Using a Photocell + MCP3008 + Raspberry Pi + Pi_piper gem

## Enabling SPI on the Raspberry Pi

To enable SPI on the Raspberry Pi, at the command-line:

1. type `raspi-config`
2. select *9 Advanced Options*, then select *A6 SPI*
3. select *Yes* to enable, then *Yes* to load the SPI kernel module by default


## Connections

-[http://www.jamesrobertson.me.uk/dynarex/2016/feb/19/photocell-mcp3008-raspberry-pi-connections.xml]

## Example

    require 'pi_piper'
    #port of the Adafruit MCP3008 interface code found @ http://learn.adafruit.com/send-raspberry-pi-data-to-cosm/python-script

    def read_adc(adc_pin, clockpin, adc_in, adc_out, cspin)
      cspin.on
      clockpin.off
      cspin.off
      
      command_out = adc_pin
      command_out |= 0x18
      command_out <<= 3

      (0..4).each do
        adc_in.update_value((command_out & 0x80) > 0)
        command_out <<= 1
        clockpin.on
        clockpin.off
      end
      result = 0

      (0..11).each do
        clockpin.on
        clockpin.off
        result <<= 1
        adc_out.read
        if adc_out.on?
          result |= 0x1
        end
      end 

      cspin.on

      result >> 1
    end
      
    clock = PiPiper::Pin.new :pin => 18, :direction => :out
    adc_out = PiPiper::Pin.new :pin => 23
    adc_in = PiPiper::Pin.new :pin => 24, :direction => :out
    cs = PiPiper::Pin.new :pin => 25, :direction => :out

    adc_pin = 0

    value = read_adc(adc_pin, clock, adc_in, adc_out, cs)

    # raw reading from value when the room light was on
    #=> 614

    # raw reading from value when the room light was off
    #=> 918

The above example results in a raw value which can be used to help indicate the ambient lighting level. 


## Resources

* Raspberry GPIO pins http://www.jamesrobertson.me.uk/dynarex/2015/sep/02/raspberry-gpio-pins.xml
* Photocell + MCP3008 + Raspberry Pi connections http://www.jamesrobertson.me.uk/dynarex/2016/feb/19/photocell-mcp3008-raspberry-pi-connections.xml
* Microchip MCP3008 I P 10Bit ADC 16DIP 8CH SPI http://www.jamesrobertson.me.uk/cpages/2016/feb/16/microchip-mcp3008-i-p-10bit-adc-16dip-8ch-spi-ebay-co-uk.html
* pi_piper/mcp3008.rb at develop · jwhitehorn/pi_piper https://github.com/jwhitehorn/pi_piper/blob/develop/examples/mcp3008/mcp3008.rb
* Analogue Sensors On The Raspberry Pi Using An MCP3008 http://www.raspberrypi-spy.co.uk/2013/10/analogue-sensors-on-the-raspberry-pi-using-an-mcp3008/
* Measuring the ambient light intensity using a photocell http://www.jamesrobertson.me.uk/arduino/2016/feb/16/measuring-the-ambient-light-intens.html

mcp3008 pi_piper analog photocell raspberrypi