ULN2003A and 7 segment display (common anode)

Forum 2005-2010 (read only) Hardware Interfacing
1

I finaly bought some material for my arduino projects and I have questions - again.

I have four 7 segment LED displays with common anode that I want to control (each segment needs 2V to work). And ULN2003A (here is a link to the datasheet: ULN2003AN Datasheet pdf - High-Voltage, High-Current Darlington Transistor Arrays - Texas Instruments ). Now I am having trouble connecting this thing. For some reasons it allways works like it should ::slight_smile: .

I only tested it with one segment because it is realy late here and I need to go to bed. Here is my wiring:
pin 1 on ULN to arduino pin 2
pin 8 -> GND
pin 16 -> segment on LED display
+5V from arduino to common anode pin on the display (with 100ohm resistor).

Is this correct? I get 2V on the segment and I can control the segment like a normal LED. The main question - will this stop working if there are more segments active at one time?

2

Is this correct?

Nope: you need a resistor on each segment, between it and the sink driver. With a single resistor, as you light up more segments, they'll get dimmer and dimmer because they share the (roughly constant) limited current.

3

Yep! It worked for one segment but not for more.

Another question - what do I do if I want to control common cathode display with external power supply (displaty needs 7V to work)?