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This is only a preview of the September 1995 issue of Silicon Chip. You can view 34 of the 104 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
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Items relevant to "Notes On The Train Detector For Model Railways":
Items relevant to "Build A Jacob's Ladder Display":
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Headphone amplifier
for PA system
After reviewing your recent articles on the 50W Stereo Amplifier
(Feb-Apr 1995) and looking at my
version of the 120W PA Amplifier
(Nov. 1988 to Jan. 1989), which
has four microphone inputs and a
line output for a tape recorder or
power amplifier, I would like to add
a headphone amplifier.
This would function in the same
way as it does in your 50W Stereo
Amplifier, having normally the
audio signal feeding the output
socket for the line output, but
when the headphone jack (mono)
is inserted, the signal is diverted to
the headphone amplifier.
Can the circuitry used in the 50W
Stereo Amplifier for the headphone
and the bistable IC2 is at fault.
There may have been others who
have experienced the same trouble
and have been able to rectify the malfunction.
I would also like to ask for a concise
explanation, maybe from one of your
readers, on how do the "LCD A/D
converters-display drivers" activate
the correct segments when they are
bundled up in groups, ie agd, bc and
ef respectively. (K. B., Forestville,
NSW).
• Jitter in the last digit of the display
could be attributed to the changes in
amplifier be used for this project?
What alterations will have to be
done? And lastly, what are the
power supply requirements for the
headphone amplifier?
Please note the headphone amplifier will only need to be mono,
not stereo. (R. T., Mundubbera,
Qld).
• The headphone amplifier to the
50W Stereo Amplifier should work
without problems. If you need to
increase the gain you can do so by
reducing the 10kW resistor connected to pin 6 (2) of IC3a.
The required supply voltage is
±15V DC. This can be derived in
the same way as the existing low
voltage supply rails in the 120W
PA amplifier; ie, via 680W 5W dropping resistors and 15V 3-terminal
regulators.
output levels of IC1 and IC2 occurring
at different times. These can affect the
power supply rails which can slightly
alter the timing sequence and thus the
capacitance reading.
The PC board design should be
carefully checked for correct ground
track layout so that current loops are
not formed. The board pattern for the
original design using LED displays
can be seen in the May 1990 issue,
where star point earthing was used.
This design did not have problems
with jitter on the pF range.
Each segment of an LCD is driven
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entirely separately. They are not
bundled together as in a multiplexed
display. A "lit" or visible segment
occurs when the signal applied to
it is 180 degrees out of phase to the
backplane. When the applied signal
is in phase with the backplane, the
segment is off.
How to design an
electromagnet
Could you please give me a simple
formula to calculate the ampere-turns
necessary for a home-made electromagnet or solenoid for various battery-operated gadgets?
For example, I wish to make an
electromagnet or solenoid to operate a
camera which requires 250 gram pressure on the trigger to operate. Hope this
is in your field. (D. H., Mosman, NSW).
• Unfortunately. we cannot help you
directly since the design involves
not just the ampere-turns produced
by the coil, but also the details of the
magnetic circuit as well. This involves
the type of steel and design of the
laminations, the design of the plunger
and its return spring, if required. The
driving circuit must also be taken into
account because many solenoids are
designed to be energised momentarily;
if energised continuously they will
quickly overheat and burn out.
Your camera solenoid application is
one requiring considerable power and
would also need to be a momentary
design otherwise it would be physically quite large.
Having noted all of the above, why
not try winding several hundred turns
of fine gauge wire onto a bobbin which
is a close fit over a 1/4-inch bol? Energised with a 6V lantern battery, this
should provide quite a reasonable
degree of thrust. At the very least it
will provide a starting point.
Notes & Errata
Fuel Injector Monitor, August 1995:
the specified LM358 op amp has been
found to be non-linear in the circuit
at low and high injector duty cycles.
The problem is corrected by substituting an RCA CA 3260E op amp which
has CMOS outputs. This op amp is a
drop-in replacement but a change to
the integration RC network at pin 3 is
desirable. Change the 4.7kW resistor
to 47kW and the 220µF capacitor to
10µF.
SC
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