Re: decoupling capacitors

Grassy Knollington wrote:
On Mon, 27 Feb 2006 12:29:48 -0500, John Popelish wrote:
(snip)
The only capacitor performing a decoupling function in this schematic is C1. And it is not bypassing the battery lines, but 2/3rds of that voltage divided down by an internal resistor divider in the 555. The CV label on the pin refers to the use of this fraction of the supply as a control voltage (that is compared to the voltage on pin 6, to decide when to reset the output flip flop). See:
http://www.uoguelph.ca/~antoon/gadgets/555/555.html
Here, the decoupling action is greatly improved by the resistors between the CV pin and the supply rails.

Thats a useful reference ta, do you mean the test circuit on this
page where 2 resistors and 2 leds cross and join the output pulse line

No. I am talking about the 3 equal resistors inside the 555 that divide the supply voltage by 1/3 and 2/3, to act as reference voltages for the trigger and threshold comparators.

On the zapschematic.gif, if I put a 4.7 uf capacitor (or what size
would you recommend) across the power terminals would that help the
circuit at all?

The 555 draws a very brief spike of current, each time the output changes state. Any load on the 555 output has to be considered, also. Your 4.7uF sounds like a fine guess as a bypass capacitor, unless you find that the circuit is showing mysterious malfunctions.

WOW, wish I had continued to study electronics 20 years ago when I
started... got into software design/prog instead... Never too late I guess!

Not till you are dead.
.

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