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from typical NTSC or PAL cameras. We published a project featuring a miniature camera and old monitor in the June 1995 issue and revisited the subject in the October 2000 issue. 500W amplifier for subwoofer use I’m considering building the 500W amplifier, described in the August, September & October 1997 issues of SILICON CHIP, for use in a subwoofer application that needs to be flat down to at least 10Hz. The specs printed in the relevant issues give a frequency response between 20Hz and 20kHz but no farther. Can you tell me the -3dB point at the bottom end, or better still, provide a frequency plot? (N. N., via email). •  The frequency response is quoted as 20Hz to 20kHz at the -0.3dB points so you can expect that the response at 10Hz is less than -1dB down. By the way, 10Hz is not audible (it can be felt at high levels) and there is no sub­woofer that goes to 10Hz – most don’t even go below 25Hz. Coil-based electric fence wanted I seem to recall you did a project for a home-made electric fence based on an automotive coil. I am trying to find this kit and have checked all my local electronics shops to no avail, so was wondering if you could give me details of this kit please. (M. P., via email). •  We published the coil-based electric fence kit in July 1995. Kits are no longer available but an almost identical kit, using the same PC board, is available for the Jacob’s Ladder, published in September 1995, from Jaycar. Cat KC-5191. We can supply the back issues for $7.70 including postage. Capacitance in house wiring Perhaps you can shed some light on house wiring voltages and leakage with regard to neon indicator lights in switch plates and power points. I have a double power point with inbuilt neons, and with nothing plugged into it and the switches off, the neons glow very faintly (only visible in the dark). I assume it cannot be leakage due to faulty wiring, as the neons are integral to the power point. Secondly, I have two neons connected to switches in one room that turn off lights in another room. If both switches are off, the neon glows very faintly but if the one nearest the light (a fluoro) is on and the other one near the neon is off, the neon does not glow at all. A similar problem occurred with the other neon when the other light was installed. The switch and neon went in first, then the wiring and light a week or so later. Once again, this neon glowed very faintly in the dark with the switch Notes and Errata Li’l PowerHouse, June/July 2000: some constructors have not been able to calibrate the current reading successfully, finding that the current reading is too high and cannot be adjusted low enough with VR2. If you encounter this problem, try reducing the value of the 15kΩ resistor connected to pin 2 of IC2. We suggest a value of 7.5kΩ. The final paragraph on page 60 of the June 2000 issue states that “IC2 is specified as an OP77GP which has off. Once the light (also a fluoro) was wired up, this neon does not glow faintly when the switch is off. The wiring to both is correct, with neutrals and earths going back to the switchboard. I assume it is the same problem as the power point neons. I also saw this problem at my father’s house. He had an external light with a neon at the switch. No problems, until he had an extension added and an electrician removed the light but temporarily left the wiring with connectors on it. Then this neon also glowed faintly in the dark with the switch off. This house had been fully rewired a few years before. In all of the above, with the switches on the neon glows fully as you would expect. Any idea as to why? (J. B., via email). •  Without devoting the time to fully analyse your letter, the explanation for your observations is almost certainly due to the capacitance of the wiring. If you have a capacitance meter, try measuring the capacitance of a 10m length of mains cable. You will be surprised. the required low input offset voltage (typically 50mV).”. This should read “(typically 50µV)”. The last paragraph of the first column on page 61 of the June 2000 issue states “In operation, VR2 sets the voltage on pin 4 of IC2 at between 0V and 1.25V”. This should read “pin 6 of IC2”. The final paragraph on page 64 of the July 2000 issue states “Note that the wire connecting the pole of S4b to the PC board (and to the 100kΩ resistor at pin 2 of IC4) . . .”. This should read SC “pin 3 of IC4”. 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. www.siliconchip.com.au August 2001  101