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 Forum index » DIY Hardware and Software » The layout factory
Help? How to isolate 555 oscillators
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Cfish



Joined: Feb 24, 2016
Posts: 473
Location: Indiana

PostPosted: Fri Jan 04, 2019 11:11 am    Post subject: Help? How to isolate 555 oscillators
Subject description: Isolate 555s to make a 555 TOG
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Hi ya all. Bean a while.
My TOG chip died in one of my home built rigs recently. When I started that project I was originally build a TOG using independent 555 oscillators, but I never figured out how to keep the oscillators from interfering with each other.

I still have the board with 12 555 oscillator circuits built on it.

Can anyone recommend a basic circuit to isolate the 555 oscillators? I honestly can’t remember what all I had even tried. I haven’t bean building much lately, so I have forgot a lot of what I had learned.
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PHOBoS



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PostPosted: Fri Jan 04, 2019 11:45 am    Post subject: Reply with quote  Mark this post and the followings unread

Something with CMOS 555's and/or a LED in series with the power but JovianPyx knows more about that Wink
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JovianPyx



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PostPosted: Fri Jan 04, 2019 1:33 pm    Post subject: Reply with quote  Mark this post and the followings unread

An old familiar subject to me.

If this is an organ board with 555 timers, they are likely bipolar and those are nasty. If that is the case, the easiest approach is to simply replace the 555 chips with CMOS ones. The pinout is identical. A lot of the CMOS ones are labeled 7555 or have 7555 in the number. The other way I've done it is more involved. You change the circuit so that instead of powering off the positive rail, you run the rail through a forward biased LED to the 555 power pin and then put a 100uF cap from the power pin to ground. Both methods work pretty well, but I prefer the CMOS 555 way (less messy). Note that the LED/cap circuit doesn't draw enough current to actually light the LED, it will remain dark. If the 555 oscillator is working, then the diode is doing it's job.

When I did this to update my Fatman VCOs, the CMOS 555 chips I used did not need any other treatment. It cured the soft-synch problem I had when trying to get the two VCOs to track closely and phase slowly. (slowly as in up to some 15 seconds per phase cycle)

The reason the bipolar 555 chips are bad is that they have a "totem pole output" structure. In this design, when the chip switches pin 3 from on to off or off to on, as it is switching, both upper and lower transistors are full on. This causes a short high current pulse to be drawn from the supply, basically a short duration short circuit. The LED and cap method works by allowing that current to be drawn from the 100 uF cap instead of the rail. The CMOS way works because it doesn't have totem pole outputs and the short circuit never happens.

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Cfish



Joined: Feb 24, 2016
Posts: 473
Location: Indiana

PostPosted: Fri Jan 04, 2019 6:25 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thank you very much JovianPix. Would be easy to swap the 555s, they are in sockets.
Thank you for the explanation. I have spent days trying to grasp why this was happening. Years ago I gave up and bought the old top octave IC. I really didn’t want to spend 200 american to replace it, and I’m not up on microcontrolers, so this really helps.
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