still don't understand 555 timer

Hi, I read all the 555 timer tutorials I could possibly find, but still don't understand how it works. I understand the basics of electronics, what else do I have to know? Any tips please?

Thanks!

The 555 uses an external R - C ramp generator. When the ramp voltage reaches 2/3 of VCC at pin 6, a transistor switch is turned on to dump the charge on the Cap. If pins 6 and 2 are connected, then when the Cap voltage falls to 1/3 VCC, the switch transistor is turned off and the Cap starts charging again. A resistor (Rb on the data sheet) can be put in series with the Cap to slow the discharge time. The output (pin 3) is high when the Cap is charging and low while it is discharging.

How do you determine the value of R1, R2, for example, if I have 3v supply and want the frequency to be 1?

There are heaps of 555 timer programs on the net.
Google "555 timer program"
or download one at www.schematica.com

The frequency of the 555 oscillator is independent of the supply voltage. F = 1.44/(Ra+2*Rb)/C1 where Ra is the resistor from Vcc to pin 7 and Rb is the resistor from pin 7 to pin 6.

To find a data sheet for a common IC, go to www.mouser.com, enter the part number in the search box, then click on the data sheet symbol.

555 timer

hi alanzhao

check my post dated jan 14, 2004. it has all the formula you can use to operate the 555 timer.