Study the Analog to Digital capabilities of Atmel ATtiny26. This tiny but mighty IC is really a miracle. One special thing is the internal 10-inputs multiplexed ADC circuit which can covert analog voltages to bytes. This check circuit uses only 3 inputs. Of course you can may changes to use more inputs.
For more detail: 10 Bit analog to digital converter using ATtiny26 microcontroller
This free embedded project also found using:
- ADC ic
- attiny26 voltmeter
- arduino seletor 4051