[sdiy] Reading two buttons with a single IO pin

[Jack Jackson]

Is there a particular reason you want to use only one pin Tom? Why not mux in using a shift register?

Best Regards,
Jack 

----------------------------------------
> Date: Fri, 20 Feb 2015 14:04:00 -0800
> From: jays at aracnet.com
> To: synth-diy at dropmix.xs4all.nl
> Subject: Re: [sdiy] Reading two buttons with a single IO pin
>
> Using an ADC pin I think you can do it with 2 resistors and 2 switches.
>
> Put a pull up resistor on the input pin.
>
> Put a switch between the input pin and ground.
>
> Add a resistor to ground and take the other end and put a switch between
> it and the input pin.
>
> With both switches open the pin is at what you have the pull up tied
> too.
>
> With the switch connected to the top end of the other resistor a you get
> a voltage divider set by the value of the resistors.
>
> With the switch connected to ground is closed you get ground.
>
> Here's a crude ascii schematic, hopefully it will make sense.
>
> Jay S.
>
> +V - Don't go over cpu ADC voltage!
> |
> |
> /
> / R pull up
> /
> |
> ---+----- \----+
> | |
> | V
> |
> +-----\-----+
> |
> /
> /
> /
> |
> V
>
> On 2015-02-20 13:16, Tom Wiltshire wrote:
>> Hi All,
>>
>> I've been considering the question of how to read two buttons with a
>> single microcontroller IO pin. There are various ways to do this. One
>> I've seen is to charge a capacitor via some resistors with switches
>> around them. You then measure how fast the capacitor charges and you
>> can work out which buttons are pressed. There are other techniques.
>> Another way is to read an ADC value from some kind of potential
>> divider, and by checking where the voltage falls, determine which
>> switches are pressed. All the versions of this I saw used a *lot* of
>> conditional statements ( if ((x>y) && (x<z)) { do summut } type of
>> thing ).
>> It seemed to me it should be possible to arrange the incoming voltages
>> so that the bits of the ADC reflected the switch states directly.
>>
>> E.g. Do it a simpler way: Just read an ADC value, reduce the ADC value
>> to the top two bits (00, 01, 10, 11) and arrange the resistor values
>> such that the bits reflected the switch states. This was what I had in
>> mind:
>>
>> http://www.electricdruid.net/images/TwoButtonsV2.png
>>
>> Having messed about with calculations and experiments for a while, I
>> determined that this arrangement isn't possible. The 00 case is easy,
>> since both switches are open and R3 acts as a pulldown, so we get 0V
>> and read 00. The R1 and R2 values that give good stable ADC readings
>> for the 01 and 10 cases are easy enough to determine, but
>> unfortunately, they don't give the right value when used in parallel.
>> Since the "ideal" value would be in the middle of the range, I did
>> some experiments to see if I could make it work by pushing the R and
>> R2 values closer to the limits, further from the ideal values. It
>> turns out you can't.
>>
>> Finally, I had a brainwave. What if I inverted the state of one of the
>> switch bits? Let's say I invert S2, so that instead of reading 1 for
>> pressed, it reads 1 for not-pressed. In this case, my required ADC
>> sequence would be 10, 11, 00, 01. Given that for code it doesn't much
>> matter whether a switch reads 1 pressed or 1 unpressed, this isn't a
>> big deal. It turns out that this *is* possible to arrange:
>>
>> http://www.electricdruid.net/images/TwoButtonsV3.png
>>
>> I produced this with a PHP script, so I can enter a R4 value, and the
>> code derives the other necessary values, finds the nearest actual
>> resistor values, and checks what potential divider ratios they give to
>> make sure they're in range. The given values are therefore a
>> demonstration, and could be adjusted to taste.
>>
>> I was quite pleased with this, since it'll allow me to double the
>> number of switches for only a couple of resistors per switch.
>>
>> Presumably I'm not the first person in seven billion to have come up
>> with this idea, but I thought I'd share it, since I haven't been able
>> to find it somewhere else.
>>
>> HTH,
>> Tom
>>
>>
>>
>>
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