Last time, I installed OpenWRT on the TL-MR3220 router and added a connector for it’s serial interface. Today, I wanted to try and connect this interface to an Arduino to make the two communicate. The router’s interface operates at 3.3V while the Arduino’s UART operates at 5V, so we need to convert between those levels. The cheapest way to do this that I could find is described here. It involves using a 74LS04 hex inverter chip to convert the 3.3V signal up to 5V and two resistors for a simple voltage divider to convert the 5V signal down to 3.3V.