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Microphone For Remote Listening (ART431E)

There are situations where the microphone can not be hidden wherever a conversation takes place. In this case, the listener must be remote and for this the detective or spy can count on some interesting resources.

This article is part of one of my spy books. A first feature greatly explored is the directional sensitive microphone. The most common type is the tubular shown in Figure 1.

 

Figure 1 - Tubular directional microphone
Figure 1 - Tubular directional microphone

 

The tube allows the pick up of sounds that comes from only one direction, then is hidden and pointed to the people who are talking. The side interfering sounds are not picked up.

Setting up such a microphone does not present many difficulties.

As shown in figure 2 we can use a very thick cardboard tube or metal, and inside it place some acoustic absorbent material.

 

Figure 2 - Construction of the tubular microphone
Figure 2 - Construction of the tubular microphone

 

The end where the sound enters must be open and at the other end is the electret microphone.

The microphone must be protected to avoid picking up the sounds behind the unit.

A shielded cable carries the signal to the amplifier or recorder. We have several devices that can be used with this type of microphone.

 

Preamplifier

To increase the gain of the microphone we can use a pre-amplifier transistorized with only one transistor. Note that the recorder usually already has its amplifier and electret microphones too, so excessive additional amplification can saturate the circuit causing distortion.

In figure 3 we have a preamplifier that can be used for this purpose.

 

Figure 3 - Microphone preamplifier
Figure 3 - Microphone preamplifier

 

 

SEMICONDUCTORS

Q1 - BC548 or equivalent - NPN general purpose transistor

 

RESISTORS

R1 - 10 k - resistor (brown, black, orange)

R2 - 470 k - resistor (yellow, violet, yellow)

 

Capacitors

C1 - 10 uF x 6 V - electrolytic capacitor

C2 - 22 uF x 6V - electrolytic capacitor

C3 - 100 uF x 6 V - electrolytic capacitor

 

SEVERAL

B1 - 3 or 6 V - 2 or 4 small batteries

S1 - Single switch

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