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Timed Vox (ART430E)

A vox circuit is an interesting resource for those who want an automatic monitoring of what is happening in a place under surveillance. The Vox is a circuit control that turns on the recorder as soon as the microphone picks up a sound above a certain level. Thus, the recorder will only be turned on when people in a room start the conversation.

This project has been included in one of my books on electronic espionage. The book had the original edition in the 80's, thus, an old tape recorder is indicated, but modern digital equivalents can be used.

In figure 1 we have an example of a timed Vox circuit.


Figure 1 - Timed Vox
Figure 1 - Timed Vox


When a signal arrives via the microphone cable, it is sufficient to trigger the 555 monostable for a time interval which depends on the setting of P1. This setting is such that the recorder does not stop between short intervals between the words or phrases of the speakers.

In figure 2 we have a printed circuit board suggestion for this project.


Figure 2 – Printed circuit board for the vox
Figure 2 – Printed circuit board for the vox


P2 adjusts the sensitivity depending on the minimum sound level required for the activation of the device.

The relay contacts act on the microphone switch of the recorder, usually on the small cassette recorders or digital used in this type of work.

Figure 3 shows how to use the appliance.


Figure 3 - Using the appliance
Figure 3 - Using the appliance




Cl-1 - 555 - integrated circuit

Q1 and Q2 - BC547 or equivalent - general purpose NPN transistors

D1 - 1N914 - general purpose diode



P1 - 1 M - potentiometer

P2 - 100 k - potentiometer

R1 - 1M - resistor (brown, black, green)

R2 - 47 k - resistor (yellow, violet, orange)

R3 - 22k - resistor (red, red, orange)

R4 - 10 k - resistor (brown, black, orange)

R5 - 1 k - resistor (brown, black, red)



C1 - 100 nF - ceramic or polyester capacitor

C2 - 47 uF x 12V - electrolytic capacitor

C3 - 100 uF x 12V - electrolytic capacitor



K1 - Relay 6V x 100mA

B1 - 4 medium or large batteries

Printed circuit board, battery holder, wires, solder, mounting box, etc.