This is only a preview of the February 1993 issue of Silicon Chip. You can view 54 of the 104 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "Build The Electronic Cockroach":
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Audio leve
meter
with LED readout
This LED VU meter can be used as an adjunct
to the mechanical meters found in older tape
decks & in low-cost audio mixers. Its main
advantages include a fast response time & a
peak hold facility so that you can accurately
set recording levels.
By BERNIE GILCHRIST
When recording audio signals, it's
important that input levels be accurately set in order to obtain good results. If the signal level is set too high,
the recorded signal will sound distorted due to signal overload. On the
other hand, if the input signal level is
set too low, the recording will be noisy,
will have limited dynamic range and
may even suffer from signal dropouts.
The traditional way of monitoring
signal levels is to use a VU (volume
units) meter. In practice, the input
signal level is normally adjusted so
that peak levels register no more than
56
SILICON CHIP
about +3dB. Any more than that and
you run the risk of signal overload.
Mechanical VU meters have one
major disadvantage though - they're
not fast enough to respond to fast
signal transients. The result is signal
clipping and unwanted distortion in
your recordings. By comparison, this
electronic VU meter can respond to
fast transients and also has a peak
hold facility to give you sufficient
time to observe these signal peaks.
Unlike conventional units, this unit
displays the signal level using 12 rectangular LEDs arranged in bargraph
fashion. The reading is scaled in volume units, which is a logarithmic scale
commonly used in audio equipment
for displaying dynamic signal levels.
Table 1 shows the absolute and relative levels for the LEDs when no input attenuation is used; ie, when the
input signal is applied direct to Cl
(see Fig.1).
The peak hold facility involves the
top five LEDs. If the signal peak
reaches these LEDs, then the highest
LED that lights will be held on long
enough for it to be observed, while
the other LEDs in the peak hold section turn off. By this means, any peaks
that are normally too quick to be observed are captured and displayed.
By making a simple circuit modification, two of these VU meters can
Below: the LM317 regulator is secured
to the underside of the PC board using
a screw & nut. The 12 LEDs indicate
the signal level. They can be mounted
separately & wired back to the main
board via a 12-way cable if required.
Of
TABLE 1
1N4002
+11-40V
i
CS +
33
3svw-.,.
ov°+
C4 +
AS
1M
1
sovw-
13
PK OSC
-20dB
LED1
15
D1 16
.,.
-15dB
LED2
D2 17
Rt
33k
-tOdB
LED3
INPUT
R2
47k
GND°+
7
VRt
50k
.,.
.,.
D3 18
IN1(·)
R4
82k
-7d8
LED4
C2 1
SOVW+
04 19
-SdB
LEDS
D5
20
·3dB
LED6
IC1
LB1412
LED
Input Level
Relative Level
1
11mV
-20dB
2
20mV
-15dB
3
35mV
-10dB
4
49mV
-?dB
5
62mV
-5dB
6
78mV
-3dB
7
98mV
-1dB
8
110mV
0dB
9
123mV
+1dB
10
156mV
+3dB
11
196mV
+5dB
12
276mV
+8dB
D6 21
10
~
AO I
-1dB
LED7
D7
OdB LEDS
R6
4.7k
14
D8
.~.
010
11
D11
I
I
I
012
+
BARGRAPH
vu
1
ILED
09
Jf
22
2
3
4
5d8 LE011
5
SdB LE012
K
A
.,.
METER
Fig.1: the circuit is based on an LB1412 display driver (ICl ). The audio signal is
fed in on pin 7, after which it is amplified & detected before being fed to 12
comparator stages. These stages compare the input level against preset
thresholds & operate the appropriate LEDs.
share a common peak hold reset li:rie,
so that the peak LEDs in each channel
switch off at the same time. This enhances the meter display and makes
it easier to adjust recording levels in a
stereo system.
To make the display easy to read,
the bottom seven LEDs are all green
and these correspond to relative signal levels ranging from -20dB to -ldB.
The eighth LED is yellow and corresponds to 0dB, while the top four LEDs
are all red and indicate signal levels
ranging from +ldB to +BdB.
How it works
Let's now take a look at the circuitsee Fig.1. It's based on a single LB1412
IC plus a 3-terminal regulator and a
handful of minor parts.
As can be seen, the audio signal is
fed in via an input attenuator consisting of Rl, R2 & VRl. This circuit
allows the input sensitivity and impedance to be adjusted (see Table 2).
