555 Astable Timers - Discharge

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Hi everyone.

Stuck on a bit of theory hoping someone can help clarify.

Have a look with me at figure 12 of the 555 datasheet here.

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If we want to charge the cap (C) we now need to go through R(a) and R(b).
But if we discharge we only go through R(b) which means the duty cycle is not symmetric.

If you want a symmetric oscillator (which is surely many applications) why not omit R(a) entirely and just charge through R(b) alone?

If I want to, can I put a zero ohm resistor in R(a)?

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If you did this you would short out gnd to vcc through internal discharge transistor. using diodes works well to get a symetric output

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Hi @MarkMakies

Thanks for your reply. :slight_smile:

Oooohhh! Yeah, obvious when you point it out.
Thanks mark.

Ah this is a great tip!
Why does the diode prevent the short circuit?

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My old CMOS handbook as the output from the 555 as the clock into a JK-Flipflop, then take the output Q

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image1029Ă—547 35.1 KB

And this is an option with diodes

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Hi Pix
Michael beat me to it.
The “real” way to get exact symmetry is to run the 555 at double frequency then divide by 2 as in the top circuit.

The next best thing is the bottom circuit using the same values for R1 and R2 and 2 diodes. The symmetry here is very close to 50%. This symmetry is largely dictated by component tolerances. You can of course do your own matching if this is worth while.

Another sneaky way which will give you a not too bad result is to make R1 a small fraction of R2. A minimum value fr R1 should be about 1kΩ. If R2 ia 100kΩ this would give a charging resistance of 101kΩ and a discharging resistance of 100kΩ. Sometimes this ratio is close enough for practical purposes.

Keep in mind that there are a few different flavours of 555 these days. I think that for the “garden variety” readily available types 5V would be the absolute minimum operating voltage. They are good to 15V or for a split supply +/- 7.5V Max. There might be a low voltage version I have not had a look. I know there are Cmos versions with slightly different specs.
Cheers Bob

There are lots of variations with a 555 astable circuit but they sometimes get a bit complex and you have to weigh up just what you need

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Oh that’s clever. :slight_smile:

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Hi Pix

Yes but the method has been around for a long time. This method of obtaining a symmetrical square wave from a 555 was probably thought of when the 555 was first invented.

As for the 555 series itself I think it would have to be the longest manufactured IC type ever produced. Been around for many many years now with no significant changes. People have just found more and more uses for it. Probably one of the most useful ICs ever made.
Cheer Bob

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This seems like another opportunity for me to drop Ben Eater’s video on the 555 Timer. I’ve never come across a resource that does a better job explaining and building intuition behind the 555.

Also, if you haven’t seen the series that this leads into, it’s well worth a watch :smiley:

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Hi Michael
Very good simple explanation of a 555 in astable mode, easy to follow.
I have a couple of small booklets on 555 applications but my reference I have been using for many years is “IC Timer Cookbook” by Walter G Jung which also describes a few other types as well. My copy is 1981and nearly falling apart but by treating it gently is still very useful.
Cheers Bob

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