# Question on 555 Duty Cycle

Discussion in 'General Electronics Chat' started by Metalfan1185, Nov 21, 2008.

1. ### Metalfan1185 Thread Starter Active Member

Sep 12, 2008
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Im wondering if it is possible to set up a 555 to have a variable frequency and keep a fixed duty cycle, for instance, 50% at 1hz, and variable to say, 50% at 100 Hz,

Or maybe set it up so it keeps an on time of 10%, so it would make a quick on pulse every second, or quick pulses 100 times a second. Both change as the frequency is adjusted.

the other side I want to try is to make one that keeps a constant on time while changing the frequency, so if the on pulse is on for .01 sec at 1 Hz, can it have the same on time at 100Hz, and only vary the off time...

you understand what im tryin to say?

2. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,047
295
Yes...if the resistor between your timing junction and Vcc is very LOW compared to the main timing resistor, it will maintain a 50% duty cycle. However, if you want to have a fixed 25% duty cycle, for instance, you have to vary BOTH those resistances.

eric

3. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,047
295
http://www.ecelab.com/circuit-astable-555.htm

In this diagram, R2 is your main timing resistor and R1 is the "duty cycle" resistor. If R1 is very small compared to R2, you will have a 50% duty cycle. You can't eliminate R1 altogether, though....usually 100 ohms is the lowest value you can use here and have a reliable oscillator.

eric

4. ### Metalfan1185 Thread Starter Active Member

Sep 12, 2008
146
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Thank You for your responses...this is a little clearer now.

So it's better to use Larger Value resistors and a small cap so you have the adjustability in both frequency and Duty cycle.

If i used one of those variable resistors that have 2 resistors on one shaft, if they were the same value, and i used one for R1 and the other for r2, would that keep the duty cycle 50%?

5. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,047
295

Actually, it's better to have large R values and small C values for the simple reason that smaller value capacitors are more precise. It's really hard to get a tight tolerance electrolytic capacitor....whereas most ceramic and mica caps have good specs..but are much smaller values.

Here's all the formulas you need for 555's
http://www.doctronics.co.uk/555.htm

Using a ganged potentiometer for R1 and R2 is the way I'd do it. However, if you're just interested in keeping a 50% duty cycle, you don't even have to do that. Just be sure the minimum value of your variable resistor (r2) is at least ten times the value of R1 at all times.

eric

6. ### Metalfan1185 Thread Starter Active Member

Sep 12, 2008
146
0
That's great, a big help,

my goal is to have one timer that say you want 10% on time at 100Hz, you can adjust the frequency and keep the 10%,

or change the 10% to 20%, and then chang the frequency and keep the 20%

-----

the other circuit im looking to do similar, but when you set the duty cycle it stays static, so you want it say, on for 1 sec, then off for 5, then change a variable resistor and get 1 sec on, and 25 sec off,

I've been try to do this for a while now.

I appreciate all your help.

7. ### SgtWookie Expert

Jul 17, 2007
22,183
1,729
You really have to watch the size of R1.
Pin 7 of the 555 is an open-collector NPN transistor which can only handle around 15mA current. In order for that transistor to be able to discharge the timing capacitor, you should value R1 to allow a maximum of about half that current.
So, R1 >= Vcc/7.5mA, or 133 Ohms per volt of Vcc.
If your Vcc is 5v, then R1 should be >= 665 Ohms.
If your Vcc is 15v, then R1 should be >= 1995 Ohms.

If you want a 50% duty cycle 555 timer, you'll need to either use diodes, or get away from using pin 7 as the discharge circuit by using pin 3 (the output) to charge/discharge the timing cap.

You might want to download Schematica's 555 Timer Pro. It can be run in the "Lite" mode as freeware.
http://www.schematica.com/555_Timer_design/555_Timer_PRO.htm

It's handy for calculating ballpark RC values.
Note that there are a number of variations on the astable MV configuration in two different groups; >52% duty cycle (what you're currently working with) and wide duty cycle.

8. ### jpanhalt AAC Fanatic!

Jan 18, 2008
5,699
909
Here is a circuit I copied from a post by Audioguru last year. It is claimed to give a 50% duty cycle as charging and discharging are through the same resistor. NB: Pin 7(discharge) is left open.

I have not built the circuit myself.

John

9. ### Audioguru New Member

Dec 20, 2007
9,411
896
The circuit that I posted almost has a 50-50 duty cycle. The output high voltage of a 555 is not the supply voltage but is 1.3V less. But the output goes almost to ground.
A Cmos 555 will have a 50-50 duty-cycle in the circuit.

10. ### Metalfan1185 Thread Starter Active Member

Sep 12, 2008
146
0
That circuit is Great, That should keep the 50% cycle time, im thinking if i make that resistor variable on pin 3 to trig, i may be able to control the frequency, i think i might have to add a fixed resistor of a small value to make sure it doesn't just short out.

You guys are great, as for the other circuit, well, i guess ill keep lookin into it.

Thank You guys so much,

11. ### SgtWookie Expert

Jul 17, 2007
22,183
1,729
Audioguru's circuit doesn't take into account the fact that the high side of the output is a Darlington, and the low side is a transistor.

You won't get a true 50% duty cycle out of his schematic. It isn't going to happen.

It's good for wishful thinking though.

12. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
It's funny you should mention that, I've illustrated this point in my latest experiment. The CMOS version of the 555 will be 50% though.

I think I've remembered a way to do it with op amps and a 555 with separate controls for each function, no interaction.

CR2 can be eliminated, but CR1 is needed. Meltdown will occur without it.

• ###### 555 Function Generator.GIF
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13. ### Metalfan1185 Thread Starter Active Member

Sep 12, 2008
146
0

I didn't know that, hmmm...So if i used a 7555 with AudioGuru's Circuit, I should get closest to a 50% cycle? I would like it to be as close as possible, but it's not imperative to have it EXACTLY at 50%. I definitely want it close enough to the point where you wont be able to tell the difference by eye at a low frequency.

Also, this circuit may be used outdoors, I imagine that variables with Temperature have an effect on the tolerances and values of the passive components. Will this too vary it to the point where you will able to tell the difference by eye? (By eye i mean, if you used 2 LED's to flash back and forth, would you be able to tell by eye that one is on longer than the other)? I mean, hottest would be 100F to as far as 0F, but i dont think it will be used at 0F, maybe 30F?

Thank You all for your contributions to my project.