wire would upset the adjustment. I
then “fixed” the turns with a dab of
nail polish. L1 is very touchy; L2 takes
maybe only 1/16 turn to reset. As I
only need to “transmit” over about
three metres I used the single wire
antenna but even so it works well over
20 metres. (K. C., Balgownie, NSW).
• The drift in frequency of transmission is probably due to the capacitance
change of the tuning capacitors with
temperature.
Make sure that the 47pF capacitors
across L1 and L2, the 15pF capacitors
on pins 9 and 10 and the 4.7pF capacitor at pin 10 are NPO types. These
will be labelled “NPO” or with a black
dot. NPO stands for “Negative Positive
Zero”. This means that the tempera
ture coefficient of capacitance is zero
for the normal operating temperature
range.
You can expect to affect the tuning
when the antenna is held with your
finger and thumb. This is because it
alters the capacitance of the tuned
circuit.
Using a transducer
with the speed control
I am interested in using the 12-24V
Motor Speed Controller featured in
the June 1997 issue of SILICON CHIP.
However, I wish to use the output from
a MAP sensor in a car to control the
speed of a 12V fan. The MAP sensor
has an output range of zero to 5V and I
wish to obtain full fan speed when the
MAP sensor output is 5V. Furthermore,
I would like to introduce an adjustable
offset, so that the MAP sensor might
produce a voltage of say, 1.5V, before
the fan operated. Can this wish list be
achieved or is it all a dream? (G. B.,
Unley, SA).
• While we did not make provision
for this sort of application it turns out
to be relatively easy to do and just by
adding one resistor. By connecting a
Don’t overdrive
the Bass Barrel
I intend to construct the Bass
Barrel Subwoofer from the August
1997 issue but I need further information. Firstly, what is the power
requirement to drive this unit? Also
what is the maximum (safe) power
the unit can handle? Can you recommend a suitable amplifier kit?
What passive low-pass filter could
be used in place of the active one
briefly alluded to in the arti
cle?
Thank you. (G. P., NT).
• The specified woofers have a
maximum rated power input of 75W
and since they are in parallel, the
Bass Barrel would have a nominal
maximum input power of 150W.
However, we do not think it would
be safe to drive them at these high
levels for anything more than a brief
interval otherwise you run the risk
68kΩ resistor from your transducer to
the pin 2 of IC1, the motor will have
12V applied to it, as set by VR1, for
an input of 5V from the transducer.
Furthermore, the initial offset will be
1.5V, so the effective input range is
from 1.5V to 5V.
By changing the resistor to 100kΩ,
the offset can be reduced to 0.3V and
if you reduced the resistor to 47kΩ,
the offset is increased to 2.5V.
Notes & Errata
Flexible Interface Card, July 1997;
Stepper Motor Con
troller, August
1997; and PC Card For Two Stepper
Motors, September 1997: in the circuit
of each of these boards the resistor
from pin 4 of IC1 is shown going to
ground. This is how the PC boards
of blowing them. In fact, we have
heard of a number of instances
where people have done just that.
Note that this unit was originally
designed for cars and is also suitable
for small lounge rooms but if you
use it in a large room and wind up
the wick, you are bound to pay the
penalty.
A suitable amplifier module
would be the 125/175W design
featured in the April 1996 issue.
This could be teamed with the
subwoofer controller featured in
the December 1995 issue of SILICON
CHIP but remember our warning
about winding up the wick. An
alternative amplifier would be the
50W unit described in March 1994.
Alternatively, if you intend using
the Bass Barrel in a Dolby Surround
system, you could use the Dolby
subwoofer output to drive the Bass
Barrel amplifier.
will be supplied. The resistor should
go to the +5V supply, which is pin 16
of IC1. On each board it is quite simple
to move the ground end of the resistor
to the positive supply after drilling
one hole.
If you are only using a single board
this may not be a problem, although
we do suggest that you move the 10kΩ
resistor so that it is wired between
pins 4 and 16.
The problem shows up when you
are using several cards with different
addresses on the same printer port and
start changing these addresses.
Low Dropout 5V Regulator, Circuit
Notebook, October 1997: This circuit
has the emitter and collector of Q1
reversed. The emitter should go to the
+9V supply and the collector should
SC
connect to the 5V output.
WARNING!
SILICON CHIP magazine regularly describes projects which employ a mains power supply or produce high voltage. All such projects should
be considered dangerous or even lethal if not used safely. Readers are warned that high voltage wiring should be carried out according to the
instructions in the articles. When working on these projects use extreme care to ensure that you do not accidentally come into contact with
mains AC voltages or high voltage DC. If you are not confident about working with projects employing mains voltages or other high voltages,
you are advised not to attempt work on them. Silicon Chip Publications Pty Ltd disclaims any liability for damages should anyone be killed
or injured while working on a project or circuit described in any issue of SILICON CHIP magazine. Devices or circuits described in SILICON
CHIP may be covered by patents. SILICON CHIP disclaims any liability for the infringement of such patents by the manufacturing or selling of
any such equipment. SILICON CHIP also disclaims any liability for projects which are used in such a way as to infringe relevant government
regulations and by-laws.
Advertisers are warned that they are responsible for the content of all advertisements and that they must conform to the Trade Practices Act
1974 or as subsequently amended and to any governmental regulations which are applicable.
November 1997 91
