[SOLVED] 555 time in astable mode giving "wrong" frequency???

[mindcrime]

Hi all, I have a problem very similar to what was discussed in this earlier thread:

https://www.eevblog.com/forum/beginners/icm7555-in-astable-mode-actual-frequency-not-same-as-calculated-frequency/

but the discussion there has not led me to a resolution.

Briefly, I have a 555 timer, wired up as an astable timer, as diagrammed on page 11 of Forrest Mims III's "Timer, Op Amp, and Optoelectronic Circuits and Projects."  As best as I can tell, it's exactly the same as this circuit, with the exception of my version not having the capacitor between pin 5 and ground. The Mims III book version doesn't show pin 5 connected to anything, and I followed that.



Anyway, I wanted about an 8 hz output, so I selected a 1uf capacitor, and two resistors for r1 and r2 that add up to 97k (I started out using the graphic in Mims III's book to pick the values, but the various online calculators show this combo coming in pretty close to what I want).  This one, for example, using my values of 82k and 15k, shows the expected frequency as 8.061 hz. 

But when I start the timer, both my oscilloscope and my handheld multimeter show an output frequency of around 2hz.  I do get a nice square wave as expected, and I can blink an LED with the output from pin 3, but the frequency is just off.

At first I had used an electrolytic capacitor, then I read that using those for timers is a bad idea, so I switched out to a tantalum capacitor of the same value (and I checked it with my bench multimeter to be sure). I also tested the two resistors to make sure the values were correct.  They both are.

At this point, my best theories are:

1. I made a simple wiring mistake and jumpered the wrong two pins together or something. But the fact that the oscillator fundamentally works makes me question this.

2. I built this on a breadboard and some combination of a bad connection resulting in extra resistance or stray capacitance is messing things up.

and bonus option 3 - "that cap between pin 5 and ground that Mims III didn't show in his circuit actually matters a lot".

I plan to pull it all apart and rewire it tomorrow when I get home from work, but in the meantime, if anybody has any suggestions for obvious stuff to look for, it would be appreciated.

And if anybody is interested in why I want an 8hz timer... I'm building a Zilog Z80 based computer and I want to get the simple NOP test circuit going first, and I want a clock speed that's really low so I can see the LED's blink as it cycles through the memory addresses.

[soldar]

The capacitor on pin 5 adds stability. I would use it.

Regarding the frequency you can try increasing the value of the resistors.

[StillTrying]

Rather than pulling it apart I usually go around every pin of the IC, make sure pin1 is connected to ground, pin 2 connected to the 2 places it should be and nowhere else.....

Is the 2Hz oscillation still giving the 54%/46% on/off time, and are the output's levels correct, about 1V below vcc.
Is vcc high enough. Is pin 8 connected, - they still work with just the reset connected to vcc.

[GerryR]

Check to see if the 15K isn't actually a 150K resistor.

[mikerj]

The capacitor on pin 5 adds stability. I would use it.

Regarding the frequency you can try increasing the value of the resistors.

Also add a decoupling cap to the supply rail.  Seems like every other post asking about 555 behaviour leaves this out.

OP, is the duty cycle close to the expected value?

[mindcrime]

Thanks everybody, I'll dig into this and try some more things out when I get home.  The good news is, I don't specifically *need* any particular frequency for the test circuit I'm putting together. Just something that's low enough that I can see the address bus LED's blink with the naked eye. Some people actually test these things with a push-button momentary contact switch and send the clock pulses by hand.

When I get ready to run the Z80 for real, I'll probably use a real crystal oscillator for the actual timing, so this is just a sidebar thing.

[boffin]

Didn't Dave do a video showing the 'easter egg' in the 555 when operated at specific frequencies ?
:-)

[mindcrime]

Didn't Dave do a video showing the 'easter egg' in the 555 when operated at specific frequencies ?
:-)

I vaguely recall him doing something about an easter egg in the 555... It was an April Fool's Day joke though, IIRC.   

[mindcrime]

OK, this is fixed.  Remember when I said that the Mims III book didn't show pin 5 connected to anything? Well, that's true. It doesn't.

And remember when I said:

1. I made a simple wiring mistake and jumpered the wrong two pins together or something. But the fact that the oscillator fundamentally works makes me question this.

Well guess what? Stupid me, for reasons I cannot even begin to fathon, had jumpered pin 5 to Vcc.  After removing that, and adding a small cap between pin 5 and ground, I now get a perfect 8.06hz square wave.

No idea what I was looking at when I inserted that jumper, but anyway, it's all working now, and life is good. Thanks for all the help!