4017 driving multiple LEDs

Jan 12, 2006 2 Replies
P

I need a circuit which lights 4 LEDs upon 4 pulses on input. 4017 CMOS counter/decoder seems the obvious choice here, and there are tons of example circuits on the web. However, every circuit I've seen only drives one output at a time (a LED connected to each of the 4017 outputs). What I want to do, though, is keep 'earlier' LEDs lit. So the following would be lit:



count 0 - LED1 count 1 - LED1 + LED2 count 3 - LED1 + LED2 + LED3 count 4 - LED1 + LED2 + LED3 + LED4



I'm thinking I could use diode OR gates (none for LED1, 2 for LED2, 3 for LED3, 4 for LED4 - 9 total), but not sure that this is the right answer. On count 4, 4 LEDS have to be lit - does the 4017 deliver enought current for this?



Do I need to throw a transistor on each output to amplify the current?



Is there a better way?



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D

Another way is to use a shift register counter sometimes called a Johnson counter. The flip-flop outputs would give you exactly what you want

Dan Hollands 1120 S Creek Dr Webster NY 14580 585-872-2606 dan.hollands@gmail.com www.QuickScoreRace.com
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D

I would use transistors or MOSFETs. In general, logic IC's don't reliably comfortably source more than a couple milliamps of current, sometimes not even a milliamp (4000 series with 5 volt supply arguably could have their worst-case voltage drop of pullup output FETs higher than desirable with much more than half a milliamp.)

I would rather use diodes than OR gates when things change infrequently enough to see things happen. With MOSFETs and with diodes rather than OR gates, add a pulldown resistor of 10K or 100K or so from gate to ground.

Diodes used for such a purpose are called "steering diodes".

Make sure the LEDs have dropping resistors. You may get away without, but I would not rely on this. 470 ohms should be sufficient with room for error with a 12V supply, and 150 ohms should do likewise with a 5 volt supply.

If driving bipolar transistors, have a resistor in series with the base to limit current to something that the IC can comfortably deliver - perhaps a 10K resistor. You can usually get away with 4.7K and possibly but nowhere near reliably with none.

- Don Klipstein ( snipped-for-privacy@misty.com)

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