Fullscreen HTML5Med Res (??MB)HTML4

10A Speed Controller I constructed the 240VAC 10A Speed Controller from the November 1997 issue and it works well. I initially tested it by dimming a light and later with an angle grinder. Everything is fine apart from the fact that there is around 39- 40V AC bet­ween the speed controller’s diecast case and the alloy blade cover of the angle grinder. I got the same result with a router. Is this normal? The angle grinder is double-insulated and does not have an earth connection. The case for the speed controller is fully earthed correctly, as described in the instructions. I haven’t been game enough to touch it – I am not sure whether or make sure the gap between the LEDs and infrared diodes is at a minimum. Kill switch for rev limiter I have built a rev limiter from the April 1999 issue but I also need to have a switch which will kill the engine completely. The car is used for motor sport where a kill switch is required by the regulations. It seems that this can be incorporated in the ignition switcher circuit in the following two ways. (1) Installing a second capacitor at C1 which can be switched in to kill a very high number of sparks, causing the engine to stall; or (2) Install a switch between terminals 8 and 3 of IC1. When the switch is closed, transistor Q3 will be turned on to kill all sparks. Solution 2 would be not the multi­met­er is misreading something? Secondly, are the motors in vertical press (Ryobi) drills brush type? (A. P., via email). •  The reading you are measuring will be due to the fact that the power tools are not earthed and that there is some ca­pacitance between the metal parts and the internal wiring. The meter will read a voltage due to its high input impedance. Try connecting a 10kΩ resistor between the multimeter terminals and do the measurement again. You should get a very low reading. Your speed controller is probably working completely normally. Drill presses usually have in­ duction motors, which are not suit­ able for use with this speed controller. easier to install. What do you recom­ mend? (D. M., via email). •  Solution 2 would be best as it completely shuts down the igni­tion. Low-cost oscilloscope probe I am interested in building a sound card adapter kit from Electronics Australia, on sale at one of the kitset suppliers for $30. However, I reckon I need oscilloscope probes so that I can use the adapter, right? When I checked prices of probes at Jaycar or Dick Smith Electronics, they were around $44 each, much more than the price of the kit. That is too much for a student budget. Can we make one probe on our own? (D. B., via email). •  We featured a low-cost, low-capacitance scope probe in the August 1989 edition of SILICON CHIP. It utilises a short length of coax cable, a resistor, trimmer capacitor and a few other bits & pieces you’d probably find in your junk box. Door alarm uses electret microphone I have searched all the indexes, including those on your website (excellent site, by the way) and I cannot find what I seek. It is an alarm using a microphone as a sensor. It does not operate on sound but on changing air pressure as a door or window is opened. I am certain it was in SILICON CHIP and I have every copy from issue number one but I just can’t find it. Can you help? (R. C., via email). • The article appeared in the July 1995 issue. It used an electret microphone as a pressure sensor. We can supply the issue for $8.80 including postage. Notes & Errata AVR ISP Serial Programmer, October 2002: there is an error on the circuit on page 75. Pins 1 & 4 have been swapped on CON3. The PC board is correct. 12V SLA Battery Float Charger, March 2003: when this charger is used with the PortaPAL (February and March 2003), the 10µF ca­pacitor connecting to the adjust terminal of REG1 should be omitted. Simple VHF FM/AM Receiver, December 2002: a short track is miss­ing from the PC board, as shown on pages 88 & 90. The track should connect the junction of the two 3.3kΩ resistors and L1 with the adjacent end of the 22kΩ resistor. The corrected PC board pattern can be downloaded from our SC website. 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. 92  Silicon Chip www.siliconchip.com.au