555 timer 40khz 50%duty cycle

THE_RB

Joined Feb 11, 2008
5,438
I just thought maybe you were used the CMOS type, but do they really can reach to 50%/50% or just close to.
Close to, or close enough. Remeber we're talking rock-bottom simplest 555 circuits here, not precision 50:50 duty generators.

If it was 49:51 I'd say that was close enough to 50:50 for a hobby circuit with 4 components. ;)


I used two TTL type, one is LM555, another one is NE555(TI), but they just approaching to 50%/50%, I'm not sure how much different for the high and low duty cycle, but I can see from the scale of O'scope, they are not equal.
...
What is the actual duty you are getting? Can you post a 'scope photo?

If the simple pin3 RC circuit is not exactly 50:50 you can bias it with a high value pullup or pulldown resistor to Vcc or gnd. If we could see your schematic and RC values (and the actual duty cycle) we could suggest a pullup or pulldown resistor (or trimpot) to tweak the duty cycle a few percent.
 

ScottWang

Joined Aug 23, 2012
7,397
Close to, or close enough. Remeber we're talking rock-bottom simplest 555 circuits here, not precision 50:50 duty generators.

If it was 49:51 I'd say that was close enough to 50:50 for a hobby circuit with 4 components. ;)

What is the actual duty you are getting? Can you post a 'scope photo?

If the simple pin3 RC circuit is not exactly 50:50 you can bias it with a high value pullup or pulldown resistor to Vcc or gnd. If we could see your schematic and RC values (and the actual duty cycle) we could suggest a pullup or pulldown resistor (or trimpot) to tweak the duty cycle a few percent.
I was used IWATSU 100Hhz O'scope to measured the circuit that you described on #6, it shows as below:
left side low level 5uS*1.45, right side high level 5uS*2.65.
(The values of Lattice just an approximate values)
R1=R2=10K, C1=103,49.030Khz(it shows on O'scope, but the last values is astable).
1000000Hz/(5uS*(1.45+2.65))=1000000Hz/20.5uS=48.780 Khz
So, the width of low level and high level are different, I was used NE555(TI).
 
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THE_RB

Joined Feb 11, 2008
5,438
Um, the circuit I described in post #6 has ONE resistor. And will give a heck of a lot closer to 50:50 duty than your 2.65:1.45 duty (65%:35%).

Why do you have R1 and R2? It's time to post a schematic. :)
 

ScottWang

Joined Aug 23, 2012
7,397
Um, the circuit I described in post #6 has ONE resistor. And will give a heck of a lot closer to 50:50 duty than your 2.65:1.45 duty (65%:35%).

Why do you have R1 and R2? It's time to post a schematic. :)
Sorry, Only R1=10K,no R2.
So if the problem is on 555 type, maybe I need to buy the CMOS type to try some day.
 

THE_RB

Joined Feb 11, 2008
5,438
That's the technique I've been talking about (since post #6).

I'm pretty sure ScottWang's result of 65%:35% duty and previous mention of two 10k resistors means he was using the standard RRC circuit, charging through two 10k resistors and discharging through one (giving roughly 67:33 duty).

If he makes the circuit properly with one resistor and one cap the duty will be very close to 50:50. :)
 

ScottWang

Joined Aug 23, 2012
7,397
That's the technique I've been talking about (since post #6).

I'm pretty sure ScottWang's result of 65%:35% duty and previous mention of two 10k resistors means he was using the standard RRC circuit, charging through two 10k resistors and discharging through one (giving roughly 67:33 duty).

If he makes the circuit properly with one resistor and one cap the duty will be very close to 50:50. :)
I already made clear on #24 - it's Only R1=10K,no R2.
You can try the 555 TTL type, and to see how's the duty cycle.

I was asked my friend to tried, he was mentioned that when the Vcc increasing then the duty cycle will getting close a little, you can also try that if you want.

