555 timer 50% duty cycle

I just googled 555 50% PWM and got :

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.

--
Ian Malcolm.   London, ENGLAND.  (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk [at]=@, [dash]=- & 
[dot]=.
*Warning* SPAM TRAP set in header, Use email address in sig. if you must.

--- 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

-- JF

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

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.

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

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

Bye. Jasen