# 555 timer 50% duty cycle

Discussion in 'Electronic Basics' started by panfilero, Dec 27, 2006.

1. ### panfileroGuest

Hi, does anyone know how I could get a 555 timer to operate in astable
mode at a 50% duty cycle? All the circuits I've seen are always for
greater than 50%. thanks.

2. ### James BeckGuest

I just googled 555 50% PWM and got :
http://www.dprg.org/tutorials/2005-11a/index.html

Look at what is done with the diodes D1 and D2

Jim

3. ### Ian MalcolmGuest

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.

4. ### John FieldsGuest

---
An easy way is to use a 7555 and let the output feed the RC:

View in Courier

.. +-------------+
.. | |
.. [Rt] +V |
.. | |8 |
.. | 6+---+---+3 |
.. +--|TH OUT|--+-->OUT
.. | 2|___ _|4
.. +-O|TR R|O--+V
.. | +---+---+
.. [Ct] 1| 7555
.. | |
.. GND GND

Another way is to use the circuit you have and run the output
through a divide-by-two circuit:

+-----------+
| +-----+ |
+--|D Q|--|--->OUT
555OUT>-----|> _| |
| Q|--+
+-----+
HC74
4013

5. ### DorianGuest

mode at a 50% duty cycle? All the circuits I've seen are always for
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

6. ### kellGuest

Connect a signal diode like 1N4148 in parallel with the resistor
between pins 6 and 7, with the cathode (stripe) oriented toward pin 6.
With this arrangement you can get any duty cycle you want. You can
even get a fixed frequency, variable duty cycle oscillator if you
replace the fixed resistors with potentiometer. Connect the ends of
the pot to pins 6 and 8, the wiper to pin 7, and the diode from pin 7
to pin 6.

7. ### Richard Seriani, Sr.Guest

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

8. ### jasenGuest

change the voltage on pin 5 or use a different circuit, how much precision
do you need?

Bye.
Jasen