View source for Multiple Buttons using one Analog Read

So here is the dilemma. You are running out of digital input pins but you still need to add six buttons. Fortunately there is a way you can add all these digital inputs as long as you have an analog pin. What you need to do is wire all the buttons and connect it to a R-2R Ladder. A R-2R Ladder is a very simple Digital to Analog converter. You treat your buttons as if they were bits and you use an analog pin on a microcontroller to read them. 

In practice the accuracy of an R-2R ladder becomes more poor as more bits are added. Most hobbiest do not go over 5 bits since the required tolerance of the resistors need to be less than 1% beyond that.


Lets say you have a 10bit A/D converter on your 5V micro controller. 2^10 = 1024 steps so that would mean you can resolve 5V into 1024 parts. 5V/1024 = 0.00488V or 4.88mV. 


If you mapped these buttons to act as bits in R2 ladder the following would be true.

2^6 = 64 steps
5V / 64 steps = 0.078125V or 78mV/step

[[File:7Bit_R2R_Sch.PNG|420px|thumb|right|Schematic exmaple of 7 bit R-2R]] 

[[File:Transistor_NOR.PNG|420px|thumb|right|N Channel NOR]]