The MR3020 from TP-Link is a really versatile OpenWRT-capable and easily hackable product.
Since the ethernet transformers are not build into the Rj45-plug and it is not a gigabit device the unused pairs 4,5 and 7,8 can be accessed without any big modifications.
- Open the case. It is glued and you may damage the cover.
- Remove the board from the case.
- Remove the following components: C45, R71, R68, R65, R64. They are between the ethernet plug and the transformators on the underside.
- Solder some wire to the pins 7,8 (-) and 4,5 (+).
You have two possible ways to continue:
- You inject 5V. In this case you can directly connect the + from 4,5 to the + of the USB port. If your injector is too far away the voltage drop over the cable may be to high. Since the MR3020 has an internal converter for 3.3V this might nit be a problem for a board. But 5V devices like webcams may crash if they get lower voltages.
- You take a linear regulator (like 7805) to get 5V from a higher voltage. I would only recommend this if you don’t plan to connect any devices to the USB port. You may need to cool the 7805.
- You take a small step-down converter. I took a 2€ LM2596-based board from china. A step down converted produces less heat, can handle up to 3A output in this case an accepts higher input voltages (up to 40V).
If you don’t want to use an external adapter for the USB port you can directly solder the cable on the circuit. You need to remove the coating from the board with a small screwdriver.
NB: The +V port of the serial pins is 3,3V. You can bypass the internal regulator and set the LM2596 to 3.3V but you won’t be able to use the USB port with devices that don’t have their own power supply.
This works on most fast ethernet devices where the transformers are not integrated in the plug.
Take a look at Wikipedia for more informations.