Simple USB KVM

[MarcinPe]

Hi, I found simple scheme to create USB KVM (one keyboard - two PC's). Can anyone look on this and tell me, should I add diodes on 5V line to prevent burn motherboard? 

[Kilrah]

Yes you should have diodes to prevent the supply from one board from feeding into the other. 

[MarcinPe]

Should I add something else before connect it to PC?

[Kilrah]

For a keyboard it might work, they usually talk USB 1.1 speeds...

[MarcinPe]

2 hours ago, Kilrah said:

For a keyboard it might work, they usually talk USB 1.1 speeds...

Yes, it's for the keyboard. Thanks for the reply. 

[mariushm]

If you want something super simple, just use a couple mechanical relays  ... each relay has 2 on-off switches so you can switch all 4 usb 2.0 wires with two relays. 

 

Example... 2 of these : https://www.digikey.com/en/products/detail/cit-relay-and-switch/J104D2C5VDC-20S/12503179

 

See ms paint stuff i just drew ...  in default relays off, the usb 1 is connected to 5v, gnd, d+ and d-

Send 5v to the two relays, and the relays switch to usb out 2

 

A basic on-off switch would be enough ... off, the relays don't get power, defaults to usb 1 out.. on, you give power to relays, goes to usb 2 out.

 

It takes about 6ms to engage, and about 4ms to disconnect... so in theory you should be fine. If you're paranoid... you could use 4 separate relays and a 3 position slider switch so that when you're on middle position of slider both usb ports are disconnected. Also, you could add a basic resistor and capacitor combo so that voltage will raise slowly across maybe 2-3 seconds and therefore the relays will be engage 1-2 seconds after you move the slider switch to the new position.

 

 

 

Note 5v relays would consume about 100-200mA to stay powered, so if you use a usb 2.0 port you'll still have plenty of juice left for a keyboard and mouse.

The two diodes (pretty much anything that can handle 5v and 100mA, 1n4001.. 1n4007 are super common, 1n581x etc etc)  are needed for protection, when the magnetic field of the relay coils breaks (you turn off the relays), don't want voltages to go back in

I'd probably also add a small capacitor between usb 5v in and gnd, something like 47-100uF 10v+

 

[MarcinPe]

1 hour ago, mariushm said:

If you want something super simple, just use a couple mechanical relays  ... each relay has 2 on-off switches so you can switch all 4 usb 2.0 wires with two relays. 

 

Example... 2 of these : https://www.digikey.com/en/products/detail/cit-relay-and-switch/J104D2C5VDC-20S/12503179

 

See ms paint stuff i just drew ...  in default relays off, the usb 1 is connected to 5v, gnd, d+ and d-

Send 5v to the two relays, and the relays switch to usb out 2

 

A basic on-off switch would be enough ... off, the relays don't get power, defaults to usb 1 out.. on, you give power to relays, goes to usb 2 out.

 

It takes about 6ms to engage, and about 4ms to disconnect... so in theory you should be fine. If you're paranoid... you could use 4 separate relays and a 3 position slider switch so that when you're on middle position of slider both usb ports are disconnected. Also, you could add a basic resistor and capacitor combo so that voltage will raise slowly across maybe 2-3 seconds and therefore the relays will be engage 1-2 seconds after you move the slider switch to the new position.

 

 

 

Note 5v relays would consume about 100-200mA to stay powered, so if you use a usb 2.0 port you'll still have plenty of juice left for a keyboard and mouse.

The two diodes (pretty much anything that can handle 5v and 100mA, 1n4001.. 1n4007 are super common, 1n581x etc etc)  are needed for protection, when the magnetic field of the relay coils breaks (you turn off the relays), don't want voltages to go back in

I'd probably also add a small capacitor between usb 5v in and gnd, something like 47-100uF 10v+

 

O my... That's too much than I expected

[cachethrash]

You can buy a switch that does the same thing, I think.

https://www.amazon.com/UGREEN-Sharing-Peripheral-Switcher-Computer/dp/B01CU4QCRS/ref=sr_1_4?dchild=1&keywords=Usb+Printer+Switch&qid=1618678717&sr=8-4

They are usually marketed as a "printer switch" but I think it's just a regular usb port.

Or, there used to be a software solution to the problem, Synchron or something like that, they used to advertise online a lot but I haven't seen anything about them in a long time.

[MarcinPe]

1 hour ago, cachethrash said:

You can buy a switch that does the same thing, I think.

https://www.amazon.com/UGREEN-Sharing-Peripheral-Switcher-Computer/dp/B01CU4QCRS/ref=sr_1_4?dchild=1&keywords=Usb+Printer+Switch&qid=1618678717&sr=8-4

They are usually marketed as a "printer switch" but I think it's just a regular usb port.

Or, there used to be a software solution to the problem, Synchron or something like that, they used to advertise online a lot but I haven't seen anything about them in a long time.

I know about ready-to-use UGREEN switcher but I want to built my own. I like to built something from time to time.