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Advertising Index ADM Instrument Engineering....... 61 Altium Ltd....................................... 7 Altronics...................loose insert, 77 Apex Tool Group....................... OBC Cleverscope................................... 6 Core Electronics............................. 8 Emona Instruments...................... 82 Hare & Forbes............................. 2-3 Ask SILICON CHIP . . . continued from page 103 LDR resistance is brought from an initial high value (when in darkness) to a low value (when exposed to daylight). So that effect can be simulated using a reed relay that closes contacts across pins 5 & 2 of IC1 when the bird sound is required to start. The LDR should be removed from circuit. Alternatively, applying power will start bird sounds after a short 5-second delay provided the LDR is exposed to daylight or shorted out. How to filter a modified sinewave inverter I had an idea for smoothing the “modified” sinewave from a cheap 12V to 230VAC inverter but I’m unable test to the idea right now as I don’t have access to an oscilloscope. Could you simply add a large suitably-rated iron-core choke in series with the secondary 230VAC circuit (eg, use either the primary or secondary winding of an iron-core transformer)? Would this round off all the sharp DOWNLOAD OUR CATALOG at www.iinet.net.au/~worcom WORLDWIDE ELECTRONIC COMPONENTS PO Box 631, Hillarys, WA 6923 Ph: (08) 9307 7305 Fax: (08) 9307 7309 Email: worcom<at>iinet.net.au corners of the modified waveform? I could see noise emanating from the transformer as a potential issue. (J.K., Emerald, Qld). • The switching frequency of a modified sinewave inverter is only 50Hz. If you were to filter out the high frequency components for a sinewave output you would need bulky components that would reduce the overall efficiency from the inverter. You will be restricted to less of the available inverter power due to the removal of the 50Hz harmonics by the filter components. In a real filter, there will also be resistive power losses that will generate heat. Pure sinewave inverters develop the 50Hz sinewave at a much higher switching frequency than the 50Hz of a modified sinewave inverter and so the filter components are smaller and SC more efficient. Issues Getting Dog-Eared? Keep your copies safe with these handy binders Order now from www.siliconchip.com.au/ Shop/4 or call (02) 9939 3295 and quote your credit card number or mail the handy order form in this issue. *See website for overseas prices. Buy five and get them postage free! 104  Silicon Chip REAL VALUE AT $14.95 PLUS P&P Icom Australia.............................. 15 Jaycar .............................. IFC,49-56 Keith Rippon .............................. 103 KitStop.......................................... 14 LED Sales.................................. 103 Low Energy Developments........ 103 Master Instruments...................... 13 Microchip Technology................... 19 Mikroelektronika......................... IBC Ocean Controls............................ 62 Quest Electronics....................... 103 Radio, TV & Hobbies DVD.......... 101 RF Modules................................ 104 Rohde & Schwarz.......................... 5 Sesame Electronics................... 103 Silicon Chip Binders.............. 80,103 Silicon Chip Bookshop................. 90 Silicon Chip Online Shop........ 96-97 Silicon Chip Subscriptions........... 98 Tekmark Australia...................... 8,63 Trio Test & Measurement.............. 10 Virtins Technology........................ 11 Vicom Australia.............................. 9 Wiltronics...................................... 12 Worldwide Elect. Components... 104 xLogic........................................... 10 siliconchip.com.au