Data is shifted on positive clock transition. The parallel outputs may be connected directly to common bus lines. 4: LED Scrolling Display: Display unitĬD4094 (IC3-IC5) is an 8-stage serial shift register, having a storage latch associated with each stage for stroking data from the serial input to parallel buffered 3-state outputs. It has 8k bytes of Flash ROM, 256 bytes of RAM, 32 I/O lines, three 16-bit timers/counters, a six-vector two-level interrupt, a full duplex serial port, an on-chip oscillator and on-clock circuitry. AT89C52 (IC2) is a low-power, high-performance CMOS 8-bit microcontroller. IC1 is a 7805, 5V regulator IC, which provides the 5V output voltage for driving the circuit around microcontroller IC2. In the circuit, an 8×8 dot-matrix has been used. In this project, PNP transistor BC558 (T1-T8) has been used for this purpose. To obviate this limitation, external transistor arrays or buffers are used. A microcontroller has low sourcing as well as sinking capabilities. For example, column C1 should be able to sink the current from six LEDs while displaying alphabet A. You must have noticed that across each row one pin is sourcing the current for only one LED at a time, but a column pin may have to sink the currents from more than one LED. Then, activate column C5 by pulling it down and deselect other columns, and apply forward-bias voltages to LEDs in rows R2 through R7.īy repeating these steps quickly (>100 times per second), and turning on the respective LEDs in each row of that column, the persistence of vision comes into play and we perceive the displayed image of the alphabet A as still. Next, select column C2 (and deselect all other columns) and apply forward-bias voltages to resistors R1 and R5, and similarly for columns C3 and C4. 3: LED Scrolling Display: Power supply and controller circuit Now, the first column is active, and you need to turn on the LEDs in rows R2 through R7 of this column, which can be done by applying forward-bias voltages to these rows. If we want to display alphabet A, we will first select column C1 (which means C1 is pulled low in this case) and deselect other columns by blocking their ground paths (one way of doing that is by pulling C2 through C5 pins to logic high). The seven rows and five columns of the array are controlled through a microcontroller. 2 shows which LEDs in a 5×7 matrix of LEDs are to be turned on to display the English alphabet A. 2: 5×7 array of LEDs Working of a dot-matrix displayįig. In this project, column scanning has been used. Alphabets and numerals can be displayed by fast scanning of either rows or columns. 1, if R4 is pulled high and C3 is pulled low, the LED in the fourth row and third column will turn on. Each LED is addressed by its row and column number.
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