LightWriter Cricket

Hard on the heels of the LightWriter Clock project, comes my latest creation – LightWriter Cricket. This little critter writes the time with light onto a phosphorescent screen, and like the LightWriter, the displayed time just fades away until it is rewritten at the next minute. And, mirroring the ephemeral perception of time, each specific moment quickly fades into the past.

Here, LightWriter Cricket is seen writing out the time 14:13. Notice how quickly the display fades away such that the hours are almost totally invisible by the time the minutes are written.

The LightWriter Cricket clock is based on the PlotClock, a clock that uses a marker to write out the time onto a piece of dry-erase board, For the PlotClock, at each minute the head picks up an eraser to remove the previously written time before writing out the new time. This ingenious design caught my attention a couple of years ago and appears to have been improved upon several times. The clock uses three servos: two to move the head and another to raise and lower the pen.

For the LightWriter Cricket, I have removed the head servo motor and redesigned the head to equip it with a purple LED. Rather than a dry-erase board, the head is positioned just above the surface of a piece of acrylic, sprayed with several coats or phosphorescent paint.

As I don’t have my 3D printer yet, the dimensions of the leg pieces were taken from the pdf of the design details on the PlotClock site. A paper copy of the pdf file was reduced to 65% size, stuck to a sheet of acrylic, and the legs and servo mount were cut out and shaped by hand.

Future plans include a simple housing, and some experimentation with different LED colours (including using actual UV LEDs), phosphorescent paints and paint thickness, and backings for the acrylic, to make the display more visible. For now, it works just fine!

Time Jumper

LightWriter Cricket: showing the two servos that control the movement of the head to which a UV/purple LED is attached

Both servos were mounted in the plastic servo mount using M2.5 nylon hardware. Each metal-gear servo connects to a pair of jointed legs, made out of acrylic, using normal servo horns. The joints are connected with nylon M3 nuts, bolts and washers. Calibration routines in the original software help to set up the appropriate geometry and travel of the two servos. The short articulated legs give the LightWriter Cricket its name.

Time Jumper

LightWriter Cricket: closeup showing both servos, horns and plastic “legs”

The hardware consists of an Arduino Nano, a 12C interface real-time clock module, the two servos and a purple/UV LED. Pins on the Arduino directly control both of the servos and the LED. While the software to control the servos to create each number was left largely intact, I removed all software associated with the pen lifting servo, simplified the real-time clock handling routines and added control of the head LED.

The software wakes every minute and writes out three or four 20mm high digits representing the current time, with the hours and minutes separated by a colon. Once written the servos “park” the head away from the written time.

Easily hackable, simply elegant and a great afternoon project.

Neat, huh!

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