66: Analog Input using an Arduino
- revised 11/18/14 -

After my frustration with the MCP3008 (post 65) I decided to try using an Arduino. I bought a new Nano from Sainsmart ($14). Here's the Arduino setup:

Note that the Nano plugs directly into the breadboard

(very handy)

I use the Arduino.app on my iMac to write and debug the Arduino program (over the USB cable). Then I move the cable to a USB port on my Pi. (Note: the Arduino uses Flash memory, so the program starts again.)

Here's the Arduino "sketch" (as programs are called) in C language:

// Rheostat has 3 pins L=GND, R=5v (or 3.3V), Mid to A1

// Readings connected to 3.3V: 0 to 730, to 5V: 0 to 1023

#define RheoPin 1

#define spl(x) Serial.println(x)

void setup() {

    Serial.begin(9600);

}

void loop() {

    int i, r;

  

    while(!Serial.available()) delay(100); // Wait for request

    while(Serial.available() && i < 10) {

          Serial.read(); // discard input

    }

    r = analogRead(RheoPin);

    spl(r); // Writes number as ASCII to the USB

}

Meanwhile, on the other end of the USB cable -- the Pi Python program:

import serial # I had to install "pySerial" for this

import time

import RPi.GPIO as GPIO

GPIO.setwarnings(False)

GPIO.setmode(GPIO.BCM)

ser = serial.Serial("/dev/ttyUSB0", baudrate=9600, timeout=1)

ser.open()

while True:

        ser.write("1") # Ask the Arduino for a reading

        time.sleep(.2)

        ct = ser.inWaiting()

        if ct > 0:

                s = ser.read(ct)

                i = int(s.rstrip()) # ASCII to Int

                print i

        time.sleep(.5)

There's good news and bad news about this approach:

GOOD: 

* No GPIO pins used (a USB port, instead).

* My Arduino program worked the first time.

* Because the Arduino only runs one thing at a time, results can be more accurate.

BAD:

* More complicated, two kinds of programming, etc.

* The cheap Nano still costs 3X the MCP3008 chip.

* The request-to-response turn-around is about 1 second, minimum.

Overall, I like using the Arduino. Better yet, the Pi model B+ should have had analog pins.

REVISED:

I have successfully added other sensors to the Arduino end of the setup: 

1. A photosensor.

2. An HC-SR04 rangefinder.

And I have checked out adding --

3. DS18B20 temperature and DHT22 temperature/humidity sensors (although they work fine on the Pi).

4. Output to a 2x16 LCD character display (saves 4 GPIOs on the Pi).