Raspberry Pi: Analog audio affecting status of LEDs in Python

Hi,

It is not possible to directly measure an analog input into the Raspberry Pi. This has to be done using an external ADC (analog to digital converter) chip. These measure an analog voltage and convert it to a digital value linearly. Important points with ADC:

• RESOLUTION: measured in bits, this is how accurately the ADC can measure the value. It will split between the supply voltage and 0V into 2 to the power of n steps where n is the number of bits. So a 10 bit ADC powered off 5V will split 0V-5V into 1024 equal steps (2^10) so an input of 2.5V will produce a digital output of 512. Assuming you are making a VU meter, make sure you have enough bits to provide enough combinations for you LEDs. That said, a small 8 bit ADC still has 255 lit combinations ( 1 of the combinations is 0) so you could have 255 LEDs and I doubt you'll be using more than that.
• SAMPLE RATE: measured in samples per second, this is how often the ADC will measure the input voltage. If your using it for audio, it doesn't need to be sampled very often so any ADC with a sample rate > 20SPS (samples per second) will do. Most range in the 1000s of samples per second so any device should be fine.
• INTERFACE: this is how the ADC will tell the Raspberry Pi what the digital value is. Common examples are serial, I2C and SPI. For the Raspberry Pi, I would recommend serial or I2C, there are many tutorials on how to set both of these up. http://www.instructables.com/id/Raspberry-Pi-I2C-Python/?ALLSTEPS for I2C or http://codeandlife.com/2012/12/27/raspberry-pi-as-arduino-hdmi-shield/ for serial

Assuming you use I2C (which is what I recommend), once you have set the RPi up as in the I2C instructions above, the code would be:

import smbus

import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BOARD)

GPIO.setup(10, GPIO.OUT)

bus = smbus.SMBus(1)

address = 0x60

def read():
        signal = bus.read_byte_data(address, 1)

        if signal>512:

               GPIO.output(10,True)

        else:

               GPIO.output(10,False)
       
Going through 1 line at a time

import smbus : import the python i2c library

import RPi.GPIO as GPIO: import the GPIO library and define it as "GPIO"

GPIO.setmode(GPIO.BOARD): set the GPIO names to their physical pin number (so GPIO 10 is pin 10 on the GPIO header)

GPIO.setup(10, GPIO.OUT): set pin 10 as an output

bus = smbus.SMBus(1): give the name "bus" to the i2c bus. On newer Raspberry Pi, the bus is number 1, on older RPi's, its number 0. To find out go into terminal, with the ADC connected and powered and type in "i2cdetect -y 0" and "i2cdetect -y 1" This will tell you what is connected to each I2C bus, use the one that shows it has something connected to it (see I2C tutorial)

address = 0x60: set the address of the ADC chip to 0x60 (found when running i2cdetect - see tutorial)

def read(): start a procedure called read
signal = bus.read_byte_data(address, 1): read register 1 on the ADC chip and store it as the variable "signal"

if signal>512: if the signal is greater than 512, in a 10 bit ADC this would be half of supply voltage e.g. 2.5V

GPIO.output(10,True): set pin 10 to logic high (turning LED on)

else:  otherwise

GPIO.output(10,False): set pin 10 to logic low (LED off)

This means everytime the read procedure is called, the Raspberry Pi will measure the voltage, if it's greater than half the supply voltage, turn the LED on, else turn the LED off. Combine if statements to turn lots of LED's on or off in different combinations depending on the signal level.

Any further questions, just ask

Good luck

Dan McGraw

M0WUT