Q&A
■ WITH RUSSELL KINCAID
In this column, I answer questions about all
aspects of electronics, including computer
hardware, software, circuits, electronic theory,
troubleshooting, and anything else of interest
to the hobbyist. Feel free to participate with
your questions, comments, or suggestions.
Sendallquestionsandcommentsto:
Q&A@nutsvolts.com
WHAT’S UP:
Join us as we delve into the
basics of electronics as applied
to every day problems, like:
✓
High Impedance Preamp
● Audio to LED Display Driver
✓
✓
FM Shutdown Problem
AUDIO TO LED DISPLAY
DRIVER CIRCUIT
QI need a circuit which will
drive an 8x8 or 16x16 LED
matrix from an audio input
and generate random
patterns. The audio input portion has
to be very sensitive.
— Robert Christopher
AA display based on the
audio input will not be
random. If the left and right
channels of a stereo signal
are applied to the X and Y inputs of a
matrix, it will show the difference in
the two signals. If the signal is mono,
the display will be a 45 degree line
that varies in length with the strength
of the signal.
The LM3914 dot/bar graph LED
driver is made for this application.
The output is a current source so
current limiting resistors are not
needed. The IC has 10 outputs but
can be expanded to 20 or more. I will
design an 8x8 matrix display to show
how it works (Figure 1).
The input range is set by the
reference voltage. In this case, I set it
to one volt, assuming that the input
would be line level. The 1/2 input pin
is biased up 1/2 volt so that one volt
p/p input will not clip. R4 and R6 set
the current sink of IC2 at 10 mA.
The current sink of IC1 is set at
1 mA and the current gain of the PNP
26 January 2009
transistors will keep them saturated.
An array of PNP transistors would
be nice but the ULN series are all
NPN. I found a four transistor PNP
array for $6 but with 2N3906s at
$.05, I went with singles. Pin 9 of the
LM3914 is open for a dot display;
you could connect pin 9 to Vcc for a
bar display.
My schematic capture program
(Eagle) does not do group copy of
line drawings so I did not show all the
diodes in the matrix.
■ FIGURE 1
DETECTION OF MOTOR
SPEED AND DIRECTION
QI have attached the image
of a plastic circular disk with
36 holes around the outer
edge, driven by a DC
motor. I need a circuit that will tell
me speed and direction of the motor
by the holes passing a detector.
— G. W. Heikkila