When he tested the 555 CMOS type, he said the result as you mentioned and it's close to 50/50, that's 51%/49%,
 
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ScottWang

Joined Aug 23, 2012
7,397
For a general NE555 astable circuit, I get used to used the R1=1K, if the frequency is fixed, when the R2 using more higher ratio of R1 then the output duty cycle will close to 50/50, as the R2 using 100K then the ratio of duty cycle will be 1K+100K : 100K, the error of ratio for a 50/50 duty cycle only has 1%.
 

THE_RB

Joined Feb 11, 2008
5,438
I already made clear on #24 - it's Only R1=10K,no R2.
You can try the 555 TTL type, and to see how's the duty cycle.
...
Sorry i was not saying you are a liar, just that in one post you mentioned 2 resistors of 10k each, whcih matches the duty cycle you are getting almost exactly.

OK I went and plugged a NE555 TTL version into a breadboard and a couple of parts. With 12v supply, one resistor from pin3 (1k8) and a 1uF electro cap I got 1.8mS HI and 1.3mS LO so it was 58%:42% duty. Definitely not as good as a CMOS 555 timer like a 7555.

To fix it to 50% duty I just added a 15k resistor from pins 2,6 to pin 8 (pullup resistor to 12v). Frequency stayed the same but duty is now 50:50.

CMOS 7555 test; I unplugged the 15k pullup resistor and swapped the NE555 for a 7555. Duty was so close to 50:50 that i can't be bothered counting scope divisions, but to the eye it looks better than 51:49. Frequency went up a little bit a few percent.

(in both tests I also had a 22nF cap from pin 5 to gnd, a good idea for stability)

So an old TTL 555 makes approx 58:42 duty with just one R and one C (and can be corrected to 50;50 with a pullup resistor about 10x larger than R). The CMOS 7555 makes a very good 50:50 out of the box.

Hope that helps. :)
 

ScottWang

Joined Aug 23, 2012
7,397
Sorry i was not saying you are a liar, just that in one post you mentioned 2 resistors of 10k each, whcih matches the duty cycle you are getting almost exactly.

OK I went and plugged a NE555 TTL version into a breadboard and a couple of parts. With 12v supply, one resistor from pin3 (1k8) and a 1uF electro cap I got 1.8mS HI and 1.3mS LO so it was 58%:42% duty. Definitely not as good as a CMOS 555 timer like a 7555.

To fix it to 50% duty I just added a 15k resistor from pins 2,6 to pin 8 (pullup resistor to 12v). Frequency stayed the same but duty is now 50:50.

CMOS 7555 test; I unplugged the 15k pullup resistor and swapped the NE555 for a 7555. Duty was so close to 50:50 that i can't be bothered counting scope divisions, but to the eye it looks better than 51:49. Frequency went up a little bit a few percent.

(in both tests I also had a 22nF cap from pin 5 to gnd, a good idea for stability)

So an old TTL 555 makes approx 58:42 duty with just one R and one C (and can be corrected to 50;50 with a pullup resistor about 10x larger than R). The CMOS 7555 makes a very good 50:50 out of the box.

Hope that helps. :)
Thank you.
Now I can sure the CMOS 7555 can do the 50/50 duty cycle, but my friend said that only the intersil product is the true CMOS type, have you ever heard that, and what's the brand of 7555 that you
frequently used?



used?
 

THE_RB

Joined Feb 11, 2008
5,438
Lucky I keep those 555's in a little bag where i could find them again! (You haven't seen my chip stores) :eek:

The NE555 TTL version was Philips Thailand
The CMOS 555 was TLC555 from texas Instruments.

Generally I just order 7555 or CMOS 555 from the hobby shops when i am ordering bits and pieces and I just get whatever the send. It should not matter, if they are CMOS ones they will be 50:50 duty, I have used that one RC circuit for years as my preferred oscillator circuit and they always look 50:50 on the 'scope.

And like I said before if you want to tweak the duty cycle a tiny bit you can always use a high value resistor as a pullup/pulldown. So don;t sweat it. :)
 
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