Part Search for an oscillator

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

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.

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

On 2008-09-19, Electronworks.co.uk Electronic Kits wrote:

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

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

See Don Lancaster's "CMOS Cookbook", pages

That's the basic oscillator, where R and C set the frequency. To vary the duty cycle as well, you use a pot for R, with the wiper connected to the output of inverter 2. The legs of the pot connect back to the C node through diodes with reversed polarities.

One problem with this is that R sets the frequency, and the duty cycle is set by the proportion of R to each diode. Just changing the single pot setting only changes the duty cycle, but not the main frequency. To vary both, you need a second pot in series with the R pot, but then you reduce the range of duty cycles.

On the other hand, if you don't need intuitive separate frequency and duty cycle controls, just replace the R pot with two separate pots as adjustable resistors. Each one will affect both frequency and duty cycle, but together you will get a huge range of both.

Best regards,

Bob Masta DAQARTA v4.00 Data AcQuisition And Real-Time Analysis

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

See

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