Pin 3 connected to pin 6&2 no discharge

For an assignment i need to understand how this 555 circuit works.
Pin 2 & 6 are connected by a 10nF capacitor to the ground. They are also connected via a variable resistance to output pin 3. Pin three is also connected by a 150 Ohmn resistor, followed by a 470 micro farad capacitor and an 8 Ohm speaker, to 0v. The discharge pin is not used - how does it oscillate? please help.

Posted by Anonymous on 29/05/2005, at 03:30 GMT

555 operation

This arrangement for a 555 astable has the advantage of giving equal mark and space times (Tm and Ts). Pin 3 (the output) does the charging and discharging.

Tm = Ts = 0.7 x RC (R = variable resistor)

When pin 3 is high current flows through the variable resistor to charge the capacitor until pin 6 senses it has reached 2/3 of the supply voltage. This makes pin 3 go low (0V) so it now discharges the capacitor through the variable resistor until pin 2 senses it has reached 1/3 of the supply voltage. This re-triggers the circuit making pin 3 high and so the cycle starts again.

Posted by John Hewes on 30/05/2005, at 18:27 GMT