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What if I hooked up the panel LED to Power positive and HDD positive?

I have a case with only one LED, and I want it to be normally on when the system is powered on but flashes off when there is disk activity.

My thinking is 5V is always present as long as the system is powered on, and 5V is only present when a disk is being accessed. Voltage is relative, so by connecting the LED anode to Power positive and the cathode to Activity positive, there will be a 5-volt difference between Power and Activity when there is power but no activity, and in theory, the LED will light up, and when there is activity, there will be 0 volts, and the LED will not light up.


However, I do not want to test this theory on my computer out of fear of frying my motherboard by having current flow into somewhere the motherboard doesn’t expect, but I would like to entertain the possibility that this might work, as I’ve witnessed fans running off of 7V by having its leads run between the 12V and 5V rails, but of course, that is done on the power supply, not on the motherboard. Has something like this ever been done in practice with no ill effect?

Edited by nelsonpong
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