LED-based moving-messagedisplays are becoming popularfor transmitting information tolarge groups of people quickly. Thesecan be used indoors or outdoors. Wecan find such displays in areas like railwayplatforms, banks, public offices,hotels, training institutes, nightclubsand shops.Compared to LEDs, liquid-crystaldisplays (LCDs) are easy to interfacewith a microcontroller for displayinginformation as these have manybuilt-in functions. But these can’t beobserved from a distance and largesizeLCDs are very costly.LED-based displays can be of twotypes: dot-matrix and segmental. If youimplement a moving-message displaywith multiplexed dot-matrix LEDs, itwill be very costly for displaying 16characters or more at a time. Moreover,programming will require a lot of datamemory or program memory space.An external RAM may be needed tocomplement a microcontroller likeAT89C51.However, if you use alphanumeric(16-segment LED) displays for theabove purpose, programming burdenis reduced and also it becomes highlycost-effective. You can make yourown display panel consisting of 16alphanumeric characters at a muchlower cost.The circuit presented here uses 16common-anode, single-digit, alphanumericdisplays to show 16 charactersat a time. Moreover, programming hasbeen done to make the characters movein a beautiful manner. A message appearson the panel from the right side,stays for a few seconds when the firstcharacter reaches the leftmost placeand then goes out from the left side.It displays 16 different messages todepict different occasions, which canbe selected by the user through a DIPswitch.
Fig. 1 shows the circuit of the microcontroller-based moving-messagedisplay. It comprises microcontrollerAT89C51, three-to-eight decoder74LS138, common anode alphanumericdisplays, regulator 7805 and a few discretecomponents.At the heart of the moving-messagedisplay is Atmel AT89C51 microcontroller(IC1). It is a low-power,high-performance, 8-bit microcontrollerwith 4 kB of flash programmableand erasable read-only memory(PEROM) used as on-chip programmemory, 128 bytes of RAM used asinternal data memory, 32 individuallyprogrammable input/output (I/O)lines divided into four 8-bit ports, two16-bit programmable timers/counters,a five-vector two-level interrupt architecture,on-chip oscillator and clockcircuitry.Ports P0 and P2 of the microcontrollerhave been configured to actas a common data bus for all the 16alphanumeric displays whose correspondingdata pins have been tiedtogether to make a common 16-bitdata bus. Port-2 provides the higherbyte of data, while port-0 providesthe lower one to light up a characteron the display. Port pins P1.2-P1.4and P1.5-P1.7 of the microcontrollerhave been used as address inputsfor decoder IC3 and IC4 (74LS138) toenable one of the fourteen alphanumericdisplays (DIS3 through DIS16)at a time, respectively. However,displays DIS1 and DIS2 are enabledor disabled directly by port pins P1.0and P1.1. Pins 4 and 5 are groundedand pin 6 is made high to enable decoder74LS138.Fig. 2 shows the pin configurationof the common-anode alphanumericdisplay.All the corresponding data pinsDis 1 through DIS16 of alphanumericdisplays have been tied together
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