555 timer 50% duty cycle

[panfilero]

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.

 

[James Beck]

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.

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

 

[Ian Malcolm]

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

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.

 

[John Fields]

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.

---
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

 

[Dorian]

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

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

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

 

[kell]

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.

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.

 

[Richard Seriani, Sr.]

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

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

 

[jasen]

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.

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

Bye.
Jasen