And now, a few notes on some optional accessories. These aren't required for anyone building their own Phoebe, but are nice to have.

The first item is a battery voltage meter and alarm. While Phoebe can monitor battery voltage in software via Roboclaw API, I also wanted an always-available physical readout of battery voltage. On Sawppy I thought I just needed to show the battery's output voltage, but the number is only good if I could read it. During Sawppy's all-day outing at JPL, California sunlight was too bright to read the number and I couldn't tell when my battery dropped below recommended level for lithium chemistry batteries.

Searching for a better solution, I found these battery voltage alarms. Not only do they display voltage, when the level gets too low they also sound a buzzer. Judging by its product description, these were designed for remote-control aircraft where it's not convenient to read a small number up in the air.

The downside is that the alarm is designed to be audible while up in the air and buried inside a fuselage. When it is on the ground and right in front of my face, it is a piercing shriek. Which isn't so bad if it only occurs during low battery... but it also sounds a test beep when I first plug it in. It is loud. Very loud. To save my eardrums, the alarm buzzer has been muffled with some cotton pulled from a cotton swab. It's still loud, but no longer gives me a headache afterwards.

I've also soldered a JST-XH connector onto the unpolarized input pins to fit my battery's balance charging plug. Having a polarized connector helps make sure I don't plug the battery in backwards. Those exposed pins are also a short-circuit risk, which I crudely mitigated by wrapping a layer of servo tape around them. Finally servo tape is used to secure the alarm to Phoebe's backbone.

Now I can drive Phoebe around the house, even out of sight, confident that if I ever run the battery too low I'll be notified with an alarm.

(Cross-posted to NewScrewdriver.com)