On 26/05/2011 13:46, V G wrote:
> Hey all, after seeing this (http://das-labor.org/wiki/Stimmmopped/en), I
> wanted to make my own very simple guitar tuner.
>
> My schematic: http://postimage.org/image/32mp0tnpg/
>
> - The chip in the middle is a PIC16F886, surface mount.
> - The six LEDs on the left indicate which string is selected (E, A, D, G,=
 B,
> e). Small, surface mount, low power.
> - The four LEDs on the right are used to shine light at various pulse
> frequencies on the strings. High brightness, through hole.
> - The push button switch selects the string.
> - The toggle switch turns the power on and off.
> - An ICSP header is placed somewhere on the board.
>
> - The circuit is intended to be powered off of a few parallel 3V coin cel=
ls.
>
>
>
> 1. If the circuit is designed to run at 3V, how will programming be done =
by
> the PICKIT2? The PICKIT2 runs on 5V as far as I remember, but I think it
> should have an option to program at 3V, but I don't remember, been a whil=
e
> since I used it.
>
> 2. The PIC16F886 datasheet says that the PIC can be run at 20MHz. Since I=
'm
> tuning strings which I don't think divide nicely to those frequencies, I
> want to run the PIC as fast as possible to minimize the error. Therefore,
> I'm going to be using a 20MHz crystal. The datasheet schematic shows the
> crystal with capacitors. What should the values of these capacitors be?
>
> 3. I didn't look much into it, but I use the 16F886 as a "jellybean" part=
..
> It's cheap, available, small, and loads of features. I think the timers o=
n
> there should be adequate for the oscillation of the pins. How would you g=
uys
> recommend generating the right frequences for the LEDs? Timers?

Interesting you should bring this up again. I just finished making a=20
little kit (16F690) which I intend to shove on our new website at some=20
point in the near future. The prototype works pretty nicely.
All pretty simple really, the trickiest part was working out all the=20
tuning frequencies, choosing which temperament and stuff. I borrowed=20
quite a bit from the Stimoppped source, although I've done a few things=20
differently. I chose high brightness red LEDs (not white as the forward=20
voltage is too much for a coin cell which I decided on to power it) as=20
they work better than standard ones for this purpose. A largish=20
capacitor and small series decoupling resistor helps to lower the surges=20
on the battery.
Don't worry too much about the error - I'm using an 8Mhz as running the=20
16F690 at 20MHz requires higher than 3V. The error you can achieve at=20
this clock rate is way past anything that will be audible if you do the=20
calculations - Wiki has a lot of good info on tunings - check out=20
Scientific Pitch Notation, Cent, Equal Temperament pages.
Caps on the crystal should be around 10-20pF, the datasheet will have=20
recommendations. I use 15pF on an 8MHz.
I generate the pulses using the 16-bit timer 1 interrupt set for 1/10=20
the note period. That way you keep the uC free to do other stuff like=20
check buttons and display notes.
If you need any more help with any of it let me know.



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