Alexander Robotics

An issue quickly became apparent after setting up the Arduino and motor driver: the robot could not roam farther than the length of the USB cable powering the Arduino. A longer AC-to-DC power adapter cable would extend the range, but still keep the robot tethered. To truly roam free, the Arduino needed a battery supply.

The Arduino features a 5V on-board voltage regulator. A voltage regulator takes a higher voltage power source and regulates the power to its specified voltage. The remaining voltage is dissipated as heat.

While the Arduino’s ideal operating voltage is 5V, the recommended external input voltage is between 7 and 12V. The higher voltage required compensates for the dropout voltage - the smallest voltage difference between the input power supply and the rated output of the voltage regulator. A dropout voltage can range from 100mV for a low-dropout regulator to 2V and more for regulators like the common 78xx series.

The robot already has a 12V battery pack for the motors. Realistically, I could plug that unregulated power into the Arduino’s Vin and Gnd inputs and be okay because of the Arduino’s built-in voltage regulator. But a fresh charge of the battery actually produces around 14V — still acceptable, but that would generate more heat on the Arduino’s board than I am comfortable with. Enter the 7809.

The 7809 is part of the 78xx series of linear regulators. The 7805 has an output voltage of 5V, the 7809 has an output voltage of 9V, and so forth. Connecting a 7809 between the unregulated 12V power supply and the Arduino moves the power regulation task away from the microcontroller and onto its own circuit. This modular approach allows for easier improvements to the power supply circuit, like a more efficient voltage regulator or reverse-battery protection.

The 7809 is rather straightforward to hook up, consisting of a positive input terminal for the unregulated power supply, a ground terminal, and an output terminal that outputs 9V to the Arduino’s Vin pin. A 100uF capacitor was added between the input terminal and ground and a 10uF capacitor was added between the output terminal and the Arduino. Capacitors act as mini-power supplies, smoothing current and providing quicker discharges when the connected circuit needs power faster than the battery can supply.

Read the specifications

I have made plenty of mistakes while learning robotics. And will continue to do so. Before writing this post, I actually had a 7805 linear regulator between the unregulated power supply and the Arduino. Even though I knew about the dropout voltages, I still thought that since the Arduino operates at 5V, I should supply 5V. The specifications clearly state a recommended input voltage of 7 to 12V. While the robot ran okay with the 7805 now, I could have had a tough time debugging an incorrect voltage from the Arduino later.