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Bulbdial Clock Mimics Sundial Using LEDs

Credits: ©2009 Evil Mad Scientist

In 2008 David Friedman published on his blog Ironic Sans a design concept for something that he called The Bulbdial Clock. It's like a sundial, but with better resolution-- not just an hour hand, but a minute and second hand as well, each given as a shadow from moving artificial light sources (bulbs). Now, there’s a working Bulbdial Clock, with an implementation somewhat different from that of the original concept. Rather than using three physically moving light sources at different heights, it uses three rings of LEDs at different heights. Within each ring, only one LED at a time is turned on, so that it only has a single, effective light source-- it can light up at different places from within the ring. The three rings are located above one another so that they each project light onto the rod in the middle, making shadows of different lengths. For fun and clarity, it uses red, green, and blue LEDs for the three rings, making each shadow hand of the clock a different color. Each ring has 12 LEDs, and the 36 LEDs are efficiently multiplexed by an AVR microcontroller that also handles the timekeeping part of the project. This clock was mostly built out of laser-cut plywood.


Bulbdial LED Sundial Clock 6

In this photo of the working Bulbdial Clock, the time is about 6:20:40. (Like many clocks, it's hard to read with high precision. Even so, the seconds help to create the sense that something is happening.) The short hand, the hour hand, is created by the shadow of the red LED, and points to 6, indicating 6 o'clock. The medium hand, the minutes hand, is created by the shadow of the green LED, and points to 4, indicating 20 minutes past the hour. The seconds hand, the longest one, is created by the shadow of the blue LED, and indicates about 40 seconds past the minute. ©2009 Evil Mad Scientist

This is 1/4" (6 mm) plywood, which holds remarkable precision with the laser cutter. There are three rings, shaped spacers that keep the rings above the base with the clock face, and a hardwood dowel rod. It's really quite a three-dimensional project! The rings each have pairs of small holes in them to mount the LEDs in place. Each of the rings gets stuffed with 12 LEDs, with the correct orientation and spacing-- long enough leads-- that they can point towards the rod (and make good shadows) once they are in place.

Since each LED is only supposed to be on one at a time, the whole array is driven easily and directly from the microcontroller, Here this array of LEDs becomes a single common-anode unit, by hooking all of the LED anodes together. Likewise, after stacking these, the cathodes are connected -- vertically for the three rings-- and the microcontroller is hooked up as well. For three rings (colors) and 12 cathodes, that means 15 pins are needed to control the array. And the stack up is complete. The next step is to add the control electronics, which include a business card boards with an ATmega168 microcontroller (programmed through a USBtinyISP), along with a 16 MHz crystal oscillator, a 3xAA battery box with switch and three tactile button switches. There are also three resistors, two 50-ohm and one 100-ohm, which are used when driving a red LED, or blue or green, respectively. Because only one LED is on at a time, only one resistor is necessary for each color. Of the three buttons, two are used for setting the time-- one is fast advance, one is slow advance. The other button turns on or off the LED, but does not turn off the microcontroller. When the LED is on, the circuit uses about 26 mA of current. When the LED is off-- just timekeeping, not displaying -- it uses about 11 mA. These could both be reduced by using larger resistors and reducing the speed of the microcontroller.

The colors of the Bulbdial Clock interplay; where all of the LEDs hit, a white color (additive RGB) is produced. Then, as you subtract one of those three colors-- as in the shadow of a single LED, the results are subtractive colors, and the hands on the clock face are in the colors cyan, magenta, and yellow. A kit version of the Bulbdial Clock is available (under license from David Friedman).


Bulbdial LED Sundial Clock Kit