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Got a technical problem? Can't understand a piece of jargon or some technical principle? Drop us a line and we'll answer your question. Write to: Ask Silicon Chip, PO Box 139, Collaroy Beach, NSW 2097 or send an email to silicon@siliconchip.com.au

PC-controlled
burglar alarm

I have a problem with the PC-Controlled Burglar Alarm. When I am in the arm/disarm menu or in the comms set-up menu, the red LEDs are always off no matter what changes I make to the program. The buttons concerning the strike doors work but the red LEDs remain off whether I arm or disarm the corresponding zone.

The PC Tx/Rx LED works fine; it flashes at varying speeds. I use a laptop with a USB-to-serial interface. Can you please help. (I. P., via email).

• Check the connection between pins 5 & 6 of IC1 to pins 11 & 10 of IC4. Check that IC4 has a +5V supply at pin 16 and 0V at pin 8. Check that the LEDs are oriented correctly. Check the connection from pin 7 of IC1 to the base of Q6 via the 1.2kΩ resistor. Make sure there is a 100kΩ resistor from pin 6 of IC1 to the 5V supply.

Using a 3G modem with the WIB

I have been rather interested in the well-written and explained article on the mysteries that revolve around the Web Server In a Box project. Could this WIB be used somehow with the 3G wireless modems, like the Huawei 169 USB unit, for example?

At present, my 3G modem is through Optus which links to Virgin Blue. I hope you can help me. By the way, this is a great project that could help with remote data-logging and do it in a way that is very economical.

I have been waiting for years for someone to do this. (A. M., Warrnambool, Vic).

• You will need a modem with an ethernet rather than a USB connection to use with the WIB. To be able to use a USB modem, you would need a USB host and the WIB has no hardware support for a USB host as it stands. However, there are 3G wireless modems which have an ethernet connection and these should work with the WIB.

Troubleshooting The High-Quality Stereo DAC Project

I’ve completed the input board of the Stereo DAC (SILICON CHIP, September, October & November 2009) and it does not recognise when digital data is being sent through optical or coaxial cable.

After I plug in the optical cable and there is no data transmitted, the yellow LED is on. After I start playing anything, the green LED should turn on but it doesn’t.

I’ve checked whether I have all the parts where they should be and I do not see any problem. Can you please advise how to debug this problem? (R. D., via email).

• The first step is to measure the voltage at pin 15 of IC4 (ATMEGA­48/V) relative to ground (pin 22). If a valid signal is present, it should read close to 0V. If it is around 5V, this means that IC3 (DIR9001) is not receiving a signal. In that case, you should check IC3’s power supply (ie, the 3.3V rail) and ensure that it is operating correctly.

If you have an oscilloscope or a frequency counter, monitor pin 13 of IC2 and make sure it is delivering a 3-8MHz square-wave pulse train. If pin 15 of IC4 reads 0V but still no LEDs are lit, then the problem may be with IC4. It could be incorrectly programmed or defective. Check the voltage at pins 5 & 6 of IC4 relative to ground (pin 8). If there is a valid signal present but no audio, pin 5 should read 5V and pin 6 should be at 0V.

If there is a valid signal and audio, pin 5 should be at 0V and pin 6 at 5V. If both are at 0V and pin 15 is at 0V, then the ATMEGA48 is not programmed correctly or faulty.

If you get a reading near 5V at either pin 5 or pin 6 but the yellow and green LEDs are not lit, the ribbon cable connecting the input board to the front panel is probably faulty. Either that or the LEDs are installed backwards.

Class-A 20W
amplifier question

For my active speaker project, I need to build two 20W class-A stereo amplifiers without the preamps (SILICON CHIP, May 2007). Unfortunately, Altronics does not supply the shielded 16V+16V, 160VA toroid transformer separately from the kit (K5125). Could you please indicate if I can use an 18V+18V 160VA toroid, or should I use a 15V+15V 160VA toroid?

I will relocate the transformer to a separate enclosure to stop the electromagnetic field from affecting the performance of amplifier modules. (D. H., via email).

• Assuming you cannot purchase the specified shielded 160VA transformer, your next best approach may be to buy a 300VA 15V+15V toroidal transformer and as you suggest, house it in a separate box. Since the load on the transformer will be less than 100W for the two 20W modules, its output voltage is likely to be close to 16V+16V and therefore the amplifier operating conditions should be much the same.

The bigger transformer should also have less tendency to buzz audibly at the reduced load although it will still radiate the same magnetic field intensity.

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