This is one of great project based on AVR microcontroller. It use ATmega32 as processor and LCD 12864 pixels for display. If you need to measure low frequency i thought this project, AVR oscilloscope, will be usefull. Maksimum frequency for AVR oscilloscope is 5 kHz (square signal), other signals (sine or triangle) the frequency is lower ( almost 1 kHz) for having clear view of the signal. Voltage input range is 24V AC / 30V DC.