The magazine “Amaterske Radio” (Czechoslovakia) proposed scheme is a simple audio probe to detect logic levels; low (logic 0) and high (logical 1). The device consists of a bitonal multivibrator, is made of two elements of D1.1, D1.2 chip D1, and two keys, collected three transistors V2—V4 (see circuit diagram).
When the probe is connected to the output check the device under voltage 2.4 V or above (logic 1), the transistor V4, the capacitor C2 is connected to ground 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 a point under the potential of logic 0 (0.8 V or less), V4 is closed, but will open the transistors V2, V3, and the capacitor C1 is connected to a common bus. In this case, the hours of Thoth of the multivibrator will be an octave (double) is lower than logic 1.
The probe is permissible to use silicon transistors КТ104, КТ203, KT361 (V2) and KT315, КТ312 (VЗ, V4) V1 is КС133А Zener diode analogue IC 7413—К155ЅТЛ1.