The attenuated signal is then AC-coupled via Cl and R3 to the inverting
input of an op amp inside the LB1412
IC.
ICl is a complete 12-dot red/green
LED level meter on a single chip. It
contains two stages of input amplification, 12 comparator stages, an inbuilt oscillator for peak hold reset,
two constant current sources, and 12
output driver stages for the LEDs. In
addition, there are five RS flipflop
stages which function as latches and
provide the peak hold logic for the
five top LEDs.
The gain of the first op amp stage is
determined by resistors R3 and R4
and, according to the manufacturer's
specifications, must not be made
greater than 4. In this circuit, R3 and
R4 have been selected to give a gain of
3.7.
The output of this first stage is coupled via C2 to the input (pin 9) of the
second stage. This stage functions as
a half-wave detector with a gain of 2.
Its output at pin 10 is filtered by capacitor C3, the value of which determines the attack and decay times of
the display.
If C3 is made too small, the display
will tend to flicker rapidly with low
frequency signals. Conversely, if it is
made too large, the display will not
respond to short peaks or troughs.
The 2.2µF capacitor used in this circuit gives attack and decay times of
approximately 0.5ms and 250ms respectively.
Internal comparators
The output of the detector is fed to
the 12 comparators (one for each LED).
These compare the input level with
the preset threshold levels for the display and operate the appropriate LEDs.
As indicated above, the peak hold
function works by using the internal
latches to gate the top five LEDs. This
latching circuit holds on the highest
LED to be lit until a reset pulse is
received. It also ensures that any lower
LEDs in the peak hold circuit are
FEBRUARY
1993
57
Fig.2: install the
parts on the PC
board as shown in
this wiring diagram,
noting especially
that IC2 is mounted
on the copper side
of the board. The
LED display section
can be separated
from the remainder
of the circuit by
cutting along the
dotted line.
protection diode and about 2V across
the regulator.
A relatively large area of copper is
provided on the PC board to serve as a
heatsink for the LM317. This allows it
to operate continuously from a 40V
supply without overheating, even
with all LEDs turned on. The LM317
has automatic thermal protection but
normally only some of the LEDs are
on, so even with a 40V supply the
power dissipation will be comfortably below its shutdown point.
Finally, link J1 (shown dotted) disables the peak hold circuit by pulling
the reset line of the internal oscillator
to ground. The top five LEDs then
function in the same manner as the
bottom seven LEDs.
Construction
turned off (ie, only one of the top five
LEDs can be lit at any one time).
The reset pulse is supplied by the
internal oscillator, the frequency of
which is determined by R5 (1MQ)
and C4 (1µF). This resets the latches
once every cycle; ie, about every 0.9s
with the values shown. Because the
oscillator is not synchronised to the
signal peaks, the actual peak hold time
depends on just when the peak occurs within the cycle and may be very
short if it occurs just before the reset
pulse arrives.
In practice though, the varying peak
hold time is not all that noticeable
since the effect averages out over time.
Constant current drivers
The two constant current drivers
inside IC1 control the current through
the bottom seven LEDs. LEDs 1-4 share
one constant current driver, while
LEDs 5-7 share the other. Resistor R6
sets the current through these drivers
to about 11mA. Although the current
decreases with an increase in R6, it is
not inversely proportional to R6.
LEDs 8-12 are individually driven
because of their peak hold function,
their current being determined by series resistors R7-R11. Because of this,
their brightness can vary if the supply
voltage to the LB1412 varies but in
this circuit, an LM317 voltage regulator provides a fixed supply voltage of
9.6V.
Actually, the specified supply voltage range for the LB1412 is 10-16V
but tests involving six of these ICs
showed that they all worked down to
8.4V without any problems.
The LM317 voltage regulator allows
input voltages of up to 40V to be used
so that the unit can be connected to
the positive supply rail of most power
amplifiers. The lower supply voltage
limit for the circuit is about 11 V, which
gives approximately 8.4V across the
LB1412 after dropping 0.6V across the
All the parts for the LED VU meter
are mounted on a small PC board
coded ZA-1363 - see Fig.2. You can
install the parts in any order but make
sure that the IC, diode and electrolytic capacitors are correctly oriented.
The LM317 is mounted on the copper
side of the PC board and is secured to
the board using a screw and nut.
If you want the peak hold facility,
link J1 should be omitted. Conversely,
install J1 to disable the peak hold
s ircuit if this feature is not required.
