Weak beeper?
Build the repeater!
Does your computer have a weak beeper? Do you
have trouble hearing it? You can solve that
problem with this Computer Sound Repeater which
turns a weak beep into a much more audible low
frequency tone.
By LEO SIMPSON & JOHN CLARKE
Some computer models do not use
a speaker but use a tiny piezoelectric transducer which puts out a
high frequency 'beep'. For people
who have severe hearing loss, this
is just about inaudible. We're thinking specifically of Amstrad computers in this instance but the problem does occur with other brands.
Our solution is the Computer
Sound Repeater. It is a little box
which sits on top of your computer.
Whenever the computer beeps, to
indicate a wrong keystroke, during
a game or whatever, the Sound
Repeater emits a low frequency
"blurt" which is much ·more audible to those with poor hearing.
The Sound Repeater is low in cost
and simple to build. It uses an electret microphone, one LM324 quad
op amp IC, one transistor, one small
loudspeaker and a handful of other
bits.
How it works
The circuitry in the Sound
Repeater has been adapted from
that in the Discolight, as published
in the July and August 1988 issues
of SILICON CHIP. The electret
microphone is used to pick up the
Left: each time your computer beeps,
this gadget emits a low frequency
"blurt". The circuit is housed in a
low-cost plastic case.
sound of the computer's internal
beeper. The electret microphone
has an internal FET source-follower
to buffer it and this is powered via
the 4.7k0 resistor.
The audio signals from the electret microphone are then fed via a
.01µ,F capacitor to pin 10 of ICla,
which is connected as a noninverting amplifier with a gain of 30
times. ICla feeds a lOOkO trimpot
(VRl) which sets the sensitivity of
the circuit.
The signal then passes to ICl b,
which is identical to the previous
stage, thus giving a total gain of
about 900 at the maximum setting
of VR1.
Following IClb is a twin-T filter
stage which is more or less tuned to
the frequency of the computer's
beep. Our circuit has values
selected to centre it close to ZkHz.
The .01µ,F capacitor and the 220k0
resistor at the input of ICla and
ICl b also function as a high pass
filter , so that the circuit does not
respond to low frequency sounds.
Any tone signal picked up and
amplified by ICla, 1b and le is then
rectified by diode Dl. The rectified
signal is filtered with a 1µ,F
PARTS LIST
1 plastic utility box, 1 30 x 68 x
43mm (Altronics Cat H-0153
or equivalent)
1 PCB, code SC07104891,
66 x 62mm,
1 electret microphone insert
1 50mm 80 loudspeaker
1 1 2V AC plug pack
Semiconductors
1 LM324 quad op amp IC
1 BC338 NPN transistor
4 1 N4002 silicon diodes
2 1 N4148 small s ignal diodes
2 1 OV 1 watt zener diodes
68
SILICON CHIP
Capacitors
2 470µ,F 25VW PC
electrolytics
1 1 00µ,F 16VW PC. electrolytic
1 1µF 1 6VW PC electrolytic
6 .01 µ,F metallised polyester
(greencap)
Resistors (0.25W, 5%)
4 X 220k0, 1 X 180k0, 2 X
100k0, 1 x 47k0, 1 x 27k0, 4 X
8.2k0, 1 x 4.7k0, 2 X 3.3k0, 2 X
1 kO, 1 x 6800, 2 x 3300 0.5W,
1 x 1 20 1 W, 1 x 1 OOkO
miniature trimpot
05
1N4002
04
1N4002
12VAC
1211
1W
.,.
06
1N4002
330!l
1k
,----..---'YWr------------...------------------.....---+10V
811
SPEAKER
220 k
220k
Li~~l~-11---""I
100k
14
180k
1
~:
c·
.,.
3.3k
.,.
C
8.2k
COMPUTER SOUND REPEATER
8.2k
1&vw·T
-,.
+
27k
T
.,.
B
EOc
VIEWED FROM
BELOW
.OlJ .01!
