# 4017 driving multiple LEDs

Discussion in 'Electronic Basics' started by pstnly, Jan 12, 2006.

1. ### pstnlyGuest

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?

2. ### Dan HollandsGuest

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

www.QuickScoreRace.com

3. ### Don KlipsteinGuest

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 ()