http://www.piclist.com/techref/microchip/sw7_on_1pin.htm

The clicks where "FAQ", "I/O", "Keyboards", "7 switches with 1 A/D pin by
Bob Axtell"

Seaching for "Bob Axtell A/D keyboard" it is the 6th entry down.

--
James.

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of
Bob Axtell
Sent: Thursday, November 29, 2007 10:12
To: Microcontroller discussion list - Public.
Subject: Re: [EE]: four buttons on one A/D pin

Gee, I delivered an actual working schematic and i KNOW James posted it 
in the archives, but I can't locate it
on this exact subject. I use 8 resistors for 7 pushbuttons. Each button 
is separated by about 0.5V.

--Bob



Eoin Ross wrote:
> http://en.wikipedia.org/wiki/Resistor_Ladder 
>
> With this you basically have a binary number from the A2D with bits
matching your pressed buttons.
> Ignore the lower couple of bits from your 8/10/16 bit A2D values
>
> If your buttons were arranged top to bottom...
>
> (ASCII art in MS Sans Serif, Regular, 8 point font)
>
> Vcc
>  |
>  |
>  |                   2R
> +-----B4-----/\/\/\/-----+--------------> PIC A2D pin
>  |                                |
>  |                                \
>  |                                /   R
>  |                                \
>  |                   2R         |
> +-----B3-----/\/\/\/-----+
>  |                                |
>  |                                \
>  |                                /   R
>  |                                \
>  |                   2R         |
> +-----B2-----/\/\/\/-----+
>  |                                |
>  |                                \
>  |                                /   R
>  |                                \
>  |                   2R         |
> +-----B1-----/\/\/\/-----+
>                                  |
>                                  \
>                                  /   2R
>                                  \
>                                   |
>                                   |
>                                 GND
>
>
>
>   
>>>> rob@silk.net 28 Nov 07 20:38:02 >>>
>>>>         
>> Think binary :)
>>     
>
> I'm trying!  Do you mean for me to binary weight the four switch resistor 
> values (R4=2xR3;  R3=2xR2; R2=2xR1)?  I'm okay with that, but how does one

> optimize the value of the common pull-up resistor to minimize bunching of 
> the resulting _voltages_, keeping in mind that V= 5 x (Rx / (Rx+Rpu))?
Is 
> some sort of scaling factor required for the four
otherwise-binary-weighted 
> resistors to offset the bunching effect?   The system needs to reliably 
> distinguish between the case where all four buttons are pressed 
> simultaneously, and any one of the possible 3-button combinations.  My 
> back-of-an-envelope scenarios yield some voltage values that are less than

> 1% apart, which is too tight for comfort.  The archived solutions some
folks 
> have kindly identified seem to be invalid for multi-button combinations, 
> which doesn't work for this situation.  Any more hints?
>
> Thanks,
> RR 
>
>
>   

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