Vox box

The electronic circuit in the VOX Box consists of three parts: a microphone preamplifier, a Schmitt trigger, and a relay driver. Input signals (MIC INPUT terminals) to the microphone preamplifier (Ul) are amplified and fed to a THRESHOLD control (R8). When the preselected threshold voltage level is exceeded, the output of the Schmitt trigger (U2) immediately goes high. The signal from U2 is rectified and the voltage developed across C7 turns on the relay energizer transistor (Ql).
Vox box  - schematic

That transistor action passes pull-down current through the coil of relay Kl. The changing of the relay SPDT contacts can be used to either make or break an external ac or dc circuit.

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