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Circuit Notebook – continued from page 74 the program to define each button. As long as your measured values for the A0 voltage fall comfortably between the upper and lower limits for each button, the buttons will be successfully decoded. However, if your measurement falls uncomfortably close to one limit, you will have to alter the upper and lower limits in the program to suit. Ideally, choose limits so that they are midway between your measured values and there must be no gaps or overlaps in the spread of the limits. So, for example, the ideal value for the upper limit for the memory button would be (2.53 + 3.21) ÷ 2 = 2.87 and for the lower limit (2.53 + 1.85) ÷ 2 = 2.19. In the software, I have used average limits that should detect which button was pressed regardless of whether a 5.6kΩ or 3.9kΩ resistor is used. If you need to alter the limits they are all found in the “instruction” subroutine but there is also one line in the main program loop: “if v <= 2.84 . . .” which may need changing. When first turned on the GPS unit will initially obtain a lock on the available satellites and then the program will display the current Australian Eastern Standard Time on both displays. If another time zone is required, the program line: “Timezone = 10.0” needs to be modified appropriately. To set the alarm times, press any of the five alarm setting keys. The alarm ON HOUR time for SUNDAY is first displayed on the LCD. Use the UP and DOWN buttons to change to the desired time. Pressing the ITEM button will change the setting shown to change successively to ON MIN- UTE, OFF HOUR, OFF MINUTE and finally cycle back to ON HOUR. All these values can be changed with the UP/DOWN buttons. When satisfied that Sunday’s settings are correct, press the MEMORY button and these values will be saved to memory and the day will advance to MONDAY. Proceed to set individual alarms for each day of the week and when each day has been saved to memory press EXIT to revert to normal time display mode. This will also save the alarm data to non-volatile memory so that after a power failure the alarm data is not lost and does not have to be re-entered. One practical point to note is that you must be firm when pressing the LCD’s microswitches. Because they form part of the voltage divider chain for A0, if they are tentatively pressed they may introduce some contact resistance of their own which will cause a false voltage to occur at A0 – so be firm! If switch S1 is set to ON, then between the alarm ON and OFF times set for each day, pin 14 on the Micromite will turn on transistor Q5, which in turn activates relay RLY1 and supplies mains power the GPO. The ‘ALARM ACTIVE’ LED also lights as a visible indication that the GPO is powered. Power comes from a transformer with a 9V secondary winding. A bridge rectifier provides 10-12V DC for the relay. Voltage regulators REG1 (7805, heatsink required) and REG2 (LM1117T) follow in series to supply the 5V and 3.3V rails. Jack Holliday, Nathan, Qld. ($85) Advertising Index 4D Systems Pty Ltd...................... 23 Altronics.........................loose insert Blamey & Saunders Hearing.......... 9 Emona Instruments........................ 6 Hare & Forbes.............................. 41 Harbuch Electronics..................... 12 High Profile Communications..... 103 Icom Australia................................ 5 Jaycar .............................. IFC,49-56 Keith Rippon .............................. 103 Keysight Technologies.............. OBC KitStop............................................ 8 LD Electronics............................ 103 LEDsales.................................... 103 Microchip Technology..................... 7 Mikroelektronika......................... IBC Ocean Controls............................ 11 Quest Electronics....................... 103 Radio, TV & Hobbies DVD.............. 8 Rockby Electronics....................... 81 Rohde & Schwarz.......................... 3 Sesame Electronics................... 103 Silicon Chip Binders......... 71,91,103 Silicon Chip Online Shop............. 87 Silicon Chip PCBs........................ 39 Silicon Chip Subscriptions........... 57 Silvertone Electronics.................. 10 Wiltronics...................................... 13 Worldwide Elect. Components... 103 that will activate a LED when voltage is applied to the speaker of a radio set. This device will allow the radio operator to identify the set that was activated, while the operator may have been distracted or temporarily out of the room. After trawling through your archives, the nearest project that may do the job, with some major modifications, is the solar powered alarm that was printed in the March 2010 edition. I look forward to your valued opinion 104  Silicon Chip as to a solution. (K. B., Hawkesbury, NSW). •  A voice activated switch (VOX) is what you need. This monitors signal across the loudspeaker and switches a relay (or LED in your case). A delay is incorporated. We published a VOX in September 1994 (Jaycar kit KC5172), in July 2011 and in “Shorts Circuits 3” (Jaycar KJ8084). For the September 1994 and “Short Circuits 3” versions, you would need a resistive attenuator to reduce the signal level to the VOX from the loudspeaker if you have a direct loudspeaker connection rather than the electret microphone. The electret bias resistor would be removed. Photostat copies or back issues can be purchased from our website. A 1-page preview of the July 2011 article is at www.siliconchip.com. au/Issue/2011/July/Build+A+VoiceSC Activated+Relay+%28VOX%29 siliconchip.com.au