It is also called a wand clock. This project was inspired by a Circuit Cellar contest project. As soon as I saw it I wanted my own version. This is probably the simplest POV clock that can be easily made. As far as I remember, the Circuit Cellar contest project used a mercury switch as position indicator but I didn't have one so I made a simple inertial switch with the leg of a diode and a ball of solder.
I've posted schematics and firmware.
This clock was actually made during year 2000. The circuit is based around an ancient Atmel AT89C2051 microcontroller. The version of the microcontroller that I used is the 24PC, and its minimum operation voltage is 4V, so the three AA batteries don't last that long. This microcontroller should be replaced by the new low power 8051 derivatives such as Atmel's AT89LP2052.
The photo shows the front of the clock. I remember that after testing a few LED colors I've chosen yellow ones because those where the ones that looked the best for a certain level of on time and current that was determined experimentally.
This photo shows the back of the clock and its battery holders.
The inertial switch is the key to the good operation and display quality of this toy. As you can see it is located near the top of the clock, in the opposite side where it is supposed to be held.
A closer look reveals that it actually is a SPDT switch and connects to the two external interrupts to trigger the beginning of the display action and to software debounce its contacts.
A side view of the inertial switch:
Simple indeed but as many of my old projects, I left it uncommented. A lot of features can be added as it takes very little memory space.
$TITLE(RTCP COPYRIGHT ROLANDO CALLA Z 12/2000)
HR EQU 8H
MN EQU 9H
SC EQU 0AH
AD EQU 0BH
A2 EQU 0CH
ACOL EQU 0DH
ADJC EQU 0EH
ADJCR EQU 100
BCNT EQU 0FH
BCNTR EQU 128
SCMS EQU 55 ;2BH (PRV = 43)
SCME EQU SCMS+25
SBTM EQU SCMS-10 ;21H
STOP EQU SCME+10
CHGF EQU 20H.2 ;CAMBIO DE CARACTER=1
ADJF EQU 20H.1
MB EQU P3.7
HB EQU P3.5
ORG 3 ;EX0
ORG 0BH ;TMR0
ORG 13H ;EX1
When reset, time is initialized to 00:00:
The only complication is that to set the time, both push buttons must be held down for a second. When the clock is in time adjust mode, the time appears underlined and the buttons set hours and minutes each. To go back to normal time mode, again push both buttons for a second.
I don't have the original schematic for this. So I've redrawn it in KiCAD's EESCHEMA.
As can be seen, the LEDs are connected to a common +4.5V only with a very small resistor. The total energy for the LEDs is limited by the time they are allowed to remain powered.
The PCB was designed in Tango PCB and it is probably lost forever. Next photo shows that it is a double side PCB.
Anyway, if you want to reproduce this clock you can do your own layout, or more easily, solder parts to a perforated board or simply use a protoboard.
The original plans called for a more compact version that fits in a pen. Maybe some day...