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Build A Voice-Activated Relay (VOX)

Triggers devices on and off with sound.

By John Clarke


•12V operation
•Electret or dynamic microphone or line input
•3.5mm jack socket or screw terminal inputs
•Mono or stereo signal
•Adjustable sensitivity
•Adjustable delay
•Hysteresis prevents relay chattering at threshold
•DPDT relay
•Power and relay LED indication

VOX stands for Voice Operated eXchange and it is also the Latin word for ‘Voice’. A VOX circuit switches on a relay whenever a signal reaches a set threshold. The relay switches off once the signal level drops below the threshold and after a short delay.

They are used in communications, public address systems, surveillance, security and general purpose electronics.

For communications, a VOX switches a transceiver from receive to transmit whenever the person speaks into the microphone. This frees the operator for other tasks as a separate switch is not needed to talk. Many intercoms and public address systems are also automated in a similar way.

A VOX circuit can be used to mute any sound until it reaches a set level. That way a public address system will ignore background noise and remain quiet, until someone intentionally speaks into a microphone. For security and surveillance, a recorder can be switched on whenever a noise is sensed by a microphone.

But it doesn’t have to be a microphone which causes the VOX action. For general-purpose use, any audio signal can used to switch the relay.

Our design

Click for larger image
As you can see from the features box, our new VOX is quite a versatile beast! It can be used in practically any application which requires triggering from a sound source - and that sound source can itself be just about anything!

In line with the above comments, our VOX design has two inputs, both of which will accept the same types of audio input. First is a stereo 3.5mm jack socket which will handle both mono and stereo signals, while the second input is for mono inputs only and is via screw terminals.

You can connect an electret or dynamic microphone. Electret microphones require a bias voltage which can be selected with a jumper link (LK1). For stereo signals connected via the 3.5mm socket, a jumper link provides mixing of the left and right channels into a mono signal.

Signal sensitivity can be adjusted to cover a wide range from microphone levels up to line levels of 2V RMS. With sufficient signal, the relay switches on and remains on until the signal level drops to below a threshold level. An adjustable delay sets the time taken for the relay to switch off once this threshold is reached.

The relay has two sets of changeover contacts which will suit a variety of switching applications. LEDs are included for visual indication of power and of relay switching.

Because of the wide variety of possible uses for a VOX, our module is simply presented as a PCB which you can install to suit your application. Or if you wish, it can be fitted into a plastic “UB3” case measuring 130 x 68 x 44mm.


Power supply:............... 12VDC at 50mA
Trigger sensitivity:    Adjustable from 2mV (microphone) to 2V (line)
Maximum signal input: ............... 50V rms
Signal frequency range: ............... 16Hz to >600Hz
Attack time:............... 10 cycles with signal at threshold voltage (faster attack if signal is above threshold)
Hysteresis:............... 0.44V at the 2.06V threshold
Delay time:............... Adjustable from 100ms to 10s
Electret bias current:............... ~320μA
Relay contacts (DPDT):............... 5A (maximum of 50V recommended)

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