Vectronics VEC-483K - Manual (Page 14)

VEC-483K Owner’s Manual Voice Activated Tape Recorder Switch
14
THEORY OF OPERATION
Electret microphone MIC1 picks up sound and generates an audio signal which
loops through to microphone output jack J1. Along the way a sample of the
audio signal is picked up by U1a, an op-amp configured as a variable gain audio
amplifier. Stage gain is established by Sensitivity control R7. U1's amplified
AF output signal is detected by D1/D2, converted to a dc level, and fed to the
gate of Q1. When sufficient voltage is developed across the detector, FET
switch Q1 conducts and pulls the recorder's "remote" line to ground--providing a
current path for activation of the transport motor. To create the unit's "delay"
function, a RC circuit on the gate of Q1 stores detector voltage and holds the
gate voltage "high" at a decaying rate. The time constant of this circuit is
adjustable by Delay Control R8. Q1 has sufficient current handling capacity to
operate a wide range of small-recorder transport motors.