# 555 50% duty cycle

Discussion in 'Electronic Basics' started by amdx, Sep 29, 2006.

1. ### amdxGuest

I thought I had seen a 555 circuit with a 50% duty cycle and variable
frequency.
It used two diodes in the charge/discharge circuit.
I can't seem to locate the schematic.
Anyone have the circuit?
Thanks
Mike

2. ### Michael BlackGuest

That sort of thing would be in all the books about the 555, and I would
have thought in the application notes.

A simpler solution for a lot of purposes is to connect pins 2 and 6 together,
with the capacitor from there to ground. The feed that junction with a
resistor off pin 3. The capacitor will be charging and discharging through
the same resistor, and any variation comes from the not quite symmetrical
nature of the output of the 555. I seem to recall the original bit about
this, I think it was in "Electronics" suggested a pull up resistor on pin
3, but I never fussed with that, and it's been so long. I think somewhere
along the way someone suggested a CMOS 555 would result in better operation.

I was always satisfied with it, and after I came across the circuit about
when it came out, I always used it when I need a simple oscillator.

Of course, if you really need exactly 50% duty cycle for some very specific
purpose, it may not fit the bill. But I suspect the circuit with the
diodes has limitations too. The problem comes from wanting it to remain
constant, while the frequency changes, so the more you want to change
the frequency, the more trouble it would be to keep to the exact 50%.
The easiest way if you need exactly 50% over a wide range is to feed
an oscillator into a flip-flop, and there you are, albeit at one half
the frequency.

Michael

3. ### Guest

If it has to be 50% duty cycle take a look at the 4047 configured in
monostable mode:

http://www.datasheetcatalog.com/datasheets_pdf/4/0/4/7/4047B.shtml

4. ### John FieldsGuest

---
Blow off the 555 and use a 7555.

See figure 1(a) at:

http://datasheets.maxim-ic.com/en/ds/ICM7555-ICM7556.pdf