How big is your motor?  An old style mouse with a ball will have inside it two small plastic discs with holes in them, and some kind of sensor that goes over it to detect when light passes through.

No they don't they have a log law which can seem like two regions but it is continuous.

I'm not saying you're wrong, as I really have no clue how pots are made, but this page, which is what I was quoting, seems to disagree with you:

http://sound.westhost.com/pots.htm

All this tapering proved a rather expensive exercise, so manufacturers economised ("they" won't notice the difference!), and worked out a method of using two resistance elements of differing resistivity, and joining them to create what I referred to as the "Commercial log" taper. In short, it doesn't work (not properly, anyway), and the discontinuity where the two sections join is almost always audible with cheap "log" pots.

Normally they are such that when the pot is turned fully anti clockwise the wiper will be at the ground end.

Hm...  Makes sense.  

Unfortunately my volume control will be on the left side of my prop.  That means turning it clockwise to increase the volume would have the user turning the knob towards them.  Bit counterintuitive.  :/  Guess I gotta decide if it's more important to have an audio taper or have the volume increase as you turn the knob towards the front of the prop.  

(And yeah, I know it's possible to alter the law of a pot so I could make a custom one, but it's not worh the extra complexity.)

Huh.  That works great!  And it looks like I won't have to change my PCB layout too much to make those connections either.  Thanks!

I left my 1K resistor on the pin and connected the center pin of the pot to that btw.  I assume when the wiper is all the way at +5v that I still need to limit the current.

If I connect an audio pot up to this, is it gonna matter which one of the outer pins I connect to ground and which I connect to the pin on the Arduino?  Or is using an audio pot with this setup not a good idea?  I'm concerned about how cheap audio pots have tow discreete regions of resistance on them and whether flipping things around might produce weird volume curves.

The linear pot varies the volume okay, though most of the loudness is on the top end.

Hi guys,

I want to add a volume control to my project where I have a piezo on a 1K resistor, drawing 5mA at 5v.  

This is the pizeo I'm using:
http://www.puiaudio.com/product-detail.aspx?partNumber=AT-1750-TFL-LW95-R

For my first attempt, I added a 10K linear potentiometer in series with the resistor, and connected to act as a variable resistor.  But when I turn the knob all the way down, my piezo gets significantly quieter... but it's still quite audible.

Now obviously the problem here is my pot isn't big enough; but I don't know how big a pot I should use.  I need a pot which is big enough that I can't hear the piezo when I have it turned all the way down, but small enough that it's not compeltely silent over half the knob's travel.

I tried calculating what size pot I'd need based on sound pressure levels and assuming that halving the current would halve the sound pressure, but I got nonsensical results like having to reduce the current by 5000x to get near silence, which would require a 5 million ohm pot.

So, any suggestions for what size pot I should go with?  I intend to use an audio pot btw, I just used a linear one because that's what I had at hand.

(maybe not as much nowadays, what with all the power regulation and conversion options available - but it is still a valid method).

It's not that it's uncommon that concerns me.  

My main concerns were that:

1) I didn't know if there was even any good location on the AAA battery holders I purchased to make a solder connection, and if I did, if it would be reliable.  (It appears there is a rivet I could solder to after all, though I still don't know how reliable a connection it would be.)

2) Going this route would force me to use AAA batteries and I don't know for a fact that in the end I will have enough space in the prop for them, and I won't learn that till after I get the PCB's manufactured.

3) I don't know how to calculate battery life with such a setup.

4) I want to learn how to use voltage regulators, because I'm sure this isn't the last time I will encounter this issue, and it will be nice to have a plug and play solution ready to go when the time comes that I need 5v instead of 6v, and I'm only powering things from one 9v.

They may not be common, but yes, they do exist. Here in Arizona we have several Batteries Plus locations:

Oh.  Yeah, we've got one of those down the street. :-)  I've only been in there once, I thought their primary business was car batteries.

Kylek:

The info I got on battery capacity I got from here:
http://en.wikipedia.org/wiki/Battery_%28electricity%29

It lists a typical 9v as having half the capacity of a AAA.

As for what the manufacturer states on the package, I wouldn't put any faith in that... they're using it for marketing after all.  Still, it may well be the AAA's have more capacity.  And it may be true they can deliver more current.  I don't know.  I wish there were some other source I could trust to confirm the mAh with that would also specify how much current they can put out.

Oh man, am I dumb.

The 350mA I quoted myself as drawing from the Arduino is for one of my projects which will have a 64x64 led array.  But the servo will be in a circuit which is very similar but uses much less power, lighting only three or four led's at a time, and playing sound on a piezo.

