VOX is a switch ordered by voice, often used at microphones instead of the classic switch button. The vox described here may be connected to almost any type of audio equipment which has an external speaker for coupling.
The threshold action is set with volume potentiometer of the audio amplifier which commands the VOX . The signal (on speakerphone) from the terminals of R2 is connected capacitive on T1 base. Resistance R3 limits base current of the transistor, in cases in which voltage input exceeds 600mV. Diode D1 hangs positive trip of the input signal, so Veb can not become less than 0.6V.
Relay form the output circuit is ordered by Darlingtonul T2. Resistance R4 maintains the relay unoperated as long as T1 is blocked. The bipolar capacitor C2 allows to work as filter together with T2. Resistance R5 is limiting the T2 current base at a security level.
The threshold of switching VOX’s is about 600mV (R2). Maximum input voltage is determined by maximum admissible dissipation on R2 and R3. As a rule entry tension should not exceed 40Vvv. VOX consumed current is primarily formed by the sum of relay coil currents and R5. At VOX’s overloading, the resistance may lead up to 100mA.
VOX Circuit Schematic