Designing and Building an ADC for the Raspberry Pi

Raspberry Pis have become a rage wherever one needs to add some electronics to a project, yet they are not a very good tool for controlling electronics. It comes with few 3v3 digital GPIO pins. Now say for instance you want to hook on a potentiometer to the Pi how would you do it? The answer is normally to use an external ADC. There are numerous prebuilt ADCs out there. I however decided to try to build my own ADC.

There are numerous ADC designs out there, however when designing such a device one must take into account a few things:

  • Sample rate
  • Hardware complexity
  • Accuracy
  • Resolution

For my home made ADC accuracy and resolution are not the highest priority – I just want to be able to read the value of a Potentiometer. Hardware complexity is very important though. Ideally I should require a minimal number of components to build this ADC.

The central component of an ADC is the comparator. The comparator is a circuit that, as its name suggests,  compares two voltages.  The circuit symbol for the comparator is:

200px-Op-Amp_Comparator.svg

A comparator

Basically if V1 > V2, Vout will be High otherwise Vout is low. One of the techniques used known as the Flash ADC takes multiple such comparators with each ones voltage held at a different level to determine the incoming voltage. However, the Flash ADC requires many comparators. Another way of solving the problem is by using only one comparator and varying V1. This does not give us the best results but its a good enough compromise for us.

In order to vary V1 I decided that I would use a capacitor and a resistor as a reference for my voltage. A capacitor stores energy while a resistor dissipates energy. What I would do is charge the capacitor fully then slowly let the voltage across the capacitor and resistor drop.

RCDischarge.png

RC Circuit – View Full Simulation

In the circuit above a short pulse of a high voltage is sent to the capacitor. The capacitor gets fully charged then discharges across the 1k resistor. The diode prevents the current from flowing back to the source. This creates a well defined pattern. The equation for the discharge is;

V = V_0 e^{-\frac{t}{RC}}

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