playing with the NE556 chip again; more questions

The control input is sensitive and any little wiggle in the common circuit for that cap is going to propagate and cause some shift in the timer.

Try attaching the low side of the bypass caps to a common trace/path directly to the (-) rail with nothing else connected to that rail between..

Your readings with the use of the bypass cap on the control pin are most likely more accurate to what you should be getting with the set up you have also, you must remember the 556 uses a single power rail inside for both units, one unit could offset the other depending on what they are doing. In cases like these, you use 2 555's instead.

Jamie

"A" works best win CMOS devices where the output goes all the way up to the positive supply voltage and all the way down to ground.

"B" is better in Bipolar (like the NE555 and NE556) where the output is not symmetrical and the two resistors can be varied to balance the ratio.

They should be fully independant. check that the powersupply pins and timing capacitors are wired correctly.

That does make them a bit more consistent --- I wouldn't have expected breadboard leads to have much effect at low frequencies like this.

OK. I expected that by keeping the output current below 20 mA on each side, the single power rail effect would be insignificant.

As it turns out, though, I get the biggest improvement in consistency by using the reset pins properly to switch the timers on & off (reset to Vcc for on, to ground for off), instead of just pulling one of the wires so the LED stops blinking. Doing it that way (with the control bypass capacitors grounded more tightly gives these:

A with B on: 1.2 sec B with A on: 1.2 sec A with B off: 1.2 sec B with A off: 1.0 sec

which could be within stopwatch measuring error.

Thanks!

It shouldn't at those frequencies.

I've never bypassed the control pin when I've used 555s.

Michael

That makes sense, thanks.

Aha, the TI data sheet shows the 0.01 µF from CONT to GND in the monostable example, but for the astable example circuit, it shows CONT open but with the note "Bypassing the control-voltage input to ground with a capacitor might improve operation. This should be evaluated for individual applications." Fairly ambivalent.

OK, I'll try those & see how they improve things.

Thanks! ISTR reading that the Rs in the R C timings can range from

...

As it turns out, I've got a couple of spice & geda packages available already on Ubuntu, which I'm working out how to use. Thanks for the example.

Sure, that's the most important part after "don't set anything on fire".

:-)

Thanks again!