The Propeller Clock

Summary

Associated with: SENAI São Paulo.

Role: Student in Electronics Technology (Associate of Science).

Where: São Paulo, Brazil.

When: 2012.

Objective: Build a propeller clock capable of displaying the time on a 6-digit display format.

Contribution: As a solo project, I've developed all software and hardware elements: the mechanical design, the circuit diagram, and the firmware in C++.

Results:

• Successfully built a working prototype.

Project Description

This was my capstone project, in 2012, which allowed me to obtain attributions to graduate with a technical degree in electronics. It consists of a linear array of LEDs rotating at a high angular velocity to generate the optical illusion of a circular screen in the air.

How It Works

The 7-LED array is controlled by the 8-bit microcontroller Intel 8051, running at ~11MHz. The 12V power is transferred from the base to the control board via 2 carbon brushes (see image below) while the control board is spinning at ~600RPM. An onboard 5V voltage regulator with a low-pass filter is used to reduce the noises generated by the brushes. The control board also has a photodiode that detects the infrared light beam on the base on each revolution, which takes approximately 100 milliseconds. The photodiode triggers an external ISR (Interrupt Service Routine) every time it detects the infrared light, and this is how the algorithm works:

1. Detect the beginning of a new revolution (photodiode is triggered).

2. Flash each of the 80 columns on the display over ~360 degrees (only 40 columns are used to form the clock with 6 digits).

3. Wait for a new trigger to mark the beginning of the next revolution.

Since all 80 columns flash in just 100 ms, our eyes are not fast enough to detect them individually. As a result, we perceive all the columns to be lit at the same time, creating the optical illusion of a screen floating in the air.

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