>>>READER-TO-READER QUESTIONS AND ANSWERS
Peak Reading Voltmeter
How can I build a peak reading
voltmeter or add-on unit for a VOM to
read the peaks of a DC pulse? When
troubleshooting ignition systems on
motorcycles and marine engines, the
peak voltage supplied by the various
coils needs to be checked. A regular
VOM always reads low. How can I
catch the peaks?
With RPM as much as 14,000 and
as many as 12 pulses per revolution,
we are talking very short pulses.
Testing is normally done at a cranking
speed of 200 to 300 RPM. There can
be as many as 12 magnets zinging by
each coil per revolution.
#2098 Tony Altobelli
USB HDTV Tuner Hack
How hard it is to rewire or modify
a USB HDTV tuner to work with a
portable DVD player to watch HDTV
programming from it or any other
applications like making a portable
HDTV tuner with a battery power
source? I've written about this idea to
many other sites and have had little to
no responses positive or otherwise.
#2099 David Kichi
Inquiring Minds Want To Know!
Transformers, motor stators use
stacks of thin (approx. 1/32") steel
plates. Is this for manufacturing
economics or is there a physics reason
for using stacked plates?
Will using 1/16" plates instead of
1/32" plates affect performance? How
much less efficient is aluminum
wire versus copper for making
transformers, stators, electromagnets?
Are there small solid-state relays
capable of handling high voltage/high
current surges through the switches
(rail gun application)?
#20910 Jim Hicks DVM
[#12084 - December 2008]
Variable Brightness of LEDs
I have two sets of eight LEDs. The
sets need to turn on alternately, with
the brightness turning on gradually,
then fading out.
As far as controlling the LED
brightness, here are two possible
solutions: one using PWM (pulse
width modulation) and the other a
variable DC power source. First, I'll
describe the PWM circuit in
which is controlled by a PICAXE 08M
microprocessor. The output from a
PWM generator is programmed to run
continuously. The timing duration is
controlled by the ADC reading on
potentiometer R1. The LEDs are
controlled by FETs Q1 and Q2.
All parameters are set by the
programming code. Figure 2 uses a
combination triangle-square wave
generator (U1) to control the variable
DC voltage from Q3. R2 controls the
duty cycle and R1 controls the minimum DC output from Q3. The setting
of R1 will depend on the color and/or
type of LEDs used. Basically, R1 is
adjusted so the LEDs just shut off
before the counter advances. The
square wave output (pin7 of U1) is
used to advance the 4017 (U2) which
is configured as a count to two and
recycle counter. The square wave
output switches low at each minimum
crossing of the triangle generator. This
is used to control the "clock enable"
(pin 13) on U2. The clock input is
connected to Vcc so the counter will
advance each time pin 13 goes low.
One nice feature about this circuit is
the 4017 allows for easy expansion (up
to 10) if more LED strings are needed.
There were no details mentioned
about the LED strings regarding how
they are wired, the color, or voltage
needed. In this case, I decided to
show both circuits operated with the
LEDs in parallel with individual current
limiting resistors. The RL values shown
are only guidelines as the specs on the
LEDs and the brightness required by
the user will vary. Both circuits require
a regulated 5 VDC power source.
February 2009 93