Technical Tips


Project to create a docking station for HP Stream tablet with Windows 8.1 . This enables the tablet to be charged while using the USB to interface a mouse, keyboard and other USB devices. The version with a relay is an option for automatic switch to DEVICE input.
When the tablet detects external 5V at the USB connector and D+ linked to D- and the battery is below 95% it charges the battery with a current of 800 - 1200 mA. This charging state is set and will persist until the external 5V is removed. When the tablet is ON the charging will go on until battery reaches 95% then charge will switch on and off to keep the battery at 95%. When tablet is shut down the battery is charged up to 100%.

OTG switch is on for battery only operation. When the OTG switch is on charge is disabled by the tablet.

When the tablet is in charging state D+ and D- can be switched to the data lines of the USB device, the tablet then communicates with the USB devices. The USB devices are powered by the external 5V (charger).
USB Device can be a USB hub.
With manual switch circuit version switch to CHARGE position then connect the charger. After the tablet indicates that it is charging switch to DEVICE to enable the use of the USB devices.

Docking Station with manual switch


Docking Station with auto switch

Circuit Description
See also Technical Tips

Charger USB connector is a Micro B Female. Tablet USB connector is a 5 pins Micro B Male. USB Device is a type A Female. To identify the 5V, D+, D- and ground pins I used these pinout drawings.
The transistor is a 5 seconds delay timer.
Relay is a 5V coil with a changeover contact. The smaller the relay the better. A relay is used because the USB bus is a high frequency signal.
Leads should be kept short because of the high currents on power leads and high frequency in the data lines.