Arduino or AVR as ADC, polled through SPI or I2C

Turn your Arduino/AVR into an Analog to Digital Converter (ADC) that you can poll via SPI or I2C in slave mode.

Quantity		Component name
1	×	Arduino or AVR ATMEGA328. Other AVRs may work with modifications.
1	×	Level Shifter
1	×	Raspberry Pi Or any other master to which you want to connect an ADC