I can't say for sure that the Arduino's onboard supply will have enough juice to power a servo in addition to that, but the chances seem much better with like 340mA available.

Guess I gotta check out some servos and figure out how much power they'll draw.

I found this 6v regulator:
http://www.mouser.com/ProductDetail/STMicroelectronics/LF60CV/?qs=sGAEpiMZZMsGz1a6aV8DcCwRipvt0X3P3GV4HfM2J2I%3d

But I've got a problem.

I found this circuit diagram showing how to hook up a voltage regulator:

http://homepages.which.net/~paul.hills/Circuits/PowerSupplies/PowerSupplies.html

And that page states:
"The capacitor between the battery and the regulator is required to ensure there are no high frequency variations in the supply to the regulator. The capacitor after the regulator is required to supply high frequency variations in the current drawn by the logic chips, which the regulator is not fast enough to react to."

Which makes sense, but it doesn't tell me what values I should use for the caps, or why.

The datasheet for the regulator does contain a few circuits:
http://www.st.com/stonline/books/pdf/docs/2574.pdf

And it would seem that most of the circuits there use 0.1uF for C1 and 10uF for C2, but I don't know why, and I don't know if they're electrolytic or ceramic capacitors.

The reason why I'm cncerned about the capacitor values is because I found another 6v regulator on Digikey:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=497-1445-5-ND
http://www.st.com/stonline/products/literature/ds/2143/l7805.pdf

And that one's datasheet contains a schematic which uses a 0.33uF cap on the input.

I think the purpouse of the larger cap is just to smooth out larger fluctuations in the input voltage, but am I going to have any fluctuations at all pulling voltage from a battery which is also connected to my Arduino, and how do I know how big of a cap I really need?  

And again, should I be looking for an electrolytic or ceramic cap here?  I'm pretty sure the 0.1uF cap is a ceramic cause I think that's the same size cap often used as a decoupling cap, and I'm fairly sure those are ceramic.  So I'm guessing these are both ceramic?


[edit]

And now I find this page which says ceramic capacitors on output are bad:
http://www.national.com/nationaledge/jul02/article2.html

"The biggest reasons many LDO's oscillate are:

...

b) Using a ceramic output capacitor on an LDO not designed for it. The typical 2.2 - 4.7 uF ceramic capacitor will have an ESR of about 5 milli Ohms. This puts the ESR zero somewhere around 6 MHz where it clearly won't help compensate the loop. Using ceramics on the output of LDO's which are not designed to work with them is presently the #1 reason for unstable LDO operation.

Why most LDO's hate ceramic bypass capacitors

The last section explained why most LDO regulators will not operate in a stable mode with a ceramic output capacitor. Their loop requires the ESR of the output capacitor to supply a zero which gives the phase lead necessary to cancel out the effects of one of the low-frequency poles.... and ceramic capacitors have almost no ESR, so they won't provide any phase lead. Most LDO's designed in the late 1980's and early 1990's were made assuming a Tantalum capacitor would be used for the output capacitor, and so they don't tolerate ceramics very well."

What else is hooked up to the arduino? 350mA seems a tad high.

An array of 64 green leds (multiplexed), an illuminated pushbutton switch with one green led, two yellow leds in series, and a piezo speaker.

Usually people add capacitors to both sides of the regulator.

Why?  I'm guessing that's to smooth current spikes.  But is that necessary if the only thing that would be connected to it is the servo?

4 AAs for the servos and a 9volt to run the arduino might make more sense.

Use two 4-cell AA battery packs connected in series; that would give you 12 VDC (or if using rechargeables, 9.6 VDC) for powering the Arduino through its voltage regulator.

Did I mention that I have limited space in which to cram all these electronics and servos?  A box about 4x3x1.  Also, I'd rather not do any funny stuff tapping off the middle of a battery pack.

If you have a good hobby shop or a custom battery shop in your neighborhood

Custom battery shop?  Such a thing exists?

Kylek:
I misspoke.  I wouldn't be using six AA's, I'd be using six AAA's.

And everything I can find indicates that two 9volts supply the same mAh as six AAA's.  There is the question of how much current they can actually supply at once, but information on what one might expect to get out of different types of batteries is hard to come by.  The numbers I did find though seemed to indicate that two 9v in parallel should be able to supply a similar amount of current as six AAA's in series.

With that in mind, would you still reccomend six AAA's over two 9v, and if so, why?

My Arduino circuit will be running off two 9v batteries in parallel or six AA batteries in series, and while either of these might supply enough current to run a servo, I'm pretty sure servos need 6v.

So what should I do here?  Do I need to use one of those three pin voltage regulators?

