how to drive 18 digits of large seven segments ?

[iugmoh]

Hi all ,
I want your idea and experience in how to drive 18 digits of large seven segments worked at 12V ( LDS-CD14RI ) without any problems such as poor lighting or fluckness, so that I can control each digit using PIC.
what about using max7219 , is their anyway to use it because it works at 5V

[sayzer]

For large displays, you should not use a scan frequency. If you do, you will get dim light.

Thus, you will need to use display drivers for each display.

74LS248 would be a good choice.

Large displays are around 9V. Since PIC output or 74LS248 output is 5V, you can not drive them directly.

Thus, you need to use ULN2803 and UDN2981 together to drive the displays.

This way, your PIC drives 74LS248, then it drives either one of ULN or UDN (cathode or anode), then finally you will drive your display.

The input voltage to UDN will be the voltage for your large displays.

Clear?
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[iugmoh]

[QUOTE=sayzer;50002]
Thus, you will need to use display drivers for each display.
QUOTE]

Ok I understand , but with your sugesstion I will use 2 IC's for each display so for 18 digits I will use 36 IC's but this is too large for our PCB design , what are you sya ?

[sayzer]

Check 74LS248 datasheet first.

74LS248 uses 4bits of MCU to drive one display (without dot).

Thus, for example, by using only PORTB, you can drive two displays with no scan frequency.

Also, if you use enable/disable thingie of these IC's, you can reduce the number of pins required for driving all 18 displays. But, this time scan frequency will be on table again. You should experiment.

For example,

You can drive 4 displays via PORTB and PORTC without any scan frequency.
IF you also use enable/disable pins of these 74LS248 ICs, then you can drive say 8 displays.
This way, you will not get flickering because you are going back and forth between two displays on each port.

May be, your experiment will give you an acceptable light level if you drive 3 displays via one port using enable/disable feature.

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