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PIC NiMH charger wanted You recently published a NiCad battery charger for power tools (April 2001). How about a similar project for NiMH batter­ ies? The only difference I’d need is 12VDC input so I could use it in the car. I’d be using it to charge four AAs for my digicam while on holiday. (A. G., via email). •  There should be no reason to change the design at all apart from a change to the 3.3kΩ sensing resistor the latest NTSC releases on DVD have an ‘improved’ version of Macrovision which isn’t easy to remove. So the results on these DVDs may be disap­ pointing. Where to get BASIC •  We occasionally get queries about where to get BASIC for use with various past projects. As some readers may be aware, GWBA­ SIC/QBASIC interpreters were supplied free until DOS version 6. The original IBM PC even had a ROM-based interpreter (BASICA). We’ve had a good look at what’s available and can thorough­ ly recommend FirstBASIC, available as shareware ($US25 to reg­ister) from http://www.powerbasic.com You’ll find it listed on their down­ loads page. FM SCA decoder/multiplexer I was wondering if you have ever published a design for a circuit to to cope with the lower voltage from your battery pack. However, we are not certain whether the PIC’s code has the resolution to cope with the re­ quired dV/dt end-of-charge detection. NiMH cells have a very similar charge profile to NiCads but they peak at around 1.8V. You need to be aware also of the C value and note that NiMH cells like to be charged slightly below 1C (about 90%). If the batteries are around 1450mAH then 1Ω should be OK for the current limiting resistor. decode the SCA (subcarrier auxiliary) channels that are transmitted by some of the FM broadcasters. I am not really interested in using it for this purpose, but I want to build a multiplex system designed around the same principle. I have several wideband (100kHz) radio links that I want to feed to up to six narrow-band (0 to 3.5kHz, phone quality) audio channels. I thought that by modulating subcarriers, as they do with SCA, I could achieve the results that I want. I planned to use subcarriers at, 25kHz, 35kHz, 45kHz, 55kHz and so on. (P. D., via email). •  We published an ACS Adaptor (same thing as SCA) in the January 1988 issue. We can supply a photostat copy of the article for $7.70 including postage. Charging circuit for a strobe lamp I am trying to solve a puzzle, on how to charge a 680µF capacitor quickly, up to 320-340V DC, suitable for a highspeed disco strobe. The existing circuit is an AC-to-DC convert­er, which then steps it up to 500V DC using a Mosfet WARNING! and an inductor. It’s way too slow and inefficient. I know that by using a high-voltage diode and some sort of current limiting device, you can charge a capacitor to 320V from the 240VAC mains supply. I have experimented with various devices to achieve this, from a fluorescent light ballast to a light bulb to resistors and even winding my own inductor. I need this circuit to repeatedly charge this capacitor quickly, providing the 320V DC needed to fire the large Xenon bulb of the strobe. The trigger circuit etc is the easy part which I can do myself. (J. W., via email). •  How fast do you want the strobe to be? We published a beat-triggered strobe in the August 1998 issue which would run at up to 20 flashes per second. Anything faster tends to be perceived as continuous light so there is not much point. Our design used two 470Ω 5W resistors in series with a bridge rectifier running directly from 240VAC. It is quite fast enough and if you use higher charge currents and faster flash rates the limiting factor becomes the ripple current ratings on the discharge capacitor. We can supply the August 1998 issue for $7.70 including postage. Notes and Errata Parallel Port PIC Programmer and Checkerboard, March 2001: the circuit diagram on page 64 indicates that IC1 is an inverter. In fact, IC1 is a 7407 hex buffer with open-collector outputs. These buffers do not invert from input to output. Some kits for this project have been supplied with a female D socket and “gender changer”. This will not work. The specified male D socket must be SC used. 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. 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