Start 555 output in OFF state

AnalogKid

Joined Aug 1, 2013
8,596
I imagined it this way.
1602822896799.png
Here's the problem - at the point when the output changes state and the DIS pin releases (goes to high impedance), the direction of electron flow into the cap does not change. The DIS pin has pulled the cap top end to GND, and when it releases, R1 also is connected to GND. The other end of the cap is at Vcc, so it isn't changing. Thus, the cap keeps charging up to a potential of Vcc volts, but at a slower rate since now it is charging through both R1 and R2 in series.

ak
 
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Ian0

Joined Aug 7, 2020
490
It requires a version of DISCHARGE with a PNP open collector output with the emitter connected to V+.
but DISCHARGE is just a NPN open collector version of OUTPUT, so the same thing could be achieved with a diode from OUTPUT.
 

sghioto

Joined Dec 31, 2017
1,453
Regardless of which way the cap is connected the TS has overlooked another problem. As I see the circuit in post #1 Vcc is always on and the circuit is controlled by the reset pin 10. When pin10 is low then the output pin 9 is low, the other reset pin 4 is low and the output pin 5 is low. Now you have a constant current flowing through the speaker. Pin 5 needs to be high when pin 10 is low.
Connecting a diode between pin 9 and the pin 2,6 junction will accomplish this when pin 10 is low.EEE LM556  Goxeman.png
 
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