acoustic logic probe

This logic probe is a little unusual, as it indicates the logic states by audio tones rather than the normal light display (LEDs, etc.). The audio tone generator used to make the high and low notes, which corre- spond to the high and low logic levels, is formed by S 1 and R4. These two parts form an astable which is tuned by Cl or C2. The signal is buffered and amplified by S2 to drive the loud- speaker.