Powering the UNO or MEGA. on/off, fron Vin while still having the USB connected

Hello,

I would like to know if anyone out there knows how to fake the USB circuit on the UNO to work with
out the USBVCC connected, the power from the USB cable. I would like to be able to power my circuit on and off from Vin. And still have the ability to use the USB when the Vin power is turned on. Now as it is, when USB cable is plug into the UNO the USBVCC powers on the UNO and I can’t turn it off without disconnecting the USB cable. I need to leave the cable connected but the UNO powered off. Any ideas?

StanK

Easiest way is to remove the PTC polyfuse, that will disconnect the USB power coming in. It's the somewhat large flat device near the USB connector. Or make a cable where you cut the 5V wire.

Note that many ICs specify to not have signals applied that are more than 0.5V above Vcc. You may find that circuits are partially powered thru the protection diodes on the inputs.

Grab a USB cable and cut the 5V wire in it.

Ok, thanks,

Well I did remove the polyfuse and added a jumper from the down stream side of it, the polyfuse, to +5 and it seems to work ok. I thought it might be that simple. I also noticed some jumper pads on the UNO reset line that goes to the the USB chip, thought that might have something to do with it. Does anyone know what they are use for?

StanK

The USB-Serial chip toggles its DTR pin just before uploading. This then causes the microcontroller to reset allowing for programming. The "Auto reset" feature.

There is a small pcb trace between the two jumper pads which can be cut if you wish to disable the auto reset. If you wish to then re-enable it, you can use the solder jumper to do so.