hi, i was wondering if any know how to build a circuit that would turn on something whenever there is a 5V or less DC voltage present, for e.g, those self power external hard drive that comes on whenever u turn on your computer

 

Do you mean up to 5v?

Are you talking about external drives that are plugged into the USB port? They're powered by the USB port's output; up to 500mA @ 5v for regular computers, lower current for laptops.

 

no, i want the ouput of the circuit to be 12volts, but i want the circuit to turn on whenever there is a 5v present.

on my external hard drive, the hard drive is connect to a adapter that provides it with 12V, there is also a usb cable going to my computer which transfer the data and also somehow tells the hard drive when to turn on and off. when i turn on my computer it turns on itself. the usb is not powering the hard drive it's the 12V adapter, cuz when i disconnect the 12v adapter the hard drives turn off.

am thinking of using a phototransistor, from the USb cable i will get a 5v from there to light a led, and whenever the phototransistor sees that it turns on the circuit, but am not sure how to wire it to give and ouput of 12V

a simpler solution would be to use a 5v to 12v Dc to Dc converter, but it's bothering me that i can't figure it out the other way i described. It's like i wouldn't rest until i firgure it out..lol

 

Why would you want to cut the 12v to the hdd? Don't you said that the computer turns on and off the drive?

 

You could use a relay with a 5v coil energized by the USB supply to switch mains power to your external supply, as long as the coil required less than 500mA. You might also use it to switch a TRIAC.

A DC-DC converter would work if your circuit didn't draw much power. Going from 5v to 12v means you'd reduce the available current from the USB from 500mA to 500mA x 5/12 x the efficiency of the converter itself. You might get one around 92% efficiency. That would give you around 191mA @12V.