In these times with
viruses and other threats from the Internet it would be nice to have
reassurance that the PC cannot be infected. That is why this circuit was
designed. It makes it possible to install multiple hard disks inside
the case of a PC, which are separated in such a way that viruses cannot
move from one disk to another. In this case there are three drives
installed, one for use of the Internet via ADSL, one for working with email and one for other applications.
If data from the Internet never arrives on the third disk, it is
effectively protected against viruses. The solution outlined here has
been in satisfactory use for a couple of years. There is an additional
benefit: if there are ever any problems with the operation of the
computer, then it is very easy to change to another hard disk to check
if the problem manifests itself there as well. In this case, fault
finding can be made much easier. The circuit operates by only switching
over the power supply voltages (5 V and 12 V) of the hard disks. The
hard disk is out of service without a power supply. This works without a
problem with S-ATA disks.
With IDE disks this only works with modern drives. There may only be a combination of hard disks on the relevant port and no CD-ROM, DVD-drive,
CD-burner or something similar. The selection of the desired hard disk
is done with a rotary switch. This has to be set to the correct position
before the computer is switched on. When the power supply is turned on,
one of three relays is driven via diode D1, D2 or D3. The relays are
provided with a hold circuit via a second diode (D4, D5 and D6). In this
way the selected relay remains energised as long as the power supply
voltage is present.
After switching on, electrolytic capacitor C1 is charged via R1, so
that the common contact of the rotary switch is quickly at 0 V. This
prevents an accidental change of hard disk while the computer is in
operation. The ADSL modem is powered from the
PC. This power supply voltage is only present if hard disk number 2 is
selected. This prevents the use of the Internet if one of the other
disks is selected.
Author: Uwe Kardel – Copyright: Elektor Electronics Magazine