Electronics Forum / Basics / December 2006

Also notice that the timing resistor is fed from the Output and the Load
 fed from the Discharge pin (pullup resistor required or directly drive
a load with low side switching).   If you dont need a variable duty
cycle, leave out the diodes and try a single timing resistor from the
Output to the junction of Threshold and Trigger.  It will probably be
very close to 50% but might be off by a little.  The circuit Jim gave is
trimmable.

The CMOS version of the 555 (e.g. 7555) is far superior to the Bipolar
version (e.g. NE555) for a variety of reasons. Reason 1 is reduced power
consumption. Reason 2 is the absence of the quirky short circuit spike (400
ma) during an output transition. Reason 3 is that output levels of the CMOS
version approach the supply rails (ground or common and VCC) as they should.
Also I believe the voltage divider resistors in the bipolar version are 1K
while they are 100K or more in the CMOS version. This is very useful if
you're modulating the pin 5 voltage level.

Hooking the output of the 555 (pin 3) to the RC network will give you a 50%
duty cycle as John mentioned and this always works best with the CMOS
version.

Dorian

If you have the space, feed the output of the 555 into the clock input of an
edge-triggered  J-K FF. Tie J and K high to create a toggle and the output
will be a nice 50% duty cylce at 1/2 the input frequency.

Richard