This is only a preview of the January 1996 issue of Silicon Chip. You can view 22 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:
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Door minder
is insensitive
I have recently assembled the
“Door Minder” kit as published in
the July 1995 issue of SILICON CHIP.
I find that it is not very sensitive
and it only works in a small room
with the windows shut and if I give
the door a hefty pull.
I would be most grateful if you
could tell me how to make it more
sensitive; the preset pot is adjusted to the most sensitive it will go.
The article says it works with open
windows in an adjoining room.
Also, where can I purchase the
Philips ETD49/25/16 transformer
components and the TEA100
nicad monitor IC for the Fast Nicad
Charger, as published in the September 1995 issue?
I refer to the letter on page 7 of
the April 1995 issue, on making
PC boards by photocopying the
original back to front and rubbing
with thinners mixture on the back. I
have had limited success and find it
is best if the photocopy is removed
whilst still moist.
I say moist and not wet, as too
much thinners gets underneath
and smudges the pattern. When
dry, doing it a second time darkens
the picture but use a fresh, new
photocopy.
monitor the output of IC1 to see that
it delivers the correct signal from its
output in response to the arm/disarm
switch.
How to reduce
preamp gain
I recently put together the preamp
section of your 50W amplifier design
and find it very satisfactory. My amplifier and speakers are of very high
sensitivity so I need much less gain
than is provided. Would you please
advise the correct way of changing the
feedback components around IC1 to
achieve closer to unity gain? I prefer
to do this rather than use attenuators.
(K. A., Moss Vale, NSW).
• The gain of IC1 can be reduced by
increasing the 4.7kΩ resistor at pins
6 & 2. To halve the gain, increase the
94 Silicon Chip
However, the result produces a
porous copper surface which has
to be heavily layered with solder.
An improvement can be made by
going over the whole pattern with
a fine felt tip pen which is water
resistant. (D. S., Caloundra, Qld).
• We are surprised that your Door
Minder is so insensitive. You
should check that the 0.1µF and
1µF capacitors on IC1a’s input
are the correct values and that the
1µF capacitor is inserted with the
correct polarity. Also confirm that
the two resistors around IC1a are
47kΩ and 3.9MΩ – a wrong value
may have been fitted. Also, did
you ground the case of the electret
insert?
Check that the regulated DC is
about 8V and the voltage on pin 1
of IC1a is 3.3V. As the circuit has
a gain of 80 it only needs 10mV
from the microphone to trigger the
chimes. It should not be necessary
to increase the gain of the circuit,
as it is quite adequate. Perhaps the
microphone insert is faulty.
You can get some extra gain by
decreasing the 47kΩ input resistor
to 39kΩ but any more variation
could alter the passband of the
filter.
Regarding the parts for the Fast
Nicad Charger, these can be purchased from Jaycar Electronics.
4.7kΩ resistor to 10kΩ. To obtain unity
gain, omit the 4.7kΩ resistor.
Frigid remote control
won’t respond
I have just built the UHF remote
switch from the December 1989 issue
of SILICON CHIP. It is operating perfectly from a distance of 10 metres but only
when the temperature is over 20°C.
Can you advise how to overcome this
problem as the temperature in Victoria
is generally under 20°C. (P. L., Spring
vale North, Vic).
• Ever thought of moving to warmer
climes? It appears as though one or
more of the transistors or possibly one
of the ICs is temperature sensitive.
As a first step, we suggest you check
all voltages in the circuit. Second,
check all soldering on the PC boards.
Cold solder joints can be temperature
sensitive. Third, with a can of freezer
spray, freeze each semiconductor
component to see if it causes the
problem. Alterna
tively, replace the
transistors one by one to see if you
can effect a cure.
Extended leads for a
digital thermometer
Recently, I bought a digital thermometer with indoor/outdoor display
from Jaycar (Cat QM-7210). I intended
to extend the outdoor twin lead by a
further 12 metres to enable the probe
to be sited in the foliage of a bushy tree
for a genuine outdoor reading.
Doing this increased the leads’
total resistance by about 0.3Ω which,
in turn, increased the readout figure
by 7°C. I tried counteracting this by
adding 0.3Ω in parallel with the probe
but then the readout for this probe
disappears.
I cannot see any internal adjustment to compensate for extra lead
length. Have any readers had similar
experience along these lines? (M. B.,
Taree, NSW).
• We doubt whether the additional
resistance in the probe leads has
caused the increase in temperature
reading. We are more inclined to think
that the long leads may be picking up
hash which is adding to the reading.
Try connecting a 0.1µF greencap or
MKT capacitor across the probe leads
where they enter the case.
Note & Errata
Dolby Pro Logic Surround Sound Decoder, Pt.1, November 1995: the anode
of diode D12 is shown incorrectly
joined to the junction of the cathode
of D14 and an associated 10kΩ resistor.
Instead, D14 and the 10kΩ resistor
should connect directly to pushbutton
switch S7.
Dolby Pro Logic Surround Sound
Decoder, Pt.2, December 1995: the
resistor connected to pin 21 of IC2 is
marked “30O” on the layout diagram
(Fig.4, p71). The correct value of this
resistor is 30Ω.
Five-Band Equaliser, December 1995:
the supply pins for IC2 on the circuit
diagram (Fig.5, p24) are shown reversed. Pin 4 should go to the +15V
rail, while pin 11 should go to -15V.
The parts layout diagram (Fig.6, p25)
SC
is correct.
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