Five sensors we're leaving room for
Here's a fun discovery: the parts already picked for a "friendly droid" accidentally add up to a platform. The IR array is a general-purpose receiver. The sonars plus odometry can map a room. The cliff sensors are line followers. So we're leaning into it — with a $56.29 expansion BOM, provisioned in rev 1 and populated in rev 2.
REV-2 SENSOR EXPANSION — $56.29
| Capacitive touch (MPR121) | $7.95 — pet the head, get a reaction. The single best "friendly" upgrade. |
| BME280 env sensor | $14.95 — temp/humidity/pressure. Mobile weather station. |
| PIR motion (HC-SR501 ×3) | $8.49 — wake-on-motion sentry mode, no all-day vision burn. |
| INA219 current sensor | $9.95 — battery health + stall detection before motors burn. |
| VL53L0X time-of-flight | $14.95 — laser-precise final inches of docking. |
One bus, zero conflicts
Four of the five speak I2C, and their addresses don't collide (0x5A, 0x76, 0x40, 0x29) — they all share one bus on the spine. The PIR is a plain digital GPIO. Rev-1 provisioning is just a spare I2C header, one interrupt-capable GPIO, and sensible mounting points: electrodes under the head shell, a PIR dome and ToF aperture on the front face, and the current sensor near the battery lead.
Hidden features this unlocks later: a universal IR remote that drives itself to the couch, follow-me mode, room mapping, sentry patrol, and a droid that purrs when petted. The friendly face was always the plan. The platform is the bonus.
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