Simple 555 timing chip question

[hack]

Trying to get my head around 555 chip here.

Can't seem to grasp pin 7 - dishcharge.

So pin 7 is connected to 0V when the timer output is low (timing capacitor charge is high (>2/3)). What is the purpose here. Is this essentially a short while it waits for the capacitor to discharge?



Thanks

 

[hack]

Found it...

The threshold pin is initially at ground because C1 is charging through R1 + R2. The output pin is at +Vcc.
As soon as the potential of the threshold pin reaches 2/3 of Vcc , the output pin switches to the off state and the discharge pin goes to ground potential.
This causes the capacitor C1 to discharge through R2.
When the trigger pin goes below 1/3 of Vcc , the output pin goes back to the on state and the discharge pin goes to a floating (disconnected) state.
The entire cycle repeats indefinitely.

 

[hack]

Actually I'm still confused here.

I understand a capacitor to have two plates inside it.

When the plate connected to positive voltage charges past 2/3rds the electrons are able to move across to the second plate thus discharging the capacitor to 1/3. This generates a pulse wave from coming out the negative lead. The discharging is the + voltage leaving the capacitor and the recharging is the 0 voltage leaving the capacitor. Is this a correct evaluation?

So when using a 555 timer - it looks like when the capacitors charge gets to 2/3rds the 555 chip notices and grounds the positive lead to the capacitor. This would make me think that rather than the capacitor discharging across to the second plate - it actually discharges out the positive lead - meaning the one that it originally charged through. I think this - becuase the pin 7 - discharge is connected to the positive side of the capacitor AND is grounded. Am I correct here - or way off?

Thanks

 

[audioguru]

The positive wire of the capacitor charges by R1 plus R2 and discharges by R2. The 555 monitors the voltage of the capacitor's positive wire and if it is at 2/3 of the supply voltage then the output goes low and the discharge transistor at pin 7 is turned on. When the discharging voltage of the capacitor's positive wire reaches 1/3 of the supply voltage then the output goes high and the discharge transistor at pin 7 is turned off.

 

[AllVol]

At **broken link removed** you can download a free tutorial on the 555 including design wizards for various configurations. Pretty helpful information.