Need to build a simple PWM generator

Litterally heaps of pwm modules on Ebay and the like, save building completely from scratch.
One could easily modify one of these to get your range requirements.

or, if you like to tinker, could be done via Arduino if you are that way inclined.

Example.......... https://www.ebay.com.au/sch/i.html?...13&_nkw=555+pwm+speed+control+module&_sacat=0

 

As well as the raft of 555 based PWM controllers out there, there is a variable PWM generator by RMCybernetics.com who also offer a schematic.
https://www.rmcybernetics.com/science/diy-devices/diy-pwm-signal-generator

 

I put together the one I linked to, as I was in need of a bench version with variable frequency as well as pulse width to test motor driver boards.
With this one, the width control does not affect the frequency.

 

Wow, Jeff! Welcome back to the forums. Looks like you are taking up electronics as a hobby too. I hope your precision machining business is also doing okay.

I am totally confused by your duty cycle description. A PWM (pulse width modulated) speed controller for DC motors should produce maximum speed when the duty cycle is 100% or "on" all the time and zero output when the duty cycle is 0% or "off" all the time.

Could you please explain what that little doobie in your video is, the one that has push-buttons for frequency and duty cycle, because it appears to behave exactly as you describe: lower duty cycle causes higher motor speed? Exactly how does this happen?

I am sure someone will post a link to a page with a schematic of a 555 producing variable duty cycle 100 Hz output pulses by selecting one of four resistors with a rotary switch having a common terminal. That seems to be the problem you are trying to solve. However, this resistor selection may need to be accomplished with small DPDT relays to allow an isolated "floating" resistor to be inserted in the 555 timing circuit.

If you look at the schematic of a typical free-running 555 oscillator, there are two resistors in series with the "timing" capacitor. The capacitor initially charges through both resistors until it reaches a voltage that causes the output to change state, whereupon the capacitor discharges through just one of the resistors until its voltage drops low enough to trigger a transition to the other output state. The transitions occur at 2/3 Vcc when charging and at 1/3 Vcc when discharging.

The process of charging through two resistors and then discharging through one of them continues as long as power is applied to the 555 timer. The duty cycle this produces is influenced by the values of the two resistors, as is the frequency. Here is link to a page that shows a variable duty-cycle, constant frequency, 555 oscillator. Note the two diodes that are used to steer the charge and discharge paths for the timing capacitor. Unfortunately, this circuit uses a potentiometer, not a single switchable resistor, to select the ratio of charge and discharge resistance. Of course you could double up on the number of relays to allow substitution of two resistors in place of the potentiometer. Or find small relays with 4PDT contacts instead of using two relays with SPDT contacts. The latter are very easy to find, but 4PDT relays are as scarce as hen's teeth. Anyhoo, relays solve a lot a problems and will get you up an running the fastest way possible.

Of course, the simplest solution is just to use a PIC microprocessor to generate your variable duty-cycle pulse. Set the speed by selecting taps at one of four series-connected resistors using your rotary switch. One end of the resistor string is connected to Vcc and the other end is connected to ground. Read the selected tap voltage using the built-in analog-to-digital converter on one of the analog input ports. Use that reading to select the duty cycle from one of four routines. each of which produces predetermined pulse-width-modulated (PWM) output. Easy peasy. If you build it, I will program it for you.

 

As mentioned the way to supply the PWM exactly what you need, but it requires knowledge of picmicro's!
A small 8 pin can be programmed to supply the Exact frequency.
No caps or resistors.
Even a PGM to do the calc, https://www.micro-examples.com//public/microex-navig/doc/097-pwm-calculator.html

 

Simple enough with Arduino.
Could use a promini (AU$4.00) or just the chip itself if you want it smaller.

https://create.arduino.cc/projecthub/muhammad-aqib/arduino-pwm-tutorial-ae9d71

 

Outstanding idea! Plus, you already know it will work, too! Is this a vintage vehicle that you are trying to restore to its former glory?

As for suggesting a PIC solution... that would be the simplest but not the easiest or quickest solution. You can always substitute a PIC solution later, if it would restore authenticity to your automobile. Don't throw away that four-position fan switch just yet!

Maybe I can find some time to prototype something for you. Sort of pay-back for all the time we wasted trying to get that flashlight to work.

I, too, have a few PICs left over from the flashlight project. I haven't set up a "proper" electronics lab since moving here to Venice, Florida, but it won't take much effort to get a PIC project up and running again. If you would like to have a continuously adjustable fan speed control, that can be easily arranged, with either a plain old potentiometer or with a cheap rotary encoder... clockwise turn to increase fan speed, counter-clockwise to decrease fan speed. At some point on the low-speed end, the output reverts to 100% "on" duty cycle, which according to your explanation will stop the motor. There also needs to be some "minimum" duty cycle, like maybe 98% or so, that will just get the fan moving after a slight clockwise movement of the "fan speed" control. And perhaps a "maximum" duty cycle, like maybe 17%, that represents maximum fan speed when the control is rotated towards maximum speed. All this through the magic of software, re-written until it works.

Of course, with the PIC solution, you lose the spiffy digital readout of pulse frequency and pulse duty cycle... but maybe just hearing the fan speed up and slow down at the twist of your fingers would be "gud enuf" for this project...

 

As with any micro, they mostly operate at 5v or less and the voltage in an auto can go to at least 14.8v and it's a noisy environment.

Best to use a small buck converter and set down to 5v output and feed the Arduino through the Vcc pin rather than the RAW or Vin.

Nice wheels.
Had a '65 4 door Pontiac for many years, replaced by my new "78 Celica when fuel prices went sky here in Aus.

 

Yes, well I was thinking more in the lines of this but anyhow......

https://www.amazon.com.au/Ultra-MP1...eywords=buck+converter&qid=1617608994&sr=8-15

Or more preferrably these.......
https://www.amazon.com.au/MUKUAI331...k+converter+mini+module&qid=1617609169&sr=8-6

Much smaller and easier to solder a small bridge and cut one track than to adjust the micro size pot.
Board is about 10mm wide.

 

I will write it for you without charge. See private conversation I started today.

Best of luck on finishing the '68 Firebird restoration. I had a brand-new, driven off the showroom floor, 1968 Mercury Cougar that I couldn't afford to keep up. It finally rusted out, parked outside next to my former home at 168 Medford St, Dayton, OH 45410. I gave it away to some kid who wanted it for parts. It wasn't a real "muscle car" like the Ford Mustang or your Pontiac Firebird, but it got me "around town" and to work.

There is a indoor classic-car retail establishment about a mile from where I live. I plan to visit it "real soon now" but have been avoiding it because of anticipated "sticker shock."

Jeff, you might be interested in this on-line magazine.