C
C
Fig.1: tone signals picked up by the microphone are amplified by ICla & IClb, filtered by twin-T filter stage ICld, and
rectified by D1. When the voltage across the lJ.lF capacitor exceeds lV, pin 7 of IClc switches high and turns on Qt.
capacitor to produce a DC voltage
at pin 5, the non-inverting input of
IClc. This stage operates as a comparator, comparing the DC voltage
at pin 5 with a reference voltage at
pin 6, which will be about 1 volt DC,
as set by the 220k!2 and 27k!2
resistors.
If the voltage at pin 5 exceeds 1V,
the output of IClc will go high and
turn on transistor Ql, via diode D2.
Q2 then drives the loudspeaker
with half-wave rectified 50Hz AC
via diode D4. By using rectified AC
we dispense with the need for a low
frequency oscillator and an
amplifier to drive the loudspeaker.
Current through the loudspeaker
is limited to a safe value (for the
transistor) by the series 120
resistor. Dtode D3 is connected
across the transistor to protect it
against inductive kickback when it
turns off.
Power for the circuit comes from
a 12VAC plugpack. This not only
provides DC for the op amp circuitry but also the power to drive
the loudspeaker, as noted above.
The plugpack feeds two half-wave
rectifiers (D5 and D6) and two
The PCB has been designed to clip into the notched ribs on opposite sides of
the case. We used super glue to secure the loudspeaker and electret
microphone to the lid. Power comes from a 12V AC plugpack.
JANUARY
1989
69
12VAC
100
ELECTRET
MICROPHONE._.---;m.l
+
Fig.2: here's how to mount the parts on the PCB. Take
care with component orientation and be careful not to
confuse the various diodes. At right is a full size
reproduction of the PCB.
~0~~~01~2~10~~~0
,n~
l~
_./iiOl~~~!
I
''
00000000000000000
oo~
00000000000000000
000
00000000000000000
an
~oooo
o oo o
~-=~~
SPEAKER
{l
'
-
0
--00
ooo
ooo
0000
0000
il!:02
~
0000
000
..
()~Q
+
~~
1pf
:i
0
if
000
ELECTRET
·MICROPHONE
If you want to save money, you can build the
circuit on a small piece of Veroboard. But be
warned - if you use Veroboard, it's very
easy to make a mistake.
470µ,F 25VW filter capacitors, to
provide positive and negative supply rails. These are regulated with
10V zener diodes D7 and D8 to provide regulated supply rails of
± l0V.
Note that you can't use a DC
plugpack to power this circuit. It
needs 50Hz AC to drive the
speaker.
Construction
Our prototype Computer Sound
Repeater was housed in a standard
plastic utility box measuring 130 x
68 x 43mm. (Altronics Cat H-0153
or equivalent). We designed a small
printed board to accommodate the
70
SILICON CHIP
+
11
!
1
o
o o
O
00
O 0
~0000
00 o O 01 o
O
'0
Fig.3: wiring diagram for the Veroboard version of the Sound
Repeater. You can make the cuts in the tracks using an
oversize drill.
circuitry. It measures 66 x 62mm
and is coded SC07104891.
Fig.2 shows the parts layout on
the PCB. Take care when installing
the polarised parts and be careful
not to mix up the various diodes.
As an alternative, you can build
the circuitry on a piece of
Veroboard. To help you do this we
have shown a Veroboard component layout. This can be checked
against our photograph of a
Veroboard version of the circuit.
After drilling a pattern of holes
in the lid of the case (to let the
sound out) the loudspeaker can be
mounted. We used a quick and dirty
method: super glue. The electret
can be mounted in a hole in the lid
too and held in place with a dob of
super glue.
When construction is complete
check all your work carefully
before applying power. Check the
supply rails ( ± lOV) with your
multimeter. Pin 4 of the LM324
should be at + 10V while pin 11
should be - l0V. Pins 1, 8 and 14
should be close to 0V. Pin 6 should
be be close to + 1V and pin 7 should
be close to - lOV.
You can test the unit, without
turning on your computer, by whistling softly, at high pitch, into the
microphone. Each time you do so,
the speaker should "blurt".
~
