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AVR, dB and
LDR collide at
D/A Junction
By Daniel Rodrigues (Elektor Labs)
Nothing exceptional about it, I thought, and in line with most of
the article proposals we have the pleasure of seeing in the earliest
stages of publication. The circuit and the idea behind an ‘Innovative
Digital Volume Control’ proposed by one of our very remote freelance authors was duly taken through our evaluation system and
considered a fine candidate for the Summer Circuits edition by the
joint editorial team. So we pressed ahead and the paperwork was
landed on my desk.
In the original circuit, the key component was an LDR (light dependent resistor) in series with the loudspeaker connection. The LDR is
illuminated by a white LED whose brightness is controlled by a PWM
In plain English, the power
variation using that finest of LDRs is, oh
dear, 16 dB, which is disappointingly low. As a practical
example, 16 dB equals the variation of raising speech from a soft
whisper to whispering, well, a little louder.
The idea proposed by our valiant author is still nice, but sadly not for
an audio volume control as he had hoped. After all, what we have
here is a nice and inexpensive gadget which can be safely called a
digital potentiometer, or ‘Digipot’ in electronics journalese. Corny
+5V
LCD 2*16
23
J1
10K
10K
10K
PC1
PC0
3
5
21
PD0
PD1
PD2
PD4
PD5
PD6
PD7
30
31
32
2
9
10
11
+5V
R1
10K
IC1
+5V
AVR - ISP
7805
7805
C1
1000µF
16V
R3 .. R6
+5V
IC2
9-12VDC
+5V
R2
10Ω
10K
PB2
P2
Down
24
VCC
VCC
PB1
ATMega8-SMD
14
GND
P1
Up
R8
220Ω
+5V
6 18
GND
S2
13
GND
L
R
R7
220Ω
AVCC
4
Out
VSS
VDD
VO
RS
R/W
E
D0
D1
D2
D3
D4
D5
D6
D7
A
K
C4
100nF
S1
C2
100µF
16V
C3
100nF
(PB3,15)
(PC6,29)
(PB5,17)
(PB4,16)
MOSI
NC
RESET
SCK
MISO
+5V
GND
GND
GND
GND
080654 - 11
Lw = 10log(Pmax /Pmin)
= 10log[(U2/Rmin) / (U2/Rmax)]
= 10log[(U2/50) / (U2/2000)]
= 10log(40) = 16 dB
elektor - 11/2007
as it may sound, the use of the LDR based DigiPot is only limited
by your imagination: control of currents, voltages, brightness,
gain, you name it. All optically — sort of — but with better resolution, stability and dynamic control than some more traditional
approaches.
Fine AVR programming, no doubt, but with insufficient attention
paid to the real world of human hearing and some basic physics,
specifically where the dB comes in.
(080654-I)
Daniel Rodrigues (1983) has a B.Eng. degree in Electronics and Telecommunications Systems from ISEL Higher Polytechnic Institute in Lisbon,
Portugal. Daniel’s fortes and hobbyhorses include radio, digital communications and microcontroller systems. Daniel started working in the Elektor Labs in June 2008. When not pondering over electronics on his desk
or computer screen, he enjoys hiking and cycling.
43
E-LABS INSIDE
signal produced by a microcontroller — there you have it: state of
the art contactless audio volume control! However, when I started
testing the device I realised the span of the volume control wasn’t
too impressive, to put it mildly. In an attempt to overcome the problem I looked for an LDR with a larger resistance variation and the
best I was able to locate at the time was rated 50 to 2,000 ohms.
Now let there be Science instead of AVR programming. If we take
into account that the human ear is able to comfortably handle sound
levels within a range of 30–90 dB we should expect something like
60 dB of span to be afforded with ease by our volume control.
Let’s not forget that power is inversely related to resistance, meaning the lower the resistance, the higher the power. So, the sound
power level ratio can be expressed as