As shown in Fig.2, the circuit is
configured for a sensitivity range of
600mV to 1.5V and a nominal input
impedance of 50kQ (ie, R1 = 33kQ &
R2 = 47kQ). This sensitivity range
should suit most line level signals;
eg, from CD players and tuners. If
required, the sensitivity can be increased to suit signals up to 600mV
by installing a wire link for R1 and
deleting R2.
The most difficult part of the construction is getting the ends of the
LEDs to line up. One technique that
RESISTOR COLOUR CODES
0
0
0
0
0
0
0
0
0
0
58
No.
Value
1
1MQ
82kQ
47kQ
33kQ
22kQ
4.7kQ
1.8kQ
560Q
270Q
1
1
1
1
1
1
5
1
SILICON CHIP
4-Band Code (5%)
brown black green gold
grey red orange gold
yellow violet orange gold
orange orange orange gold
red red orfenge gold
yellow vio et red gold
brown grey red gold
green blue brown gold
red violet brown gold
5-Band Code (1%)
brown black black yellow brown
grey red black red brown
yellow violet black red brown
orange orange black red brown
red red black red brown
yellow violet black brown brown
brown grey black brown brown
green blue black black brown
red violet black black brown
TABLE 2
R1 (ohms)
R2 (ohms)
Sensitivity
0
-
110-600mV
15-50k
33k
47k
600mV - 1.5V
50k
100k
47k
1.5V, - 3V
100k
100k
10k
3V-8V
100k
100k
3.3k
8V- 20V
100k
100k
1.2k
20V- 50V
100k
can be used is to first carefully solder
the two end LEDs so that they are the
same distance from the PC board. The
remaining LEDs can then be installed
and the display pushed against a flat
surface to align the ends of LEDs before soldering.
It is a good idea to initially solder
only one lead of each LED until you
are satisfied that they are all correctly
aligned. Make sure that the LEDs are
correctly oriented - the anode lead is
the longer of the two. Use green LEDs
for LEDs 1-7, a yellow LED for LED 8,
and red LEDs for LEDs 9-12.
To allow for different mounting configurations , the display end of the
board can be separated from the rest
of the circuitry using a fine-bladed
hacksaw. The display can then be connected to the main board via flying
leads, or directly soldered to it at right
angles.
If you are soldering the two boards
together, lightly solder tack the two
end connections first. The angle between the two boards can then be
easily adjusted if necessary before the
PARTS LIST
Input Impedance
1 PC board, code ZA-1363
1 10mm x 3mm-dia. screw, nut &
washer
4 PC stakes
1 S0kn miniature vertical trimpot
Semiconductors
remaining connections are soldered.
If two of these VU meters are to be
used together in a stereo system, then
the following modifications should
be included so that the peak hold
circuits reset together: (1) disable the
p eak hold oscillator on one meter by
removing C4 and R5 ; (2) connect pin
13 of ICl in this meter to ground (ie,
to OV); and (3) connect the reset pins
(11) of the two LB1412 display driver
ICs together.
The oscillator in one unit will now
reset both units simultaneously.
Adjustment
Trimpot VRl is the only component on the PC board that requires
adjustment. If you are using the unit
as an adjunct to a mechanical VU
meter, adjust VRl so that both meters
read the same on a lkHz sinewave
signal.
If you have access to an audio signal generator, you can calibrate the
circuit so that the OVU LED corresponds to a signal level of 775mV at
lkHz.
SC
1 LB1412 display driver (IC1)
1 LM317T regulator (IC2)
1 1N4002 silicon diode (D1)
7 green LEDs (LED1 -LED?)
1 yellow LED (LED8)
4 red LEDs (LED9-LED12)
Capacitors
1 33µF 35VW PC electrolytic
1 4.7µF 25VW PC electrolytic
1 2.2µF 25VW PC electrolytic
2 1µF 50VW PC electrolytic
1 0.1 µF monolithic
Resistors (0.25W, 5%)
1 1MQ
1 82kQ
1 47kn
1 33kn
1 22kQ
1 4.7kQ
1 1.8kQ
5 5600
1 270n
Where to buy the kit
A kit of parts for this project is
available from any Dick Smith
Electronics store or by mail order
from PO Box 321, North Ryde
2113. Phone (02) 888 2105. The
price is $24.95 plus $5 p&p. Quote
Cat. K-5370 when ordering.
Note: copyright of the PC artwork
for this project is retained by Dick
Smith Electronics.
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10/92
FEBRUARY
1993
59
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