Simple Audio-Frequency Vco

A VOX is a voice-operated switch that is often used as a substitute for the press-to-talk switch on` a microphon
Simple Audio-Frequency Vco - schematic

e. This VOX 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 Tl. Resistor R3 limits the base current of this transistor when the input voltage exceeds 600 mV. Diode Dl blocks the positive excursions of the input signal, so that Veb cannot become more negative than about 0.6 V. The output relay is driven by Darlington T2. Resistor R4 keeps the relay disabled when Tl 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 2 to a safe level. The switching threshold of the VOX is about 600 mV across R2. The maximum input voltage is determined by the maximum permissible dissipation of R2 and R3. As a general rule, the input voltage should not exceed 40 Vpp. The current drawn by the VOX is mainly the sum of the currents through the relay coil and through R5. The resistor can carry up to 100 mA when the VOX is overdriven.

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