555 - Adjusting frequency without affecting duty cycle

[miyayoung]

This seems to be some easy thing someone figured out already, but here I am after an hour of searching - and fearfully asking the question.

My aim is to build a relatively simple 555 circuit using as basic elements as possible (suitable for newbies like me) that provides a square wave with duty cycle of either >90% or <10% (adjustable duty cycle is a bonus, if it doesn't affect the complexity too much), and with the frequency adjustable in a range of about 1-200Hz by a potentiometer. (It would be used in a simple strobe, the output drives a transistor that turns on/off some 1W LED)

Here is the datesheet of ne555n

In the standard 555 astable oscillator circuit (see below), R2 determines the off-time, and R1+R2 the on-time. Adjusting R1 thus alters the duty cycle that I would like to avoid.

[dos]

As far as I remember it's not really possible, just not how the 555 works. I think I once needed two different frequencies with the sameish duty cycle and I used a switch and two different capacitors (I know this is not what you're looking to do). Varying the resistors will always alter the duty cycle to some degree (IIRC)

EDIT: Also I think you will have more luck and a better answer if you post this in the beginner questions forum

[tszaboo]

Place a binary counter after it, and use that's output. With a 555 its not really possible.
Doh!

[Ian.M]

... or use a dual gang pot and two 555 chips (or a 556) with the first producing the frequency, and triggering the second configured as a monostable that has a timing capacitor of 1/10 of the first, and uses the other gang of the pot for its timing resistor so its pulse width tracks at 10% of the period.

[Zero999]

Place a binary counter after it, and use that's output. With a 555 its not really possible.

But how do you change the duty cycle? That will give a nice variable frequency output but the duty cycle will be fixed at 50%.

It's not possible with the plain old bipolar 555 timer, but easy with a CMOS variant such as the ICM7555 or TS555 and a comparator.



R1 controls the frequency and R2 the duty cycle.

If you don't need to adjust the duty cycle, then just use the CMOS 555 timer on the left hand side of the circuit. Don't load pin 3, as it will affect both the duty cycle and frequency. pin 7 can be connected to the positive supply, via a suitable pull-up resistor and the output taken from there.

[Cyberdragon]

Place a binary counter after it, and use that's output. With a 555 its not really possible.

But how do you change the duty cycle? That will give a nice variable frequency output but the duty cycle will be fixed at 50%.

It's not possible with the plain old bipolar 555 timer, but easy with a CMOS variant such as the ICM7555 or TS555 and a comparator.



R1 controls the frequency and R2 the duty cycle.

If you don't need to adjust the duty cycle, then just use the CMOS 555 timer on the left hand side of the circuit. Don't load pin 3, as it will affect both the duty cycle and frequency. pin 7 can be connected to the positive supply, via a suitable pull-up resistor and the output taken from there.

I've been needing one of these too, and this should work.

[Seekonk]

A 555 is crap and should not be used for anything.  A TL/LM494is old and very common.  It will do exactly what you want to do with even less parts and be stable.

[Zero999]

I've been needing one of these too, and this should work.

You could also replace the 555 with another comparator. R4 can be used to adjust the frequency. Unfortunately, it will also affect the duty cycle, which can be mitigated to some extent by making R4 1M in series with a 100k resistor. Alternatively, remove the 1k pull-up and use a comparator with a push-pull output, such as the TSX3702 or the LMC6482.

http://pdf.datasheetcatalog.com/datasheets/stmicroelectronics/TSX3702_TSX3702IST_TSX3702IYDT_TSX3702IQ2T_to_TSX3702IPT.pdf
http://www.ti.com/lit/ds/symlink/lmc6482.pdf