The pump speed is actually controlled with an external 0.3 to 5.0 volt signal. Nice feature also is the tachometer out signal.
Ao yes, you can use a low pass filter which amounts to a resistor and a capacitor RC Network where the capacitor charges through the resistor reaching the average voltage of your PWM. That's slightly over simplified.
Another option is to use a DAC (Digital Analog Converter) like a common MCP4725 where the 4725 gets a digital input from your Arduino and outputs an analog voltage value based on input. A Google of DAC4725 Arduino will get you plenty of results and examples.
Finally if you just want to control speed a potentiometer is all that is needed. A 10K , 10 Turn pot would give you a nice transition over speed. One side of pot to VCC one side to 0V and the wiper to an analog in of your pump Orange wire.
Note the caution on the pump as to Never connect power ground and 0 volts together.
You can also use the Tach Out from your pump to read pump speed.
Thank you so much 2112 and Ron - I really appreciate your assistance. I'm starting to get the hang of this, but thought of two questions.
From my understanding, other folks were using the PWM signal from the Arduino to power the pump speed control directly - no DAC or Low Pass Filter required? I believe that this pump actually has a PWM controller in it (FYI - all of the "FG" series pumps operate the same, the 209 vs. 304 refers to seal material, internal pressure relief, and gear size, but the MG pumps are slightly different):
I'll read up on the DAC and low pass filtering just for my knowledge. I learned about logic gates last night which may be helpful... but every bit I can learn will be great!
Wouldn't let me put two pieces of media in one post as a new user...
My last question for now is, if the pump that I purchased only has red, black and orange wires coming out, can I use the "Alternative Connection" instead of the standard one, with the potentiometer wiper still wired to an analog in pin on the Arduino, and no scope wired up?
As shown in the manual there is a voltage divider consisting of a fixed value 15K resistor and a variable resistor (a pot). Looking at the image when the pot is at the bottom end of travel Vout (orange wire) will be about 5.7 volts. When the pot is at the top of its travel Vout will be about 0.0 volts. So the speed control is an analog signal of about 0.0 to about 5.7 volts. This gives a speed range of about 300 to 5,000 RPM.
Nothing I read indicates using a PWM signal for speed control. You can also see that the speed command verse true speed is not quite linear. This is why we see: NOT connect the 0V speed command (brown) (in this case the speed linearity vs command can be > 5%).
So when all is said and done where you get your speed control is a matter of choice. A simple pot and resistor as shown, Getting the voltage from a D/A converter as mentioned or outputting a PWM from an Arduino or other source, running it through a low pass filter and deriving a speed control voltage. This is where you, the end user, decides how they want to manage the speed control command.
I will add this. While the drawing in the manual reflects 0 to 5.7 volts using a 5.0 volt Arduino will not get you up to 5.7 volts. If you have a 5.0 volt supply available you can try applying 5.0 volts to the orange (speed command) line and see what you get.
Luckily, I don't need the top end of the pressure range from this pump (I need 9 bars max, and I think at about 5v this pump can provide 12 bar). So hopefully 5v max from the Arduino will be perfect.
I'm going to try what the folks on Home Barista did - use the PWM signal directly - and if that doesn't work as expected, I'll figure out how to get a simple low pass filter wired in properly. Before I attempt the PWM route I'll check with the manufacturer to make sure I won't damage the pump electronics.
Thank you again for all of your help, and I'll update my post with progress I make!
