Computer Off Switch

How often does it happen
that you close down Windows and then forget to turn off the computer?
This circuit does that automatically. After Windows is shut down there
is a ‘click’ a second later and the PC is disconnected from the mains.
Surprisingly enough, this switch fits in some older computer cases. If
the circuit doesn’t fit then it will have to be housed in a separate
enclosure. That is why a supply voltage of 5 V was selected. This
voltage can be obtained from a USB port when the circuit has to be on the outside of the PC case.

It is best to solder the mains wires straight onto the switch and to
insulate them with heat shrink. C8 is charged via D1. This is how the
power supply voltage for IC1 is obtained. A square wave oscillator is
built around IC1a, R1 and C9, which drives inverters IC1c to f. The
frequency is about 50 kHz. The four inverters in parallel power the
voltage multiplier, which has a multiplication of 3, and is built from
C1 to C3 and D2 to D5. This is used to charge C5 to C7 to a voltage of
about 9 V.

The generated voltage is clearly lower than the theoretical
3×4.8=14.4 V, because some voltage is lost across the PN-junctions of
the diodes. C5 to C7 form the buffer that powers the coil of the switch
when switching off. The capacitors charge up in about two seconds after
switching on. The circuit is now ready for use. When Windows is closed
down, the 5-V power supply voltage disappears. C4 is discharged via R2
and this results in a ‘0’ at the input of inverter IC1b. The output then
becomes a ‘1’, which causes T1 to turn on.

Computer Off Switch Circuit

Computer Off Switch Circuit Diagram

A voltage is now applied to the coil in the mains switch and the
power supply of the PC is turned off. T1 is a type BSS295 because the
resistance of the coil is only 24R. When the PC is switched on, the
circuit draws a peak current of about 200 mA, after which the current
consumption drops to about 300 µA. The current when switching on could
be higher because this is strongly dependent on the characteristics of
the 5-V power supply and the supply rails in the PC. There isn’t much to
say about the construction of the circuit itself.

The only things to take care with are the mains wires to the switch.
The mains voltage may not appear at the connections to the coil. That
is why there has to be a distance of at least 6 mm between the
conductors that are connected to the mains and the conductors that are
connected to the low-voltage part of the circuit.

Author: Uwe Kardel – Copyright: Elektor Electronics Magazine

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