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by a gradual delay which plays havoc
with recording and signal processing
systems.
The idea is to limit this initial transient and have minimal effect on the
rest of the signal. Most commercial
compressor/limiters have extra controls such as independent attack and
delay. This would seem to be easy to
do by just installing potentiometers on
the front panel wired up to the attack/
delay circuit, presumably in place of
R1 and R2.
However, commercial units also
have a threshold control which is
adjusted so that up to a predetermined
level, no compression occurs. It would
seem possible to install this feature in
the CD Compressor. Could VR6 be the
correct control to use?
Also I would like to install a “compression meter” as used in upmarket
compressors. It is simply a meter connected to the VCA control input so that
one can see how much compression
is being applied at any one moment.
Would pin 11 of IC2 be the right place,
buffered via an op amp and rectifier,
etc. The reason for this is so that the
input level can be adjusted for the
desired amount of limiting.
I have asked quite a lot so here’s
hoping you can help. (R. H., Mullum
bimby, NSW).
• The attack and decay resistors R1 &
R2 could be changed for potentiometers wired as variable resistors to allow
adjustment of these parameters. The
threshold control VR6 is actually there
to operate the downward expander
and is not suitable as it stands for use
as a threshold control whereby there
is no compression until the threshold
level is reached.
However, you could forego the
downward expansion facility and
change the 2.2kΩ resistor at the output
of diode D5 to a value that does not set
downward expansion; eg, 22kΩ. Then
Notes & Errata
PC-Controlled VHF FM Receiver,
June 2000: the LM385Z isometric
drawing on the circuit diagram
shows the “ADJ” and “-” pins reversed.
40V/1A Adjustable
Power Supply, June/
July 2000: the LM336Z
isometric drawing on
the circuit diagram in
the June issue shows the “ADJ” and
“-” pins reversed.
The correct pinout drawing for
both the LM336Z and LM385Z is
shown above.
Loudspeaker Protector & Fan Controller, August 2000: the base resistor for Q8 on the wiring diagram
(page 59) is incorrectly shown as
2kΩ. It should be 1.5kΩ, as shown
VR6 control can be used to adjust for
compression threshold.
A compression meter could be added to the pin 11 control for IC1 and
IC2, as you suggest.
Vehicle loop
detector wanted
I am looking for a loop detector circuit, similar to those found at traffic
intersections, to use in the control of
an electric gate. I have built up a simple (inductive) metal detector circuit
from a recent issue of SILICON CHIP
but I’m guessing it will need far more
gain and probably a very large pickup
loop for it to detect the presence of a
vehicle reliably.
I may yet have to build up a large
diameter loop and try it but thought
I would ask you the question first.
Do you know of, or have come across
some sort of metal detection circuit
on the circuit diagram (page 55).
Also, the connections to LED1 and
TH2 were incorrect. The connections should be as shown in the
diagram above.
that could be used in this application?
I realise I could take the easy way
out and simply use something optical
but this then may also be triggered by
objects other than cars. I have found
lots of interesting websites with commercially available detectors advertised but, of course, they want an arm
and a leg for them and they provide
no circuit details. Any thoughts or
suggestions? (P. W., via email).
• We have not published a large loop
detector circuit. The recent metal
locator may well form the basis of
a viable circuit and while you will
have to increase the area of the loop
to, say, 0.5m2, it may not be necessary
to increase the sensitivity; after all,
you are going to detect a rather large
lump of metal. In fact, we have seen a
traffic light loop being tested and they
used a 60cm length of steel which they
SC
dragged over the road surface.
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SEPTEMBER 2000 91
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