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two 555 frequency interference

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Goldsphere

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I have two 555 timers running off the same voltage supply.
I want them to give two different frequency outputs, but they are summing together somehow to both give the same output frequency. How can I separate them so that I can have two different isolated outputs?
They are running as astable multivibrators and I tried putting a diode on the input of one of them but to no effect.:(
 

Mikebits

Well-Known Member
You need to post a schematic, hard to know what you are asking.
 

Goldsphere

New Member
Well, if you imagine a simple run of mill astable 555 circuit with two resistors and a capacitor creating a square wave output on pin 3. Now imagine 2 of them running off the same 12V power supply. The frequencies are adding together for some reason, and I don't know why. Anyone have any ideas?
 

ericgibbs

Well-Known Member
Most Helpful Member
Well, if you imagine a simple run of mill astable 555 circuit with two resistors and a capacitor creating a square wave output on pin 3. Now imagine 2 of them running off the same 12V power supply. The frequencies are adding together for some reason, and I don't know why. Anyone have any ideas?
Have you any decoupling on the +V upply lines.??

Are they TTL or CMOS 555's.?
 

AllVol

New Member
Well, if you imagine a simple run of mill astable 555 circuit with two resistors and a capacitor creating a square wave output on pin 3. Now imagine 2 of them running off the same 12V power supply. The frequencies are adding together for some reason, and I don't know why. Anyone have any ideas?
Do you have both outputs driving the same load? In other words, are both pin 3's connected to the same LED, or whatever?
 

Goldsphere

New Member
Both pins are unconnected as yet (no load), just scoping the frequency coming out. They are just your standard NE555 type timers. I have tried decoupling with 10uF and 2200uF to some effect but not enough, one frequency drifts by up to 25% when I adjust the pot on the other timer.
 

Mikebits

Well-Known Member
Like I said already. Post a schematic.
 

Goldsphere

New Member
I don't see the need to post a schematic, I told you exactly what it is. The schematic can be found in any electronics book or website that discusses the 555 timer. Type in 555 into google and look at the first link. The only difference is that one of the resistors is variable, and that there is two of the same circuit running off of the same 12V source. Why could this not be working? How can they interfere with each other?
 

ericgibbs

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Most Helpful Member
I don't see the need to post a schematic, I told you exactly what it is. The schematic can be found in any electronics book or website that discusses the 555 timer. Type in 555 into google and look at the first link. The only difference is that one of the resistors is variable, and that there is two of the same circuit running off of the same 12V source. Why could this not be working? How can they interfere with each other?
Hi,
I did ask, are there any decoupling caps on the pcb.??

Very important with 555's especially the TTL version.:)

EDIT:

Just saw your post, ref caps... sorry.
 
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Mikebits

Well-Known Member
I don't see the need to post a schematic, I told you exactly what it is. The schematic can be found in any electronics book or website that discusses the 555 timer. Type in 555 into google and look at the first link. The only difference is that one of the resistors is variable, and that there is two of the same circuit running off of the same 12V source. Why could this not be working? How can they interfere with each other?
Okay, so you said you have decoupled your two 555 chips. Your two devices have different frequency determining components yet output the same frequency. Maybe your using your test instruments wrong. I dunno.
Hope ya figure it out.

make sure you have like .1uf caps and maybe 1uf cap on each vcc pin.
Like Eric said, bypass caps.
 
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Goldsphere

New Member
They don't output the same frequency, but somehow the frequency is affected by the other frequency. I will try better bypassing caps tomorrow and let you know.
Thanks for the help everyone, and excuse me if I sound frustrated, I just didn't expect this little hiccup to something I thought was so simple.
 

Mikebits

Well-Known Member
They don't output the same frequency, but somehow the frequency is affected by the other frequency. I will try better bypassing caps tomorrow and let you know.
Thanks for the help everyone, and excuse me if I sound frustrated, I just didn't expect this little hiccup to something I thought was so simple.
Sounds like cross modulation maybe. Bypass caps for 1, second make sure your grounding is good. Big beefy grounds.
 
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audioguru

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Most Helpful Member
On Intersil's datasheet for their Cmos ICM7555 they say that an ordinary 555 (they call it a bipolar device, not a TTL device) causes a supply current spike of up to 400mA each time its output switches. That will cause a voltage drop in the supply and ground wiring which will affect another circuit using the same supply and ground.

To avoid interference then each 555 should have its own wires for its positive supply and ground to the main filter capacitor of the power supply. Called the STAR wiring method.
 

Rodney2

New Member
Without a schematic, I would not attempt an answer--to many possible variables not recognized by the creator; the above descriptions leave a lot to the imagination.
 

Goldsphere

New Member
Thanks Audioguru, I rebuilt the two circuits on breadboard using 10uF bypass caps on each supply and the supplies fed with star configuration. This has seemed to work.
Unfortunately, the pcb I designed and etched is now scrap. Back to ExpressPCB I go. :)
Thanks again to everyone.
 
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