So it's for providing 10% or 100% power to a motor.
And you don't want to use PWM for a simple thing like that, but rather a mosfet construction that probably won't work.
I'm out.
Leo..
duinoMike:
I need build or find an off the shelf item that will allow me to switch between two step down supply voltages, i.e., 1.5 volts and 12v via a logic signal. Supply would be 3A max.For example a high signal would have the device supply 1.5 volts and low signal would have the device supply 12v. Any ideas?
LM-317 and a little BJT or mosfet to ground or not ground a resistor?
That is, make the circuit with the proper resistors to give you 12 volts, then add another resistor from the CTRL pin of the 317 to ground to give you 1.5, then use the transistor to ground or not the "1.5 volt" resistor.
Place a capacitor from CTRL to ground to give a "smooth transistion" from one voltage to the other if you like.
Switching with two MOSFETs won't work, due to the built-in diodes. The lower voltage driver must be protected from reverse current by a diode.
I wonder how expensive the intended circuit will work out, compared to a (half) H-bridge and PWM.
I'm driving a motor at 10% and 100%. With the motor I'm using PWM creates a high pitch sound under load that's why I'm trying to come up with a power source switching solution. The high pitch sound is not present when I use a 1.5v source.
duinoMike:
I'm driving a motor at 10% and 100%. With the motor I'm using PWM creates a high pitch sound under load that's why I'm trying to come up with a power source switching solution. The high pitch sound is not present when I use a 1.5v source.
Hi,
What model Arduino are you using?
The have different PWM frequencies on different PWM pins.
Tom.... 
LM-317 and a little BJT or mosfet to ground or not ground a resistor?
This seems viable thanks, may try it.
I'm using Arduino Uno R3.
I actually may need other voltage levels as well maybe 4 total. I may also change the motor in the future so I can use PWM. I have to admit I'm a tinkerer so coming up with a solution without using PWM is a challenge.
Thank you all for your input.
I'd like to float another idea, what if I use a buck step down converter and take out the pot and replace it with say 4 similar external pots set at different levels then use 2N2222 transistors as switches to selectively pick the desired pot hence the voltage.
duinoMike:
I'd like to float another idea, what if I use a buck step down converter and take out the pot and replace it with say 4 similar external pots set at different levels then use 2N2222 transistors as switches to selectively pick the desired pot hence the voltage.
Yes you may be able to do that.
Depending on spare pin count you could just have a range of fixed resistors that you can switch in and out
or use a Digital Pot that you can talk to with SPI.
https://www.arduino.cc/en/Tutorial/DigitalPotentiometer
Or similar
PWM'ing on motors will always create some squeal, some can be removed with the right caps.
Actually that is not 100% true.
https://forum.arduino.cc/index.php?topic=188309.0
also perhaps have a look around at what the DCC projects do.
Great!
I like the idea of switching pots.
Does any one know how the pot is wired in a buck like:
In other words the the portion of a likely circuitry containing the pot. I assume its varying a duty cycle.
I have the same (similar) unit here.
I suspect that the pot maybe part of the feedback circuit.
Will a digital pot work.??
Who knows such things.
Ahh, just buy one or two and see I guess.
Just the first one shown on flebay,
Hiddenvision:
PWM'ing on motors will always create some squeal, some can be removed with the right caps.
Caps will kill the motor drivers, they only make sense in RC low pass filters. Find out yourself :-]
duinoMike:
I'm driving a motor at 10% and 100%. With the motor I'm using PWM creates a high pitch sound under load that's why I'm trying to come up with a power source switching solution.
Then use a (lower or higher) PWM frequency that's outside human hearing range.
Leo..
krupski:
LM-317 and a little BJT or mosfet to ground or not ground a resistor?That is, make the circuit with the proper resistors to give you 12 volts, then add another resistor from the CTRL pin of the 317 to ground to give you 1.5, then use the transistor to ground or not the "1.5 volt" resistor.
OP mentions a motor, without specifying current. Yet motor currents tend to be high, and a drop from 12V to 1.5V means even with a pretty big heatsink you can't draw more than about 200 mA. While possible, not likely to be a working solution, and much more complicated than PWM.