Author Topic: 555 issue  (Read 2175 times)

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Offline iceisfun

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555 issue
« on: September 06, 2013, 02:53:24 am »
I have two 555 examples wired up and I'm getting this weird issue, it happens at the same time on the scope for both. I'll have to draw the diagram for what I have on board but I suspect something weird with the psu

Any thoughts on it?



 

Offline c4757p

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Re: 555 issue
« Reply #1 on: September 06, 2013, 03:01:08 am »
Where are you probing? I wouldn't suspect the PSU, because the oscillation appears to be in phase with the main frequency (at least, as far as I can see from two periods). But I have never seen a 555 exhibit parasitic oscillations and can't really imagine how it would.
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Offline iceisfun

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Re: 555 issue
« Reply #2 on: September 06, 2013, 03:09:01 am »
Pretty sure this was pin 6 or 7

I've seen this before and I thought I solved it, ill do some more hunting and see if I can make a minimal example.

Just probing the floating psu output shows nothing suspect.
 

Offline G7PSK

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Re: 555 issue
« Reply #3 on: September 06, 2013, 07:40:10 am »
Have you got pin 4 tied to +v. I once made up a 555 circuit that worked for years and then stopped when I looked at it I found that I had left the pin 4 floating.
 

Offline M0BSW

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Re: 555 issue
« Reply #4 on: September 06, 2013, 12:26:16 pm »
  is the blue trace the capacitor charging and discharging, if it is that  those two blocks, could that be a fault in the capacitor,  either on the charging up side, or on the discharge side ,as someone learning, I wonder if that could be a possibility
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Offline Andy Watson

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Re: 555 issue
« Reply #5 on: September 06, 2013, 12:47:37 pm »
It looks like a burst of high frequency occurring when the blue trace is about to switch. Somehow this is being picked-up on the yellow trace - suggesting a problem with the integrity of the ground - a very likely problem if you're using bread-board. 

The 555 chips were notorious for crowbarring the supply as they switched - some manufacturers were worse than others. Ensure that each chip has good decoupling close as possible to the chip. Also, if the voltage control pin (pin 5? I think) is not being used try decoupling to ground with 100nF.

Edit: Just to be clear, what I was suggesting with the HF on the yellow trace is that it's an artefact from whatever is happening on the blue trace.
« Last Edit: September 06, 2013, 02:34:59 pm by Andy Watson »
 

Offline M0BSW

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Re: 555 issue
« Reply #6 on: September 06, 2013, 12:58:53 pm »
It looks like a burst of high frequency occurring when the blue trace is about to switch. Somehow this is being picked-up on the yellow trace - suggesting a problem with the integrity of the ground - a very likely problem if you're using bread-board. 

The 555 chips were notorious for crowbarring the supply as they switched - some manufacturers were worse than others. Ensure that each chip has good decoupling close as possible to the chip. Also, if the voltage control pin (pin 5? I think) is not being used try decoupling to ground with 100nF.
I've just been looking at a 555 1Hz circuit that I built from  pin1 is neg /earth, pin5 100n to earth spot on Sir pin4 & 8postive rail 5 to 15 volts.
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Offline iceisfun

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Re: 555 issue
« Reply #7 on: September 06, 2013, 03:44:14 pm »
Thanks for the advice I will investigate these issues

The yellow is from a breadboard and the blue from a soldered project board and the scope was probing both, also power was coming off the breadboard for both.


 

Offline JoeO

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Re: 555 issue
« Reply #8 on: September 06, 2013, 04:41:17 pm »
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Today, only 26,000 remain.
 


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