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555 astable circuit

The simulator in LTSpice looked OK ... so I moved to the breadboard with an LED (eventually replaced the LED with a buzzer) at pin 3. One more difference in the assembled circuit is the condenser at pin 5 is not electrolytic.

The supply is a retired AT SMPS from my old computer.

The waveform in LT Spice is initially LOW, and subsequently HIGH after a few hundred seconds. The assembled prototype however begins with the buzzer audible.

My impression is, therefore, that cycle has somehow got inverted in assembly. Is this impression correct? What need be done to have the output on the prototype behave as designed?

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  • \$\begingroup\$ You've verified all the connections and part values? \$\endgroup\$ – Ignacio Vazquez-Abrams May 18 '13 at 4:09
  • \$\begingroup\$ @IgnacioVazquez-Abrams: Yep. Kaz' post down there looks on track - my bad \$\endgroup\$ – Everyone May 18 '13 at 9:07
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You did something wrong. I reproduced your circuit in LTSpice and got this:

enter image description here

Output is high for the better part of an hour. Somewhere before 2800 seconds, it dives to zero. The blue trace tracks the capacitor voltage, by the way.

I suspect you forgot to checkbox "Skip Initial Operating Point Solution" in the Edit Simulation Command dialog box!

If I do the same, I get this:

enter image description here

Look familiar in any way?

For reference, the circuit:

enter image description here

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  • \$\begingroup\$ Very familiar indeed... the other system (with LTS) went off it's rocker sometime ago; When it is back I'll try it with the 'Skip Initial Operating Point Solution'. Thank you (+: \$\endgroup\$ – Everyone May 18 '13 at 8:58

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