The Arduino will be pulling about 350mA itself, and I suspect its onboard regulator won't be capable of powering a servo as well.

If I do get one of those regulators, I just need to hook a couple pins up to the battery and connect the servo to one pin and ground, right?  No resistors or anything?

I just saw this thread which indicates servos are designed to run off 6v:
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1267069898/15

So I guess if I've got 9v batteries I gotta also include something in the circuit to cut the voltage somehow if I want to drive the servo(s) directly from my power source.

So anyone got any ssuggestions in regards to that?

Hey guys,

I got a question for ya.

I was looking at the circuit for a stepper motor:
http://arduino.cc/en/Reference/StepperUnipolarCircuit

And I noticed the 4 pin version of the circuit is really similar to the circuit I'm building to control an array of LEDS:


Hi Res: http://raccoonrocket.com/gb/egbschematic-final.png


With the main difference being that I don't have anyhting connected to Pin 10, which is Com on the 2803A.

Now, as stated in this thread by GrumpyMike, connecting something to that pin isn't necessary with my LEDs:
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1269283709/7

But I know that with the motor it is necessary because... well I don't remember the term for it, but when the magnetic field collapses in the motor it creates a high voltage spike, and connecting the positive lead of the power source for the motor to that Com pin (which on my darlington IC is pin 10) will protect my circuit from those spikes.  Somehow.  I think.


Anyway, so I've got this circuit here, which is the PCB version of the shcematic above with a few modifications.  (Shifted the column pins down by one so I could make everything nice and neat, changed a few resistor names to better match the components they go with, added a potentiometer for volume control, and added support for dual 9v batteries in a parallel config.)


Hi Res: http://raccoonrocket.com/gb/egbpkecircuit.png

And I guess this circuit is fine without that pin 10 connected on the 2803A.  

But I'm making another circuit, almost identical to this one.  One where I'd like to have the option of controlling a unipolar stepper motor with four pins.

The other circuit will have no LED array.  Instead, those 8 row pins will be connected to 8 leds which go directly to ground.  And the column pins... four of those would be connected to my stepper motor.  The other four... I'd like to leave my options open for.  Maybe connect some more leds to them.

So my question is this...

Presumably, I want to connect the motor directly to my battery's positive lead, which is +9v.  I also want to connect pin 10, aka Com, to +9v as well.  

But what if I also want to put some leds on some of those pins.  And I want the leds to go to +5v on the arduino for whatever reason, via a resistor.

Will that even work?  Or is sinking +5v on some of the pins of the darlington array, and +9v on others, a no-no?



Also, let's say that instead of running a stepper motor, I want to run a servo.  To run a servo, I suppose I need to connect the power wire to +9v, and the ground wire to ground... but then there's the signal wire.  I'm pretty sure I can't hook that up to the darlington array.  That's gotta be pulsed high by the servo lib right?  And my darlington array will only pull pins low.  So I can't use the same connection for
the servo and the motor.  

If I want the option to connect a servo then, and I want to use those same pins the darlington is on, then I guess I gotta have some pads in there on the traces between the arduino and the darlington, to which I'd connect the servo signal wire.  I assume that would effectively bypass the darlington array, since while the traces would still go to the inputs, I wouldn't have anything on the outputs for those pins particular pins.  

At least that's the theory.  What I'm unsure about is if that +9v connection on Com for the motor would somehow feed back through the inputs to the servo or Arduino if I do that... but... I don't think it would.  I think there's diodes in there to prevent that.   I would like confirmation though.


The reason I'm going to all the trouble to design for both a stepper motor and a servo btw, is because at this time I don't know which I'll need in my final design.  Either one could potentially perform the task I need, depending on how I engineer the mechanical connections.

Okay.  Well based on that it sounds like I ought to be safe even if I go with an 82ohm resistor to give 20mA of current with these leds.

Anyway as for head room, I probably have enough head room to do 20mA or even 40mA; I'm just keeping my options open.  I haven't calculated exaclty how much current I'll be using in my array because I haven't decided what resisitors I'll be using for that, so that'll affect what I have left to drive these leds.  I know I'l be under what's safe for the Arduino, I just don't know by exaclty how much.  I've got a few places I can cut current if needed.

Hm, just looked at my spec sheet.

The particular led's I'll be using here have a forward voltage of 1.7typ, 2.6max.  

So when wired in series, I can provide them with 10mA with a 180ohm resistor, or 20mA with an 82ohm resistor.

But I don't know if 82ohm is too small for a current limiting resistor because I don't know how you determine what is too small.

[edit]

Well, I've googled every term I can think of, but I can't find any information/warnings about using small value resisitors for current limiting.