Hard Drive Latch

Thread Starter


Joined Oct 31, 2022
Cleaning up files and came across this little gem. I built it and it worked pretty well, but there was a gremlin I never uncovered, so I abandoned it. The need was for a dead silent fanless PC where you want to use a spinning hard drive only occasionally for backup. The idea is to set a switch before powering up a PC, controlling a locked state for power to a hard drive. You don't want a switch that can operate while the PC is powered up; that is bad news all around. You also don't want a buggy latch circuit, which is what this may be. I'm posting it here for enjoyment, discussion, or improvement, as I may eventually destroy the design files. Here is the original write-up:

HDD Latch Notes
The purpose of the circuit is to create a switch which cannot change position after power is applied to the circuit.
The position of the switch must be set before power is applied. After power-up, the position of the switch is locked.
The schematic shows a SPDT "RUN/OFF" switch on the left, and the coil of a DPDT relay on the right.
If the switch is set to RUN, the relay will actuate roughly one-half second after power-up. If the switch is set to OFF, the relay will not actuate.
In either case, the switch will have no further effect after the power-up initializing period.
This circuit has been built and tested. It currently works reliably in a computer with a second hard drive. It prevents power from being applied to or removed from the hard drive while the computer is on.
The switch contacts of the relay control both +5V and +12V to the power input of the hard drive. There is an LED indicator on each of these rails to confirm power.
The heart of the circuit is a set-reset relay constructed from gates B and C of a quad nand gate.
The timing diagram shows waveforms at points labeled in the schematic for the two conditions RUN and OFF.
The assembly diagram shows the top side with components in the top half of the picture and the bottom side with wiring in the bottom half of the picture.
This design is completely original by yours truly and it is free use (at your own risk of course!)
Comments and criticism welcome.



Joined Jan 23, 2018
I have thought for some time that some scheme to inactivate the write enable of a hard drive could be very handy: enable the hard drive, switch on, do a backup, switch off, disable write, switch computer back on. In the event of malware attack, including an encryption, restore from the drive that could not be written to. Although microsopht probably has an intentional back door for malware.
My wondering is if there is a flaw in my thinking about a hardware control line lockout.