AVT 732 - Serwis AVT

Whisper detector.
High sensitive wire bug.
A series of "do-it-yourself" sets
for beginners.
The kit makes an incredible impression on the user. Low-pitched whispers or sounds, normally
undetected by human hearing get amplified several-fold. The sound effects the user gets are
Recommendations: the kit is recommended to people who are fond of listening to allegedly
undetectable sounds, to people who are hard of hearing and for monitoring infant sleep.
The kit is ideal for a variety of experiments related to amplifying different sounds. It may be of
some use to people with hearing problems. It will probably turn out to be indispensable as a
monitor of infant sleep. We hope it will also be highly appreciated by nature lovers..
Electrical characteristics
built-in microphone
headset or loudspeaker output
smooth gain regulation
frequency characteristics shaping
battery-fed power supply
3…6VDC power supply
AVT 732 Whisper detector. High sensitive wire bug.
The M1 electret microphone signal drives the U1A non-inverting amplifier. The gain set by the
R5 and R6 resistors is constant and amounts to 23x (27dB). The pre-amplified signal is delivered to
the inverting amplifier with the U1B IC. The negative value gain is determined by the ratio of the
PR1 potentiometer and R2 resistances. It can be adjusted within the 0…1 range. The circuit is
powered by single voltage while the R7, R8 and C5 components constitute a floating ground circuit.
The C9, R2, C6 power supply filtering components as well as R1and C4 are necessary at high gain
and they protect the circuit from self-oscillating caused by signals penetrating through power lines.
The popular U2 TDA 7050 power amplifier is used. It works in the typical application mode as a 2channel amplifier with gain fixed at 20x (26dB).
Figure 1. Schematic diagram
Assembly and test
The circuit schematic and the PCB layout is shown in figures 1 and 2. The components should be
soldered to the PCB. The best way to do this is to follow the order of soldering given in the
component list. The three wire jumpers should be soldered in first. During assembly a lot attention
should be paid to the correct polarity of the components when soldering them in namely, the
electrolytic capacitors, transistors and diodes. The pin 1 mark on the socket and IC must correspond
to the component layout printed on the PCB. The handset plug and potentiometer must be connected
as shown in the photo. Correctly connecting the potentiometer will cause the increase in volume
while turning the volume knob clockwise. The electret microphone can be connected by short links
of wire leads. This can also be done with the help of a long cable. In each case the correct polarity of
the parts used must be observed (see the schematic and PCB). The negative terminal inside the
microphone is connected to the metal can. After assembling the circuit it must be thoroughly
checked that the components used have not been reversely soldered, that they are in their correct
places and that the solder pads do not touch each other as this might lead to short circuits. After
checking the correctness of assembly both the headset and the circuit can be plugged in. Once
AVT 732 Whisper detector. High sensitive wire bug.
properly assembled, the amplifier will work instantly. Before using the circuit the potentiometer
knob should be set at low volume (by turning it anti-clockwise) and then it should be gradually
turned clockwise in order to increase volume. Too high gain will cause the circuit to oscillate (in the
headset-to-microphone path) and screech unbearably. The circuit should operate when powered by
the two AA or AAA batteries or even by a single 3V Li-Ion battery. It can also be powered from four
batteries or from the 4.5…6V plug-type power supply.
ATTENTION! The supply voltage should never exceed 6V!
Figure 2. Components layout
Component list
In the order of soldering:
below the U1IC wire jumper
P2 - P3 connected by a wire jumper
The boxes with drafts
P4 - P5 connected by a wire jumper
of pins and symbols of
R1 - 1kW
the components used on
R2 - 100W
the PCB as well as the
R3 - 2,2kW
photos of assembled sets may
R4…R10 - 10kW
come in useful.
R5 - 220kW
The U1 IC 8-pin socket!
The U2 IC 8-pin socket!
C10 - 470pF
(also marked as 471)
C1…C3 - 220nF
(also marked as 0.22)
C4…C9 - 100µF/6,3V
(or higher voltage)!
wire/connect the 10kW B potentiometer
mount the knob on the potentiometer
wire/connect the stereo headset (see the photo)
wire/connect the electret microphone
connect the battery snap-in terminal (red wire +; black wire -) to points P.O!
insert the U1-LM358!
insert the U2-TDA7050!
connect the 3…6V power supply!
AVT 732 Whisper detector. High sensitive wire bug.
The kit was made on the basis of a project bearing the same trade name published in "Elektronika dla Wszystkich" 12/04
The sale offer of our "do-it-yourself" sets is available on our website www.sklep.avt.com.pl
AVT-Korporacja, Sp. z o.o.
Leszczynowa 11
03-197 Warsaw
fax: (+48 22) 257-84-55
phone.: (+48 22) 257-84-50 Poland
Technical Assistance Dept.
Phone: (48 22) 257-84-58
AVT 732 Whisper detector. High sensitive wire bug.
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