All that is required, besides a second drive, is a specially-designed hard drive switch. This can be achieved, as has already been described in this blog Electronics, by switching the two drives between master and slave modes using the IDE cable, and only activating the master drive in the BIOS. However, the IT skills of children should not be underestimated: the BIOS is easily changed back. The solution described here is a bit more secure. Both drives are bootable and configured as masters. One is connected to IDE bus 1, and the other to IDE bus 2. The power supply voltages (+12 V and +5 V) are, however, only applied to one drive at a time. In principle, a simple double-pole changeover switch would do the job, but that has the disadvantage that it is possible to forget to reset the switch to child mode after use, especially if the switch is hidden.
A better solution is to have to press a (hidden) button during boot to put the machine into parent mode. We will now see how this is done. If the button is not pressed when the PC is switched on, then, after a short delay of about 0.7 s (determined by R1, C1, D2 and the base-emitter junction threshold voltage of the Darlington pair formed by T1 and T2) RE2 pulls in. RE1 remains un-energized and hence the children’s drive connected to K2 is active. Subsequently pressing S1 has no effect since RE1 has been isolated by the contacts of RE2. If the secret button is pressed, either briefly or continuously, during the 0.7 s sensitive period after the computer is switched on, RE1 pulls in immediately and holds itself in this state.
D3 now prevents RE2 being subsequently activated. Since the contacts of RE1 have changed over, the PC now boots from the parents’ drive. It is impossible to forget to return the computer to child mode, since the computer will always start up in this mode if the secret button is not pressed. A 12 V miniature relay with contacts rated for 100mA is suitable for RE2. The contacts of RE1 should be rated for the currents taken by a typical hard disk drive (say 2 A to 3 A). A key switch can be used instead of a secret button as a last resort against resourceful children, since the circuit will continue to operate correctly if S1 is left in parent mode permanently while the computer is on.