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[on:]

Hey all. 

I'm slowly getting through a laser spirograph project that's controlled by an Arduino (save/load patterns etc  ) and I have 4 100ohm pots that I'd like to use for controlling the speeds of the motors. Now, I bought these because originally I planned for this circuit to be completely analogue - they're wire wound and can take up to 2W, which is perfect for driving motors, but now I'm using the Arduino I'd love to adapt them for it.

A single one works fine with the standard setup for a pot, but of course because this is only a 100ohm pot it constantly draws 50mA from the Arduino (on the 5V output). This isn't desirable at all, especially since I need 4. I've tried several combinations of resistors with the analogue reference pin and the pot itself, but so far haven't got anywhere useful.

Is there an easy way to make these draw less current? I'd love to not have to buy 4 more, however cheap they are. I'm quite poor at the moment  I've tried making the overall voltage drop less and changing the analogue reference, but I've been having issues with it.

Thanks in advance!

[Reply #1 on:]

There is no easy way to do this.
Probably the best way is to wire the + end of the pot to a PNP transistor then to +ve and only switch on the current to that pot while you make the measurement. In that way there is only one pot connected at any one time and only breafely at that.

[Reply #2 on:]

There is no easy way to do this.
Probably the best way is to wire the + end of the pot to a PNP transistor then to +ve and only switch on the current to that pot while you make the measurement. In that way there is only one pot connected at any one time and only breafely at that.

Sadly I don't have enough ports for that - 4 switches, 4 pots, 8 motor control (motors go both ways). I'm just glad that someone's confirmed it for me so quickly - it's too difficult. Thanks!

[Reply #3 on:]

How about putting a high-side resistor on the pots such that they form a voltage divider with approx 1v at the junction. Then use the internal 1v1 reference.

______
Rob

[Reply #4 on:]

How about putting a high-side resistor on the pots such that they form a voltage divider with approx 1v at the junction. Then use the internal 1v1 reference.

______
Rob

I'm not 100% sure I understand - so put an appropriate resistor so that the highest voltage is 1.1v between 5v and the left pot pin?

Aka

5v----resA-----1.1V here----100ohmPot-----0v (middle pin goes to A0)

Calculate resA=354.545 ~ 360ohms

That's still 10mA per pot - is it small enough?

Also because my brain is refusing to cooperate, is that one 360 ohm resistor per pot or one for all four? Is a resistor fine to cope with 40mA? Pretty sure, but want to check...

[Reply #5 on:]

Your numbers seem correct.

That's still 10mA per pot - is it small enough?

Small enough for what? that's a total draw of 40mA for the lot, well within the bounds of the Arduino regulator. Less would be good but as you say that's what you've got.

is that one 360 ohm resistor per pot or one for all four?

One per pot.

Is a resistor fine to cope with 40mA?

Yes, you can get resistors to handle 100A. It's just a matter of getting them the right size. That's still only 40mW (10mA * 4V) per resistor so just about anything will do.

______
Rob

[Reply #6 on:]

Excellent, thanks for the clarification. I think I should be right using the 360 ohm resistors then - I probably have some laying around somewhere...

Thanks again!

Will

[Reply #7 on:]

@ Graynomad
Good idea, I should have thought of that. Mind you I was in bed and had just woke up when I answered. That's the tyranny of an iPad for you.

[Reply #8 on:]

That's the tyranny of an iPad for you.

he he, so far I've resisted i-everything.

______
Rob

[Reply #9 on:]

Wire them in series and it'll be 12.5mA for the lot, but you'll only get 8 bits analog resolution as each will only range over 1/4 of the analog range.