Build failure - uncontrollable noise

[Steve Anderson]

Hello all

I had a go at building an Auduino over the weekend, but I've not had
much success. I built it all on a breadboard, hooked up to a
Freeduino, using a bunch of 470k pots I had to hand. I've left it
plugged into the USB port for power, and used a speaker pulled from a
trashed pocket radio for output. I've checked, double checked, triple
checked, torn apart and rebuilt it, but all I'm getting is a scratchy
noisy tone that I can't control in any way.

I'm completely new to this. Can anybody provide an insight, help,
advice, etc.?

Steve

[Peter Knight]

Hi Steve.

Why are you using 470k pots? No point in triple checking the wiring if
you're using the wrong components. Get some 4.7k pots. Actually,
anything between 1k and 20k should be fine.

From the Atmel ATmega328P data sheet: "The ADC is optimised for analog
signals with an output impedance of approximately 10kohm or less.".
4.7k pots have an output impedance 2.35kohm or less. 470k pots have an
output impedance of 235k or less. That is why 4.7k pots are specified.

As for output - while a speaker will work, it won't be very loud and
is quite a strain for the Arduino to drive. I suggest you use an audio
amplifier of some kind. Try PC speakers, personal stereo speakers, or
the aux in on your hifi.


Peter

[Steve Anderson]

Thank you ever so much Peter!

Like I said, I'm an aching n00b. I'm fine with software, but
hardware's a new an interesting challenge for me. I remember Ohm's law
from school, and I know that having 5V over a pot means you can tap
off 0-5V (and that's what I understood was happening, which is why you
could use varying resistance pots) but I've never actually got any
further than that!

The 470k pots were some I found and thought I'd put to good use. I'll
get some lesser pots and try them as soon as I can find some. I used a
speaker because I wanted a result I could experience as quickly as
possible before continuing (my making time is at a premium!); if I can
get something working it will have a 1/4" jack to run through my bass
effects box and mixer.

I'll let you know how it goes!

Steve

2009/5/28 Peter Knight <cath...@gmail.com>:

--
Irregular Shed - http://www.twindx.com
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455FE10422CA29C4933F95052B792AB2

[Peter Knight]

On May 28, 10:27 am, Steve Anderson <irregulars...@gmail.com> wrote:
> ... I remember Ohm's law

> from school, and I know that having 5V over a pot means you can tap
> off 0-5V (and that's what I understood was happening, which is why you
> could use varying resistance pots) but I've never actually got any
> further than that!

Yup - and that is accurate, provided no current flows through the pot
wiper. When current flows, the voltage changes. The lower the pot
resistance, the less it moves. But also - the less the pot resistance,
the higher the current you're wasting flowing straight through the
pot. Somewhere between 1k and 20k is a sensible compromise between
those two.

Another thought occurs - I'm presuming you're connecting the pots to
the analog pins - but if you are connecting them to the digital pins
by mistake, that could explain everything you describe.

[Steve Anderson]

2009/5/28 Peter Knight <cath...@gmail.com>:

> Another thought occurs - I'm presuming you're connecting the pots to
> the analog pins - but if you are connecting them to the digital pins
> by mistake, that could explain everything you describe.

I'm not *that* wet behind the ears =) it sounds like the pot
resistance is key because I know that the pots themselves are fine
from my multimeter readings, that the Freeduino's analogue inputs work
correctly (I ran an LED-dimming sketch with a 10k pot when I got it)
and that there's nothing shorting under the breadboard.

I'll get it going!