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I am using a 74HC154 to control 10 LEDs in a design. Since only one LED will be on at a time I wanted to use one resistor rather than a resistor per LED. Is there any difference between the two designs? Is one better than the other? Or both will work correctly?

Here is an examples of what I mean. Resistor values are not accurate.

enter image description here

PotatoSalad
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    Also, note the 74HC154 is rated for 5.2mA output current (with Vcc at 6V, down to 4mA at 4.5V), be sure to select the current limit resistor accordingly. – CrossRoads Aug 03 '18 at 12:43

2 Answers2

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There is a small difference.

In the lower circuit, the LEDs see no reverse bias at any time.

In the upper circuit (assuming one LED is 'ON') all of the 'OFF' LEDs see a reverse bias of Vcc-Vf where Vf is the forward voltage of an LED. That is because the outputs of the 74HC154 are push-pull and the p-channel MOSFET turns on when the outputs are high.

Provided the LEDs can withstand that reverse voltage there is no other significant difference. Most LEDs are rated to withstand at least -5V, so with a 5V supply there is no issue. There are a few with Vf = 1.8, say and Vr = 3V so you would be exceeding the maximum rating.

Spehro Pefhany
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    Your answer is more to the point, reverse voltage which LED's do not like. +1 –  Jul 26 '18 at 03:00
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As long as only one LED has current flowing, there should be no problem. For that matter, even if multiple LED's were turned on, they'd both illuminate, but just be dimmer than a single one, as long as they are of the same color (i.e. voltage drop).

DrMoishe Pippik
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