Q&A
using one channel from each chip for
your stereo preamp. The “spare”
channels can be used for rear speakers, if you wish.
The power amp requires a +12-
volt and a -12-volt power supply —
both able to supply four amps of
peak current — about three amps
average if you use a hefty output
capacitor to absorb the peak. A typical power supply can be built using
LM350 and LM333 voltage regulators. Although the preamp also runs
off + 12 volts, it should have its own
power supply separate from that of
the power amp. An LM78L12 voltage
regulator will provide this.
is expensive — plus, each footprint
requires a different tip.
The hobbyist solution is something called ChipQuik, a low-temper-ature solder that replaces the solder
holding the chip to the board.
ChipQuik solder has a low melting
temperature and is very brittle —
meaning it crumbles easily. You can
buy it from most electronic supply
houses, including Jameco (800-
831-4242; www.jameco.com). Get
a free sample at www.chipquik.
com
Model RR Sounds
Un-Solder Solder
QI want to remove and replace a
120-pin IC, but my 30 watt pencil-type iron tends to cause damage to
the circuit board and the solder flows
between the leads. How does the
manufacturer solder all the small surface mount parts with such precision? What type of de-soldering
equipment should I use?
Dave
Reno, NV
AThe manufacturer uses a technique called “reflow soldering” to
attach the component to the printed
circuit board. It begins by putting a
dab of tacky reflow solder on the
pads of the circuit board. The component is then placed on the pads,
where it is held in place with the solder paste. After all the parts are in
place, the board is sent through a
“hot box” with a temperature that
melts the solder and makes the electrical connections.
To unsolder the part, they often
use a reverse process of heating the
selected component and its pads to
about 350 degrees F for 10 seconds
and then lifting it off. Of course, this
is way beyond the means (as in
money) of the hobbyist. On a smaller
scale, the component can be
removed using a special solder tip
that heats all the pads at once and
lifts the part off the board. This, too,
APRIL 2005
QI recently built the LED Flasher
from the September 2003 issue
and would now like to add the sound
of a railroad crossing in sync with the
flashing LEDs. Any ideas?
Oscar Moon, Jr.
via Internet
AThe simple answer is to use the
output of the LED Flasher to trigger a one-shot 555 circuit to sound a
crossing bell. Search through the
RadioShack catalog (www.radio
shack.com) for a buzzer with a
sound that pleases you. The clinker is
that the 555 is negative-going triggered. That is, Pin 2 must be pulled to
ground to start the timer. That means,
if we tap off, the output of the LED
Flasher would chime half the time,
skipping the positive-going voltage
excursion.
This is easily solved by inserting
an inverter in the output and summing the two through 0.01 capacitors. I said an inverter, right? So how
come the schematic (Figure 5) shows
three NOR gates? This is to put a
small delay in the outputs of the
inverters so that the capacitor signals
don’t cancel each other as one is
going positive and the other is going
negative. I used a series of NOR gates
rather than a string of inverters
because they have more propagation
time and better triggering. If you
experience timing problems, add
more gates, but just make sure the
count is always odd.
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