PROPELLER CLOCK The Micro-controlled POV clock is a unique “GADGET” that is quite intriguing. You may have already seen it in operation and wondered just how it works. With only eight light emitting diodes (LED’s) it is possible to display text and/or graphics in mid-air right before your very eyes! Before discussing the operation of the project that you will make or have already made in class, you should understand the concept of “persistence of vision”. According to Merriam-Webster’s Medical Dictionary, persistence of vision is “a visual phenomenon that is responsible for the apparent continuity of rapidly presented discrete images(as in motion pictures or television) consisting essentially of a brief retinal persistence of one image so that it is overlapped by the next and the whole is centrally interpreted as continuous”. If you have ever looked at a bright light and then closed your eyes or looked away, you probably noticed that you can still “see” the light for a short period of time. This project is based on that principle. Hardware First of its kind, made using the at89s52 microcontroller. This propeller clock is mechanically scanned and displays the time in digital format. Microcontroller check for the incoming interrupt (active low signal on INT0 pin),which is detected whenever the clock passes through the IR transmitter LED, IR light arrives to IR photo diode which set the interrupt by pulling the pin voltage down. In this way clock know about its scanning junctions. After this timer interrupt are used to show the display data on by one in a 8 bit frame to the led outputs.Although right now there is no privilege to set time , but one can use switches to change the clock variables.We are updating this circuit soon with a RTC IC HT1380. Have a look at the screenshots ,these were taken during the development stage of the clock. Now , take a look at how we proceeded and ended up making this clock a success. First this was just an idea we saw one day on the internet ,it was Bob Blick who made it first using the PIC 16C84, but I was not having the same microcontroller so I decided making the same from the 8051 microcontrollers. Firstly select a high speed motor, you can select the one from your old tape recorders, that would be sufficient enough to drive this clock.
Then coming to the circuitry part our clock just consist of the at89s52 microcontroller ,8 bright red led, a perforated PCB, 8 resistances (220 Ω), a photo diode, an infrared transmitter, a motor,a pull up ladder,a 10µf cap, 12 volt battery . The clock is on a spinning piece of pcb, but it must get power so to give the required power a 9 volt battery is applied which works very fine as u can see the results are super fine.
The circuit can be made either by designing the pcb or it can be made on a zero pcb as we have done , because doing it on a zero pcb was very much easy. Firstly a 20 pin socket for the AT89S52 was attatched,then connecting the power supplies leds and resistances. The circuit is very easy as it took me only 2 hours and by evening the clock was running. For the leds I used the 220 ohms resistors for proper brightness. A crystal of 11.0592 MHz was selected to provide the clock to the microcontroller. Connect the motor properly between the pcb so as to maintain the centre of gravity so that the clock does not wobble and the display is crystal clear. So much done, now only thing left is programming the microcontroller. I used the at89s52 microcontroller which belongs to the 8051 family of microcontrollers. so I used the keil compiler to write the program code. The code is written in c language and the code finally burned to the controller and the clock is running. For programming the microcontroller you will require a programmer. You can use a parallel port programmer or make a serial port programmer as per your requirements and availability. SOFTWARE Use keil compiler to compile the code. Now as we see what runs this clock….yes the code. Firstly we generated the look up table for the digits we want to display on our clock . These values will be stored in a double dimensional array and timely called to generate the display. The clock works using interrupts and timers. The external interrupt used tells the microcontroller that a single revolution is completed and tells the timer to generate the display and then stop after displaying. One timer i.e. the timer 1 is used to generate the clock internally And the external interrupt call the values of this timer every time to display the time. We also use priority , that is priorities are assigned to the interrupts and timers. Timer1 keeps the count of the time and timer0 performs the task of displaying maintaining appropriate frames that would be displayed and create a persistence of vision