RN42 Bluetooth Serial Bridge
Drop an RN42 module on your UART and turn any sketch’s serial output into a wireless terminal session from a phone or laptop.
Difficulty: Breadboard. Estimated build time: about 30 minutes. Estimated parts cost: about US$15.00. 5-line bill of materials. Compare supplier offers when available. A wireless serial bridge that replaces the USB cable between your computer and your microcontroller.
Wire RX, TX, V_CC, GND
RN42 takes 3.3 V on V_CC. Connect Arduino 3.3 V to RN42 V_DD, GND to GND. Cross the UART: Arduino TX (D1) → RN42 RX (pin 13) via a 1k/2k divider to drop 5 V to ~3.3 V, and RN42 TX (pin 14) → Arduino RX (D0) directly (3.3 V is a valid logic-high to a 5 V AVR). Add 0.1 µF decoupling at the module.
Pair with your phone
Power up. The status LED on the RN42 blinks slowly at 1 Hz until paired. On your phone, open Bluetooth settings, look for “RN42-XXXX”, pair with PIN 1234 (the default). The LED switches to fast blink when paired.
Open a Bluetooth serial app
On Android, install “Serial Bluetooth Terminal”. On iOS, “Bluetooth Terminal”. Pick the RN42 as the target, connect, and any Serial.println() from your Arduino sketch appears live in the terminal. Type back and the bytes land in Arduino’s Serial.read().
Reconfigure with AT commands
Within 60 seconds of power-up you can drop into the RN42’s command mode by sending $. From there, SU,57.6 changes baud to 57600, SN,MYROBOT renames the device, and SP,9876 changes the pairing PIN. Type --- to exit command mode.