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Circuit Notebook

Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates.

mains power switch

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Many new computers now come with rapid data transfer 6-pin Fire­Wire ports (IEEE-1394). Alternatively, you can add them by purchasing a PCI card for about $30. They are commonly used for video transfers and sending data to external hard drives.

Both these and USB ports provide power to run the device attached to the port. However, when the computer is powered down the voltage at the FireWire port goes to zero whereas the voltage at the USB typically remains at +5V. In theory, if you want to switch off peripherals, all you have to do is wire a relay across the FireWire power and use it to control the mains power for the external devices.

The only drawback with this approach is that the output voltage from the socket isn’t defined and can vary between 12V and 25V, depending on the brand of computer. This problem can be solved with an external 12V power supply to power the relay switched by a transistor that monitors the FireWire socket voltage.

When the computer is on, the voltage at pin 1 of the FireWire port switches on transistor Q1. This energises the relay and provides power to the peripherals. Conversely, when the computer powers down, Q1 switches off, de-energising the relay which removes the power to the peripherals.

The circuit shows a conventional DC power supply using an 18VAC centre-tapped transformer, two 1N4004 diodes and a 2200µF filter capacitor. However, you could substitute a 9V or 12V DC plugpack.

Note that you will need to disable “resume by USB” and “Wake-up by PME# of PCI” in the power management set-up of the computer’s BIOS. If you don’t, the computer will start up again when the power to the peripherals is switched off.

Les Kerr,
Ashby, NSW. ($30)

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