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I read a few threads explaining how to drive a 1-digit (7-segment) LED correctly. After all, most threads recommend that I should connect 1 resistor for each segment (which make it the total of 8 resistors) rather than using only 1 resistor for all of them. Now I agree with this view-point, as the characteristic of each LED is different and therefore using only 1 resistor may result in uneven brightness and other issues.

However, when I think about the case of driving a 4-digit (7-segment) LED. The problem seems to repeat itself.

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With the above 4-digit LED, each segment is connected to one common pin. For example, all 4 A-segment are connected to pin number 11. Since I can only wire 1 resistor to this pin, this turn out to be the case of 1 resistor for 4 parallel LED again!

So I wonder how can I drive this 4-digit LED correctly without causing uneven brightness in any segment at all? Moreover, I exclude the case we use 4 separate 1-digit LED because that results in too much pinouts and resistors.

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You would drive such a display using persistence of vision techniques. Only one of the 7-segment displays is actually on at any given instant in time. This is achieved by driving only one of the common anode pins high at a time, and keeping the others in a tri-stated or grounded state.

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  • \$\begingroup\$ Are you suggesting that I should use PWM technique and drive each LED in sequence? \$\endgroup\$ – ReVl27 Sep 6 '16 at 2:42
  • \$\begingroup\$ No, not PWM, but scan through the display - light digit 1 briefly, then digit 2, ..., then back to digit 1 after all are scanned. PWM could be included to control the brightness of the whole display. \$\endgroup\$ – Peter Bennett Sep 6 '16 at 3:10
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You could use dedicated LED controllers that combine current regulation and multiple channels, for example the TI TLC59281, which provides 16 channels, enough to drive two digits.

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  • \$\begingroup\$ I'll try your suggestion out, Simon. \$\endgroup\$ – ReVl27 Sep 6 '16 at 4:57
  • \$\begingroup\$ Be aware that there may be cheaper or better suited options for your use case. This IC is what I use because I have several LEDs in series and need the error reporting. \$\endgroup\$ – Simon Richter Sep 6 '16 at 5:04

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