A Simple Vox

053 AUDIO & HI-FI A vox is a voice-operated switch that is often used as a substi- tute for the press-to-talk switch on a microphone. The one des- cribed here can be connected to almost any audio equipment that has a socket for an external loudspeaker. The actuation threshold is set by the volume control on the AF amplifier that drives the vox. The (loudspeaker) signal across R2 is capacitively fed to the base of T1. Resistor R3 limits the base current of this transistor when the input voltage exceeds 600 mV. Diode Dl blocks the pos- itive excursions of the input signal, so that Ue_b can not become more negative than about 0.6 V. The output relay is driven by darlington T2. Resistor R4 keeps the relay disabled when T1 is off. The value of bipolar capacitor C2 allows it to serve as a ripple filter in conjunction with T2. Resistor R5 limits the base current of T2 to a safe level. The switching threshold of the vox is about 600 mV across R2. The maximum input voltage is determined by...