Logic indicator circuit / logic probe is used to identify logic level at any point of logic circuitry. The indicator can be visual (using LED, LCD, of 7 Segments) or auditory (using beeper or speaker). This circuit has a audible indicator, with the help of  a loudspeaker to produce indication tone.  Using the opamp,U1, as schmitt trigger, the Audio oscillator Q3-Q1-Q2 is isolated from TTL. Beside that, the op amp also acts as high input impedance inverter. The R8-R9 is used to set reference level at +16.6V which is a midway between low and high logic level. The Q3-Q2 will generate tone that indicate a high logic when the probe voltage is above +1.6V. If the probe voltage is below +1.6V then the  OP-amp output will saturate the Q1 and disables Q3-Q2 to cut off tone. Here is the schematic diagram of the circuit:

Audible Logic Indicator circuit schematic