Part Search for an oscillator

[lerameur]

Hi,

anybody know of an oscillator or multivibrator than I can crontrol
both frequency and duty cycle. I tried to find a chip like the CD4047
but with variable duty cycle. My output needs to be about 12v.
I also looked at the 555 timer, but it is hell to tune the duty cycle
as the frequency changes.
Maybe there is a circuit out there with a 555 that has fixed frequency
and variable duty cycle?


thanks

Ken

 

[Rich Grise]

On Thu, 18 Sep 2008 08:13:49 -0700, lerameur wrote:

anybody know of an oscillator or multivibrator than I can crontrol both
frequency and duty cycle. I tried to find a chip like the CD4047 but with
variable duty cycle. My output needs to be about 12v. I also looked at the
555 timer, but it is hell to tune the duty cycle as the frequency
changes.
Maybe there is a circuit out there with a 555 that has fixed frequency and
variable duty cycle?

You can add a couple of steering diodes (in series with Ra and Rb), the
right direction, of course, and have independent control of Ton and Toff.

Or, for one, where I needed a fixed freq. but variable duty cycle, I made
an ordinary astable with discretes, but used a pot for the base resistors,
with the supply to the wiper, and each end of the pot to a 1K to the base
of each transistor.

Good Luck!
Rich

 

[Jasen Betts]

On 2008-09-19, Electronworks.co.uk Electronic Kits <electronworks@hotmail.co.uk> wrote:

What about a 556 timer (dual 555).
Wire one in monostable mode (to give a single shot output that is
controllable) - this is your duty cycle.

it won't go close to 100% duty cycle reliably.
(it'll tend to break into frequency divider mode)

Trigger it with the other 555 running in astable mode - this is your
frequency

Let me know your thoughts
Bill Naylor
Electronworks.co.uk - electronic kits for education and fun

the 555's got 2 comparitors in it and generates a triangle (ish)
wave on the timing cap.

could possibly use the second 555 to measure the first's state.

advantages of the 555/556 - stronger totem-pole output drive than
common op-amps, stronger open-collector output (on the discharge pin)
than commom comparitors. disadvantages 555/556 is larger and consumes
more power than low-power op-amps. input impedance on the CV pin is
rather low (about 6K to 2/3 vcc).

eg: here's an all-555 variable frequency capable PWM 0-100% circuit I drew up
in ltspice

Version 4
SHEET 1 900 680
WIRE 48 -64 32 -64
WIRE 80 -64 48 -64
WIRE -400 -32 -416 -32
WIRE -384 -32 -400 -32
WIRE 48 -32 48 -64
WIRE 64 -32 48 -32
WIRE 160 -32 144 -32
WIRE 560 -32 544 -32
WIRE 576 -32 560 -32
WIRE -400 -16 -400 -32
WIRE -208 0 -240 0
WIRE 48 0 48 -32
WIRE 48 0 16 0
WIRE 304 0 272 0
WIRE 560 0 560 -32
WIRE 560 0 528 0
WIRE -240 16 -240 0
WIRE 272 16 272 0
WIRE 160 32 160 -32
WIRE 160 32 32 32
WIRE -208 64 -336 64
WIRE 32 64 32 32
WIRE 32 64 16 64
WIRE 224 64 208 64
WIRE 304 64 224 64
WIRE -400 80 -400 64
WIRE 32 80 32 64
WIRE 48 80 32 80
WIRE 144 80 128 80
WIRE 144 128 144 80
WIRE 144 128 16 128
WIRE 208 128 208 64
WIRE 208 128 144 128
WIRE 304 128 240 128
WIRE 560 128 560 0
WIRE 560 128 528 128
WIRE -240 176 -256 176
WIRE -224 176 -240 176
WIRE 272 176 256 176
WIRE 288 176 272 176
WIRE -240 192 -240 176
WIRE -208 192 -240 192
WIRE 32 192 16 192
WIRE 272 192 272 176
WIRE 304 192 272 192
WIRE 560 192 528 192
WIRE 576 192 560 192
WIRE -336 256 -336 64
WIRE 208 256 208 128
WIRE 208 256 -336 256
WIRE 240 256 240 128
WIRE 656 256 240 256
WIRE -336 272 -336 256
WIRE 32 272 32 192
WIRE 560 288 560 192
WIRE -336 352 -336 336
WIRE 32 368 32 352
WIRE 560 384 560 368
FLAG -336 352 0
FLAG -240 16 0
FLAG 48 -64 vcc
FLAG 560 -32 vcc
FLAG -400 -32 vcc
FLAG -400 80 0
FLAG 272 16 0
FLAG -240 176 vcc
FLAG 272 176 vcc
FLAG 560 384 0
FLAG 32 368 0
FLAG 656 256 PWM
IOPIN 656 256 Out
FLAG 576 192 input
FLAG 224 64 triangle
SYMBOL Misc\\NE555 -96 96 R0
SYMATTR InstName U1
SYMBOL Misc\\NE555 416 96 R0
SYMATTR InstName U2
SYMBOL cap -352 272 R0
SYMATTR InstName C1
SYMATTR Value 100n
SYMBOL Misc\\battery -400 -32 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 5
SYMBOL Misc\\signal 560 272 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value SINE(3.5 1.5 100)
SYMBOL res 16 256 R0
SYMATTR InstName R3
SYMATTR Value 8K6
SYMBOL res 160 -48 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R2
SYMATTR Value 1.5K
SYMBOL res 144 64 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R4
SYMATTR Value 1.5K
TEXT -434 376 Left 0 !.tran .02


Bye.
Jasen

 

[John Larkin]

On Thu, 18 Sep 2008 08:13:49 -0700 (PDT), lerameur
<lerameur@yahoo.com> wrote:

Hi,

anybody know of an oscillator or multivibrator than I can crontrol
both frequency and duty cycle. I tried to find a chip like the CD4047
but with variable duty cycle. My output needs to be about 12v.
I also looked at the 555 timer, but it is hell to tune the duty cycle
as the frequency changes.
Maybe there is a circuit out there with a 555 that has fixed frequency
and variable duty cycle?


thanks

Ken

The usual way to do this is to make a triangle or sawtooth oscillator
whose amplitude is constant but whose frequency can be varied; a 555
does that. Then run the sawtooth into a comparator working against an
adjustable DC voltage level. The comparator output is a constant duty
cycle independent of the sawtooth frequency.

Or do it digitally, in an FPGA, with a DDS frequency-set loop and a
counter/comparator for the duty cycle.

John

 

[Robert Monsen]

On Thu, 18 Sep 2008 08:13:49 -0700 (PDT), lerameur
<lerameur@yahoo.com> wrote:

Hi,

anybody know of an oscillator or multivibrator than I can crontrol
both frequency and duty cycle. I tried to find a chip like the CD4047
but with variable duty cycle. My output needs to be about 12v.
I also looked at the 555 timer, but it is hell to tune the duty cycle
as the frequency changes.
Maybe there is a circuit out there with a 555 that has fixed frequency
and variable duty cycle?


thanks

Ken

See http://pwp.att.net/p/s/community.dll?ep=16&groupid=385588&ck=

Click on '555_timer.jpg'. You can use any old comparator, but high
input impedance ones, like FET input comparators, will be best. The
two pots control the frequency and the duty cycle. You can control
duty cycles between 0 and 100% using the comparator input pot.

If you like programming, a microcontroller can do this kind of thing
quite easily, and is trivial to program for these sorts of
applications.

Regards,
Bob Monsen