At 12:06 PM 12/17/98 , you wrote:
>"Fehrenbach, Robert J" writes:
>>The app notes for the 555 timer generally have a configuration using a
>>single resistor which produces a 50% duty cycle.
>
>        I stand corrected.  I don't have the application notes in
>front of me, but I can almost imagine what the circuit looks like.  If
>you place a resistance in series with a capacitor, the same time
>constant applies for charging as for discharging so that  one should
>get a 50% duty cycle if the capacitor is charged through the very same
>resistor  that ultimately discharges it.

To generate a square wave with a 555 timer you need to use the 7555 (CMOS)
version.  Hook up 4(/reset),8(Vcc) to +V, 1 (GND) to GND, 5(control
voltage) to GND through a 0.01 cap, connect pin 2 (trigger) to pin 6
(threshold) to a capacitor to ground. Connect a resistor between pin 2 and
pin 3 (output)  In this configuration, the output serves as both the
current source and sink for charging and discharging the capacitor.  since
the output circuitry in the CMOS version is low impedance (with respect to
the resistor), you charge and discharge the capacitor through the same
resistor, so you get equal on and off times ==> 50% duty cycle.

Using the tradiditonal 2 resistor method you cannot get an exact square
wave since you are charging the resistor through R1+R2 and you are
discharging the resistor through only R2. There are other ways to square up
a 555 output but they depend on putting another resistor in the discharge
path.

Matt Bennett