01 / Abstract
Abstract
This project explores a configurable timer for a small USB desk fan, including repeating intervals such as three hours on and one hour off. The design emphasizes low-voltage switching, manual override, reset behavior, and unattended-use safety.
02 / Use cases
Use cases
- Run a USB fan on a repeating interval without daily intervention.
- Provide an obvious manual override and disable path.
- Demonstrate a compact hardware-and-firmware safety review.
03 / Theory
Operating theory
Low-side switching
A logic-level MOSFET can switch a low-voltage DC load efficiently when gate drive, grounding, current, and inductive transients are handled correctly.
State recovery
The controller must define whether a reset resumes, restarts, or disables the cycle. Predictable recovery is more important than retaining every elapsed second.
04 / Requirements
Requirements and success criteria
- Identify the fan connector, voltage, and current before choosing the switch.
- Support a configurable repeating interval and manual override.
- Return to a documented safe state after reset or power restoration.
- Keep wiring enclosed and suitable for unattended low-voltage operation.
05 / Architecture
System architecture
06 / Current status
Current status
The behavior and safety goals are captured. The fan electrical load, switching approach, enclosure, and test procedure have not been selected.
07 / Next steps
Next steps
- Measure the fan voltage and operating current.
- Choose an off-the-shelf timer or microcontroller switching approach.
- Define reset and manual-override behavior.
- Prototype the low-voltage switch under supervision.
08 / Risks
Risks and constraints
- Unverified current can overheat a switch or connector.
- Unattended wiring near a pet requires enclosure, cable, heat, and airflow checks.
09 / Artifacts
Engineering artifacts
- Planned behavior specification
- Switching schematic
- Firmware or timer configuration
- Reset, override, and temperature test log