Symmetric 555 oscillation

Discussion in 'General Electronics Chat' started by lotusmoon, Jun 14, 2013.

1. lotusmoon Thread Starter Member

Jun 14, 2013
203
4
Please do not hijack other members' threads.

I am new to electronics, but wanted to make a strobe with different frequencies.
I have this formula for a 555 timer cycle

t1 = .693 x (R1+R2) x C
t2 = .693 x R2 x C

t1 = time on and t2 = time off
from this formula it does not seem possible to have an equal t1 and t2. could some one let me know if there is a way to have them equal?

Last edited by a moderator: Jun 14, 2013
2. WBahn Moderator

Mar 31, 2012
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Please don't hijack someone else's thread as it creates massive confusion. Instead, start one of your own. Don't do that now, though; I've reported your post and a moderator will probably split it off into its own thread in the near future.

This is a question that has been addressed many times and in many places here. I don't have a specific link to give you, but search the forum (and also the blogs) for something like "555 equal on off" or "symmetric 555 oscillator" and you will probably find something quickly.

3. lotusmoon Thread Starter Member

Jun 14, 2013
203
4
sorry about that I thought I had posted a new thread, I will look for things you have mentioned thank you

4. THE_RB AAC Fanatic!

Feb 11, 2008
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It's best to use a different circuit. There is an astable 555 timer circuit driving the threshold cap from pin 3 (does not use the discharge pin 7).

That circuit is simpler and gives a good 50% duty cycle at all speeds (especially with CMOS 555 chips like the 7555).

5. ErnieM AAC Fanatic!

Apr 24, 2011
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Correct, but if you make R2 much greater then R1 then the on and off time get very close in time.

6. lotusmoon Thread Starter Member

Jun 14, 2013
203
4
"It's best to use a different circuit. There is an astable 555 timer circuit driving the threshold cap from pin 3 (does not use the discharge pin 7).

That circuit is simpler and gives a good 50% duty cycle at all speeds (especially with CMOS 555 chips like the 7555).
thank you"

thank you - do you have a link to this circuit circuit?

7. JoeJester AAC Fanatic!

Apr 26, 2005
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Here's one from Williamson's Lab ....

File size:
29.9 KB
Views:
81
8. KL7AJ AAC Fanatic!

Nov 4, 2008
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Indeed it is not, but you can make it very close to symmetrical by making R1 very small relative to R2.

Eric

9. MrChips Moderator

Oct 2, 2009
12,446
3,362
Symmetrical clock signals are rarely a necessity.

To create a symmetrical signal from a 555 timer circuit, output a signal at twice the desired frequency and feed it into a toggle flip-flop (T-type flip-flop).

You can use a D-type or J-K flip-flop to create a toggle flip-flop.

10. THE_RB AAC Fanatic!

Feb 11, 2008
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Nah, you don't need those messy diodes.

Here's the circuit, it only needs a 555, 1 resistor and 1 cap;

If you replace the resistor with a pot and resistor in series you get variable frequency but still has 50:50 duty cycle.

lotusmoon likes this.
11. lotusmoon Thread Starter Member

Jun 14, 2013
203
4
That's brilliant I wanted a variable frequency. is possible to work out and fix the beginning and end of the variable frequency? ie 1Hz - 100Hz

12. THE_RB AAC Fanatic!

Feb 11, 2008
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Yep. If you use a 1k resistor in series with a 100k pot, the frequency adjustment range will be approximately 100:1.

As for the actual Hz, try plugging in different caps (and you may need two caps in parallel) until you get roughly 1Hz to 100Hz.

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13. lotusmoon Thread Starter Member

Jun 14, 2013
203
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I have been told capacitors are up to 20% inaccurate which would make it hard to accurately calculate the frequency is this true?

14. THE_RB AAC Fanatic!

Feb 11, 2008
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That's part of it, and the 555 output will have some impedance that will also affect the calcs.

Just plug in some different caps, it's very fast to test and settle ona cap value. Example; If your first cap runs 5 times too fast you know you need a cap about 5 times larger.

15. k7elp60 Senior Member

Nov 4, 2008
478
69
Not true use a mylar film capacitor and this formula to determing the value of resistor and capacitor: f= 0.722/R1C.