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SILICON CHIP SOFTWARE Now available: the complete index to all SILICON CHIP articles since the first issue in November 1987. The Floppy Index comes with a handy file viewer that lets you look at the index line by line or page by page for quick browsing, or you can use the search function. All commands are listed on the screen, so you’ll always know what to do next. Notes & Errata also now available: this file lets you quickly check out the Notes & Errata (if any) for all articles published in SILICON CHIP. Not an index but a complete copy of all Notes & Errata text (diagrams not included). The file viewer is included in the price, so that you can quickly locate the item of interest. The Floppy Index and Notes & Errata files are supplied in ASCII format on a 3.5-inch or 5.25-inch floppy disc to suit PC-compatible computers. Note: the File Viewer requires MSDOS 3.3 or above. ORDER FORM PRICE ❏ Floppy Index (incl. file viewer): $A7 Smoke detector beeps ❏ Notes & Errata (incl. file viewer): $A7 ❏ Alphanumeric LCD Demo Board Software (May 1993): $A7 I have a small problem that you may be able to help me with. I have a smoke detector which is working fine but it gives a low beep at regular intervals at night time. I don’t hear it during the day. I have tested the unit by pushing the button and the alarm works in its usual loud and raucous fashion. What’s wrong with it? (D. S., Berala, NSW). • This is a common problem with smoke detectors but it is entirely normal. As we pointed out in our February 1996 article entitled “Fit A Kill Switch To Your Smoke Detector”, these units produce an audible beep to tell you when the battery is low. So the answer is simple: replace the battery. Use an alkaline type because these have a longer life than normal carbon-zinc batter­ies. ❏ Stepper Motor Controller Software (January 1994): $A7 ❏ Gamesbvm.bas /obj /exe (Nicad Battery Monitor, June 1994): $A7 ❏ Diskinfo.exe (Identifies IDE Hard Disc Parameters, August 1995): $A7 ❏ Computer Controlled Power Supply Software (Jan/Feb. 1997): $A7 ❏ Spacewri.exe & Spacewri.bas (for Spacewriter, May 1997): $A7 ❏ I/O Card (July 1997) + Stepper Motor Software (1997 series): $A7 Notes & Errata Radio Control 8-Channel Encoder; March 1996: in the circuit on pages 56 & 57, R19, the 10kΩ resistor at pin 6 of IC3b, should connect to pin 5 instead. It comprises a voltage divider with R13, a 22kΩ resistor. POSTAGE & PACKING: Aust. & NZ add $A3 per order; elsewhere $A5 Disc size required:    ❏ 3.5-inch disc   ❏ 5.25-inch disc TOTAL $A Enclosed is my cheque/money order for $­A__________ or please debit my ❏ Bankcard   ❏ Visa Card   ❏ MasterCard Card No. Signature­­­­­­­­­­­­_______________________________ Card expiry date______/______ Name ___________________________________________________________ PLEASE PRINT Street ___________________________________________________________ Suburb/town ________________________________ Postcode______________ Send your order to: SILICON CHIP, PO Box 139, Collaroy, NSW 2097; or fax your order to (02) 9979 6503; or ring (02) 9979 5644 and quote your credit card number (Bankcard, Visa Card or MasterCard). ✂ along these lines was published in another Australian electronics magazine many years ago. In principle, the circuit needs to measure the frequency of the local oscillator in the radio and then offset the reading by the intermediate frequency, to get the actual incoming frequency. In an AM broadcast radio with digital readout, the local oscilla­tor usually covers the range from about 1MHz to just over 2MHz and the intermediate frequency is 450kHz (not 455kHz as in older AM radios). By contrast, the local oscillator in an FM radio covers from 93.5MHz to 113.5MHz while the intermediate frequency is 5.5MHz. Since the FM local oscillator is such a high frequency, a prescaler IC is generally used to divide the frequencies down to a more manageable frequency in the region of about 1MHz. In practice, digital radios solve the problems of pushbutton tuning, frequency readout and so on by using a dedi­cated microprocessor. It is unlikely that we will publish a project along these lines in SILICON CHIP. April 1996  93