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question about signal transfer along cable

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rodbare

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Hi!
Driven common anode seven segment led,+12V power,scanning display,6 meters RV cable(impedance unknown).it is not work properly,it is always display "8".The signal is properly at input pin of uln2003 and the base pin of 8550,but the signal is very distortion at the output pin of uln2003 and the colletor pin of 8550.I know it is about due to transfer cable error,but I don't know how to solve it!Pls give a hand!Thanks!
 

What about resistors for LEDs current limiting?
 

the seven segment LEDs is work under 12v,so there is not resistors for limit current!
 

Distortion? What voltages do you see at various points? Remember that the output of the ULN2003 must go high to turn off the LED.

Maybe you accidentally grounded pin 9 (diode common) of the ULN2003. That would turn on all the LEDs.
 

Hi,

At which frequency do you scan the LEDs? Usually this is a low frequency and then the cable shouldn't cause any problem more than a little voltage drop due to the cable's resistance.

cyberblak
 

Hi,Thank you!
The question is about solved!I add resistors 10KΩ at the collector of 8550 to GND,and 1KΩ at the output pin of uln2003 to +12V,the signal shape is properly.
It is 4" size the seven segment LED ,and work under 12V.Is it have much capacitors? So if the resistors can sink some charge from LEDs? If it is right?
 

"Distortion? What voltages do you see at various points? Remember that the output of the ULN2003 must go high to turn off the LED.
Maybe you accidentally grounded pin 9 (diode common) of the ULN2003. That would turn on all the LEDs. "
the high voltage at the segment pin(eg,seg A, seg B,etc)about 3V,but the peroper value is 12V.
 

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