Mar 3, 2016
45
Thanks for the replies, I need the duty cycle to be low at all times, simplicity would be nice. For instance it doesn't need to be a precise 5% duty cycle all time I just need the pulse to be short, and gopherT i mean high 5% of the 100% cycle, so short pulses.

To clarify my design, I need constant short pulses, where the frequency is variable using a potentiometer, but using the pot has no effect on the pulse width, i.e. changing frequency while maintaining a constant duty cycle.

#### dannyf

Joined Sep 13, 2015
2,197
An RC oscillator with a discharge can have very short duty cycle - it is a typical rc oscillator + a diode.

#### bertus

Joined Apr 5, 2008
22,264
Hello,

You have a contradiction in your requirements.
You say you need short pulses that remain constant while the frequency changes.
With a constant pulse width and changing frequency, the duty cycle will change.
With constant duty cycle and changing frequency, the pulse width will change.

Bertus

#### atferrari

Joined Jan 6, 2004
4,751
@Zotto,

Also, when you mention duty cycle, do you mean the ON time (e.g. 10 mSec) or do you mean the percent of time it is on vs. time of a full on/off cycle 40%).
If you take time to answer the question above you could then say exactly what you would like to have.

See if you could get a precise definition of what duty cycle is and go from there.

Thread Starter

#### Zotto

Joined Mar 3, 2016
45
I am sorry, I am misusing the term duty cycle. Thanks bertus. I need constant pulse width with a changing frequency.

#### bertus

Joined Apr 5, 2008
22,264
Hello,

Take a 556.
Make an astable oscillator with the first half and a monostable with the second half.

Bertus

#### HitEmTrue

Joined Jan 25, 2016
32
Can this be done with one 555, with a small capacitor (and parallel bleed-off resistor?) at the output to generate the pulse?

#### GopherT

Joined Nov 23, 2012
8,009
I am sorry, I am misusing the term duty cycle. Thanks bertus. I need constant pulse width with a changing frequency.
Things are gettingn confused and inconsistant with the answers. Again, when you say "constant" do you mean

(a) the easy way for a 555, meaning a constant 0.05 second pulse and change the interval of that 0.05 second pulse? (constant time, variable frequency)
(@bertus' answer will work here)

-or, do yu mean,

(b) the difficult way with a 555, meaning constant percent (example, 5%, when the frequency is slow) and stay the same percentage when the frequency is fast. (constant duty percentage, variable frequency)

#### Alec_t

Joined Sep 17, 2013
14,228
I need constant short pulses
Please specify how many mS, and the tolerance in mS, e.g. 10mS ± 1mS

#### dannyf

Joined Sep 13, 2015
2,197
It can be done with a 555, a resistor, a capacitor, a diode and an optional diode.

#### Bernard

Joined Aug 7, 2008
5,784
You could try connecting 555-pin 2 to junction of R1-R2. C1=2 uF, R1= 1k, R2= 2 M pot,, R3 20k.
Pulse about 2 msec. Clock period about 4 sec to 40 msec.

Thread Starter

#### Zotto

Joined Mar 3, 2016
45
@GopherT
Things are gettingn confused and inconsistant with the answers. Again, when you say "constant" do you mean

(a) the easy way for a 555, meaning a constant 0.05 second pulse and change the interval of that 0.05 second pulse? (constant time, variable frequency)
(@bertus' answer will work here)

-or, do yu mean,

(b) the difficult way with a 555, meaning constant percent (example, 5%, when the frequency is slow) and stay the same percentage when the frequency is fast. (constant duty percentage, variable frequency)
The first one you are saying is what I think I need. I dont have a 556 timer, can I implement it using a single 555, and a 10k pot? Sorry for the constraints but I dont have access to other parts at the moment.

Thread Starter

#### Zotto

Joined Mar 3, 2016
45
You could try connecting 555-pin 2 to junction of R1-R2. C1=2 uF, R1= 1k, R2= 2 M pot,, R3 20k.
Pulse about 2 msec. Clock period about 4 sec to 40 msec.
Can i use this design with a 10k pot?

#### dannyf

Joined Sep 13, 2015
2,197
It can be done with just 1 555 timer - think of it as an inverter (a NOT gate).

Why use two 555s if one 555 does the job?

#### AnalogKid

Joined Aug 1, 2013
10,949
I agree with danny. If Rb is fixed and Ra is variable, then the discharge output, Rb, and C set one pulse width almost completely independent of Ra. (Ra does contribute to the current through the discharge output, changing Vce, changing the discharge period *slightly*.) The other pulse width includes Ra and is variable. A 100:1 span in 1 range will have very poor adjustability, and will be logarithmically compressed at one end of the range, but a single CMOS 555 will meet the requirements in post #1.

ak

Thread Starter

#### Zotto

Joined Mar 3, 2016
45
I need decent adjustability with my pot, can I achieve this?