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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. 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. SEPTEMBER 2000  91