This is only a preview of the September 1994 issue of Silicon Chip. You can view 30 of the 96 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:
Items relevant to "Automatic Discharger For Nicad Battery Packs":
Items relevant to "Build The MiniVox Voice Operated Relay":
Items relevant to "An AM Radio For Aircraft Weather Beacons":
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Items relevant to "Dual Diversity Tuner For FM Microphones; Pt.2":
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Using Polyswitches for
loudspeaker protection
I wanted to build the 50W audio
amplifier published on the March 1994
issue and I figured the 2-way bookshelf
speakers published in January 1993
issue would be an excellent match.
I would like to protect the speakers
using Polyswitch protectors but the
thing is, how would I connect the
Polyswitches on the crossovers and
what sort of Polyswitches do I need?
I also want to know what kind of
Polyswitches would suit a “Nippon
America” 10-inch 100W RMS sub
woofer with an impedance of 4Ω?
I would like to run it with the 50W
amplifier. One last question: can Polyswitches be connected in parallel to
double their maximum current rating?
(M. C., Yarraville, Vic).
• We featured Polyswitches in the
50W and 100W amplifier designs
published in the December 1988 issue
of SILICON CHIP. Suitable devices are
stocked by Jaycar Electronics. You
can insert them individually in series
with the tweeter and woofer or just
have one to protect each system. For
the latter approach, we would suggest
the Cat. RN-3415 from Jaycar. These
could also be used to protect your 10inch speakers.
You cannot connect Polyswitches
in parallel as they will not share the
current equally.
Another vote for a signal strength meter
In reference to the letter from B. P.,
of Port Macquarie, in the April 1994
issue, I too would be blessed if you
would design a signal strength meter
and present it as a project. As he states,
commercial units are expensive and
no doubt this is due to their flexibility
and accuracy; a design based around
a tuner/front end from a VCR should
be just as good.
I have included with my letter the
circuit of a tuner/front end which is
made by Sanyo and are easy to come
by secondhand – most of their Beta
machines are fitted with them. The
beauty of this unit is that it only requires +12V for the supply and 0-12V
to tune across the entire band. Band
switching is achieved by taking one of
three lines high as required.
This makes it an ideal choice for
a portable unit as it can then be
Notes & Errata
Microprocessor-Controlled Nicad
Battery Charger, September 1993: a
number of errors have come to light
in the circuit on page 17. Pin 6 of
IC2 should be labelled pin 1 in Fig.1
and the 330Ω resistor associated
with Q3 should be in series with the
emitter resistor. VRI and the 30kΩ
resistor are reversed compared with
the PC board layout on page 20. The
lower of the two series resistors to
earth from pin 9 of IC1 should be
2.2kΩ. Finally, the 100µF capacitor
at the output of the 5V regulator is
shown with incorrect polarity on
the wiring diagram on page 20 and
the 12V relay has been omitted from
the parts list.
4-Bay Bow Tie UHF Antenna, July
1994: readers in eastern states who
are having difficulty purchasing
4.74mm dia, 0.91mm wall thickness
aluminium tube for this project
should note that it can be purchased
from the Alcan Aluminium Centre,
Lidcombe, NSW (Phone 02 647
9900) or ordered from them through
their local Alcan Centre. Readers
in other states may also do this but
they will have to pay freight from
Sydney to their location.
The reflectors and dipoles may
be made from thicker tubing or rod;
eg, 6.35mm diameter with suitable
adjustment being made to the dipole
mounting clip dimensions. The
reflector, dipoles and the connector
harness pieces could also be made
powered by eight 1.5V cells or a 12V
sealed lead acid battery. In designing
the meter, the following should be
taken into account: (1) It needs to
be able to detect and display signal
strength from 1µV up to 2-3V (via a
switched attenuator); (2) It should
have a large, easy to read display,
either a moving-coil meter or a digital
readout (if not both); (3) It should be
easy to calibrate without the use of
expensive test equipment and have
optional outputs at the rear for IF (to
use the meter as a substitute tuner)
and Video/Audio to feed to a monitor
for picture quality evaluation.
Judging by the comments I have
had from others, a project like this
would be extremely popular, as most
from 4.74mm dia aluminium rod.
For the harness pieces this material
will need heating and hammering
on an anvil in the areas where holes
have to be drilled. The connector
pieces could also be made from
3.2mm aluminium rod which is
readily available from CIG welding
centres.
Discrete Dual Supply Voltage Regulator; April 1994: the PC board
pattern and the overlay pattern
featured on page 31 has an error in
that pin 3 of IC1b is not grounded.
The board can be corrected by connecting a short link across to the
adjacent GND track.
Fast Charger for Nicad Batteries,
May 1994: this circuit has caused
confusion to many constructors
because of its method of dV sensing
to end the fast charging mode; it
will not work unless it is actually
charging cells. If you attempt to test
the circuit without a nicad battery
load, the output voltage will rise until pin 7 reaches +4.25V whereupon
the circuit will switch off.
For further background on this
circuit, refer to the article on using
the TEA1100 IC on page 6 of this
issue.
Dual Diversity FM Tuner, August
1994: the varicap diode D5 is
shown on the circuit the wrong way
around. It is shown correctly on
the wiring diagram of this month’s
issue, on page 68.
commercial units start around $1500.
There have been other designs presented before but in general they
were uncalibrated or involved lengthy
construction. (S. W., Hamilton, NZ).
• The problem with designing such an
instrument is not so much the tuner
front end but that of calibration. Without calibration, such instruments are
fairly useless. Tuners do not respond
equally over their band and therefore
it is necessary to produce a calibration
curve for each individual instrument
if the results are to be accurate. This
really does make it difficult for us to
produce a useful product unless we
can come up with a simple calibration
procedure. At the moment, we don’t
SC
have a solution.
August 1994 93
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