Relay switch to detect power source

I need to find a switch that can detect whether it should go to 9v source or to a 5v Source. I am making a tool that has a battery power of 9V ( from a Alkaline Battery) and it can also get powered from a USB device, which is 5V.

The Whole system runs at a 3.6V. So i had to use a fly back to step down 9v to 3.6V and a variable regulator to step down 5v to 3.6v. 2 separate circuits.

If the customer need to connect the tool to a computer while its running from the battery, will it harm the system? i was asked to use a relay switch.

the issue is, i have to use a USB to connect the device to the computer and the USB gives 5V. At the same time the system is running at 9V with a alkaline battery.

Please Help

Thank you

vikum123 said:

If the customer need to connect the tool to a computer while its running from the battery, will it harm the system?

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it depends on the current that it drains . typical usb can deliver 1A to 1.5 A. add some protection circuits like fly-back diodes etc to make to safe.

vikum123 said:

the issue is, i have to use a USB to connect the device to the computer and the USB gives 5V. At the same time the system is running at 9V with a alkaline battery.
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so you want to switch over to usb mode? try this , use and opamp circuit for voltage identification ans switch a relay accordingly.