There are many situations where you need to activate a certain device but you just cannot do it because your hands are busy or
just cannot let go. A classic example is operating a radio transceiver while driving a motorcycle. Although you may try to drive and operate the
transceiver simultaneously, this will distract and is very dangerous. The best way to solve such problem is to connect a vox (voice operated relay) to
the transceiver. With the vox installed, you can remotely switch the transceiver to transmit mode by just speaking on the mic (preferably built in inside
the helmet). One such vox is featured here. This relay circuit is operated by voice. The voice signal may come from a mike amplifier, an audio amplifier,
or similar devices. It can be used to automatically control the transmit switch of a transceiver (VOX), a voice operated repeater, or a slide projector film changer.
Obviously, the application area is not limited to motorcycling.
The sensitivity can be set through P1. Set it to a level where the vox is immune to the background noise and will only activate with your voice. The analog meter M1 is
used in the circuit as a simple audio meter. Potentiometer P2 adjusts the maximum deflection of the meter. You can remove this meter if you want. However if you do so,
you have to readjust the P1 to set the desired sensitivity level.
Here's a tip on one practical application of this circuit: this vox can be used as the autoswitch to activate the transmitter module of a vhf repeater system. The vox's input
is simply plugged in to the receiver's audio output and its relay terminals are connected to the ptt line of the transmitter module
Parts Placement Layout
External Wiring Layout
Electronic Circuits volume 1.0 - Circuit Nr. 44
The complete data of the electronic circuit described above can be found in the following book and is available from Amazon.com.
Click on the image to view the book.