READER-TO-READER TECHFORUM
near the fixed one.
Mike Hardwick
Decade Engineering
Turner, OR
not indicate what brand they were. If
they are the no-name generics, they
would be more prone to external
noise.
[#1104 - January 2010]
Speaker Hum
I have a problem with my speakers
humming on my desktop PC. I think
this may be wall wart related but not
100% sure.
Is there a way to fix this problem or
do I need to buy a new set of speakers?
Ralph J. Kurtz
Old Forge, PA
[#1105 - January 2010]
Automotive Speed Sensor
I recently replaced the trans in my
Ford van. Somewhere between 00 and
02 model years, Ford changed the
speed sensor output to the PCM,
but I have an analog gear driven sensor.
The newer sensor is digital (variable
reluctance). I need to change the signal
back to analog so the speedometer,
ABS, and cruise will work. Is there a
device to do it, or is it even possible?
(see the National Semiconductor and
Linear Technology sites), giving
further insights on capacitor selection
and much more. Alternatives are
the LM331 (±0.06% non-linearity);
Linear Technology has some precision
circuits going into the PPM region.
Walter Heissenberger
Hancock, NH
You could look at several areas to
solve the speaker hum with your PC.
First, it could be a poorly filtered
wall wart and you might want to
replace it as the filtering that it has is
just not enough to overcome hum in
the system. You could also check/
replace the cable that goes between
the sound card in the PC and the
speakers. Could be that the shield is
cracked and this is where the hum is
being introduced into the system.
Also, try to use as short a cable as
possible between the two as the
longer the cable, the more likely it will
act as an antenna picking up any
electronic noise anywhere near the
system. Try moving the speakers to
another location a short distance away
from the PC and see if that improves
the sound quality. If the sound card is
external to the motherboard, try
removing and reseating it as there may
be some contact issues between the
card and the motherboard slot that
the card plugs into. Lastly, trash the
speakers you have and go with a
better quality speaker system. You did
[#1106 - January 2010]
Design/Test Software
I've had electronic engineering
schooling back in the ‘80s and have
tinkered a little since then. I've never
had a computer until now and would
like to know some free or reasonable
circuit design/test software download
sites to tinker with.
All you need is a Frequency-to-Voltage (F/V) converter with a
Schmitt-trigger input. There are many
published circuits available to pick
from. Figure 2 shows an adaptation of
a National Semiconductor circuit
(AN-140). A comparator was used —
instead of the CMOS-Schmitt inverter
— to obtain better control of the input
switching thresholds. R1 and R2 adjust
the level, and R3 the hysteresis. A
clean 50% square wave must be at the
output of U1, swinging almost rail-to-rail (if necessary adjust R1, R2, and
R3). Current flows on a negative output swing from the inverting node
through D1 and C1. The current is
only a function of the frequency and
size of C1, since the output swing of
U1 is fairly constant. C2 and R6
provide a voltage output. There are
many different variants of this circuit
Circuit Simulators: LTspice IV
(free, no limitations, excellent; Linear
Technology) and Tina-TI (free scaled-down version; Texas Instruments).
CAD/Layout: ExpressPCB (free)
and ExpressSCH (free) are PCB
layout and schematic software. Very
consistent and easy to use. QuickCAD
mechanical CAD software is from
Autodesk.
Miscellaneous: AppCAD (free;
Agilent) calculates transmission lines,
S-parameters, microwave designs,
amplifiers, biasing, reflections, noise,
etc. MPLAB (free; Microchip
Technology, but needs a plug-in for
C or MBasic, and a programmer)
software is used to program PIC
microcontrollers. There’s Microsoft
Office which includes Word, Excel,
PowerPoint, and Access for general
office type applications such as
documents, spreadsheets, presentations,
and databases.
FilterPro (free, Texas
Instruments) software
is used to design
active filters. Last
but not least is
SpectrumLab (free;
uses sound card to
digitize and analyze
spectrum).
Figure 2
Walter
Heissenberger
Hancock, NH
80
May 2010