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Schematic (from here):

enter image description here

Why are four legs connected to the green LED and two to the amber LED in the 4017 traffic light circuit? Shouldn't it be able to do with only one leg per LED?

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Have a look at the LED truth table on the same page:

enter image description here

Each output is consecutively active for the same time. By wired-OR-ing outputs you can create longer active times. So the green LED will be on during 4 time units.
Normally you would have to wired-OR Q0 through Q4 for the red LED, but we have a carry output (here called ":10") which does just that, see the bottom trace of the timing diagram:

enter image description here

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  • \$\begingroup\$ That's curious. In Spain, the amber light is not lit when going from red to green but, in this circuit, it is. I just found out that's done in several EU countries. \$\endgroup\$ – Telaclavo May 4 '12 at 9:57
  • \$\begingroup\$ @Telaclavo - In Belgium neither. I believe the Germans do it like this. (A few weeks ago I did 600km in Germany, but only Autobahn, so I didn't see any traffic lights.) \$\endgroup\$ – stevenvh May 4 '12 at 12:29
  • \$\begingroup\$ In The Netherlands they do it as well. \$\endgroup\$ – radagast Nov 13 '13 at 7:50

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