Magazine "Amaterske Radio" scheme offers a simple audio probe for detection of logic levels; low (logic 0) and high (logical 1).
The device consists of a two-tone multivibrator in two the elements of D1.1, D1.2 chips D1, and two keys collected on three the transistors V2-V4 (see circuit diagram).
When the probe is connected to the output check in the device under voltage 2.4 V or higher (logical 1), the transistor V4, the capacitor C2 is connected to GND and the multivibrator starts to work. And since the capacitance C2 is twice less than that of C1, radiated dynamic head B1 the sound will be high.
When connecting the probe to the point, under the logic 0 potential (0,8 In or less), V4 is closed, but will open the transistors V2, V3, and the capacitor C1 is connected to the shared bus. In this case, the frequency multivibrator will be an octave (twice) is lower than when a logical 1.
The probe may be possible to apply silicon transistors CT, CT, KT361 (V2) and KT315, CT (V3, V4) V1 - Zener XA, analog IC 7413-К155SТЛ1.