Mini Logic analyzer is ATmega8-based electronics tool that can be used to watch and analyze logic transitions 0 or 1 of a digital data signal. It comes with Nokia 3310/5110 LCD to display signal and it has 4 channel inputs.
A digital data signal can be found on the output pin of TSOP-1730 Infrared Receiver, on the Transmit and receive pins of MAX-232 chip (RS-232), on Clock and Data pins of I2C data bus and many more electronic components. This circuit supports capturing for up to 400 kHz digital signals.
For more detail: Mini Logic Analyzer using ATmega8 microcontroller