Is this how to decouple 555 timers?

Thread Starter

diversifiedmultirobert

Joined Jan 5, 2012
3
Hello. I am decoupling a series of five LED's tied to 555 timers. Each LED-555 grouping is identical except for the position of the potentiometer dial and hence, the blink rate. I am concerned as to if I am laying this out correctly. Someone mentioned to me a ten ohm resistor could reduce de-coupling. Confused. Any insight would be appreciated. Thanks.
 

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praondevou

Joined Jul 9, 2011
2,942
Are you talking about the decoupling capacitor "H"? That should be ok.
I don't see any necessity to put the capacitor "G" near the battery. The battery already has low impedance. You could put it near the 555s or omit it. I assume the 9V battery is also near the circuit and not 100m away, right ? :)

What "10 OHM" resistor do you mean?

If you have a voltage drop of 3.6V at the diode, that means there will be a current flow of 11.4mA with the resistor B. That's 55mA which is a lot for a 9V battery (although the mean value depends on the blink rate of course...)
 

Wendy

Joined Mar 24, 2008
22,146
Close enough. For low current projects, such as LEDs, it is good enough. The real problems enter when two oscillators are near the same frequency, for example. One of them will "lock" onto the other frequency as the spike travels through the power supply line in the other 555.

The resistor in the power supply line is a good answer. It uses a RC circuit to drop the pulse to negligible value. There are many other things you can do also.



This circuit is a good example of what I was talking about, the two oscillators kept syncing in.

So I add the RC filter as shown, it should fix the problem.



BTW, not shown is a 0.1µF cap attached to pin 5. I ran the experiment on the first schematic. I was demonstrating how heterodyne and phase detectors work.
 

praondevou

Joined Jul 9, 2011
2,942
@Bill,

I'm impressed that this is such a serious problem. I would have thought that just the proper decoupling capacitors at each 555 would be enough to avoid "travelling voltage spikes" influencing the others. The wire inductance of the power supply lines between oscillators would help too.

I'm almost tempted to try this out.

Why would the 555s be THAT sensitive? In your upper schematic no bypass caps are shown, is this on purpose or does the circuit not have them?
 

Audioguru

Joined Dec 20, 2007
11,249
On the datasheet for the ICM7555 which is a Cmos 555, Intersil shows a graph of the 400mA supply spike when the output of an ordinary 555 changes state. The current spike of the Cmos 555 is almost nothing.

A little 9V battery cannot supply 400mA at 9V so its voltage drops which affects all timers powered by it. A decoupling capacitor close to the 555 helps hold the voltage up when it conducts 400mA but it is not perfect.
 

Thread Starter

diversifiedmultirobert

Joined Jan 5, 2012
3
7555 is a CMOS version of the 555. It has different identifications according to the manufacturer. The number "7" or the letter "C" identify the CMOS version.

LMC555, LM555CN : National Semiconductors
TLC555 : Texas Instruments
ICM7555 : Philips
ZSCT1555 : Zetex
ICM7555 : Maxim


Some info I found online..

:cool:
 

Wendy

Joined Mar 24, 2008
22,146
Thing is, they are all different. Unlike the conventional 555, where everyone tries to get close to the same page on the data sheet (and occasionally fails) the CMOS versions vary wildly from part to part. As always, datasheets are your friend, and for the most part they get close enough to be interchangeable, but it is not a given.
 
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