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# oscillating LED circuit

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#### daviddoria

##### New Member
SCHOOL PHYSICS PROJECT

i made a cool thing for my computer, seen here: http://www.bandtank.com/stuff/movies/MVI_1646.AVI

its just a 555 timer that is speed controlled by a 100k pot (i think i got the diagram from this forum!)

i was wonder how to make this into something that would make the LEDs "run" back and forth instead of blink 4 at a time.

can anyone give me any input on this?

i got a reply about using a decade counter... not sure how to do this though. I have an NTE74192 right here if its possible to use that...

any input would be great!

thanks
david

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Yes I have

Me too 8)

well i dont know ...

What is it anyway?

Use combination of two 4017 decade counters to achieve up-down running effect.

to john1: It's just a small movie showing flashing LEDs placed in an unused 5,25" bay on a computer case...

haha kinjalgp thats what you said in the other forum too hehe.
as you have probably gathered i am new to this stuff, so i was really looking for some tutorial on counter or an example circuit of some sort.

anyone?

david

page cannot be found.

hmmm

i decided to use this schematic

http://www.bandtank.com/~calculusap/leds.gif

it says the NPN transistors increase the current to 20ma. what is the current coming out of chip without the transistor?

also, what does the 120 resistor do? like what is coming out of pins 2 and 6 of the 555?

The 120 Ohm resistor limits the current flowing through LEDs otherwise they will burn out.
The current sourcing capacity of HCF4017 (SGS Thomson make) is typically 2mA per output.

David - The schematic you have selected seems too complex (all those transistors) for what it does. But, the reason for the note is that your circuit will light the LEDs in order (1 - 10) and then repeat the sequence. This was not the way I understood your original question. I thought you wanted the LEDs to run back and forth. If you are still interested, here is a simple little circuit:

Hi Gene,
I doub't whether the given "Knight Rider Lights" circuit on the site will produce the reverse effect because the counter IC seems to be doing only the up-counting whereas you require another 4017 for reverse effect.

thanks man those are great

kinaljp, i read up on the 4017 counter and it said that there are built in resistors so you can connect unprotected LEDs. is this incorrect.

also, what is a normal current requirement for LEDs? The point being can i omit the transistors on the first circuit we were talking about.

also, gene, on the night rider circuit, the diodes between the 4017 and the LEDs seem unnecessary. it says in the paragraph that they are to prevent shorts in the outputs... can you not use simply shielded wire to do this?

thanks
david

Normally LEDs are allowed to eat around 10mA of current ans here optput sourcing capacity of 4017 is 2mA, if you connect them directly, they won't glow very bright. Also I hav't read that it has in-built resistors so I can't comment upon that but its always safer to use resistor if you are connecting LEDs directly to any digital IC.

diodes are use to prevent short of adjacent pins of 4017 when one is high and ohter is low. In this case all current from high state pin will flow directly into the low state pin thereby leaving no current to turn-on the LED. The diodes acts as on-way devices preventing reverse flow of current into the chip during low state on one of the paired pins.

look, it is a problem in the datasheet, or i found a "super 4017". i made a 10 led circle wich turns. i put a current limiting resistor, and mesured the current at aroun 12-14ma. the problem is that the circuit worked well at a low voltage around 4.7V, and it worked non stop for a few weeks till i got bored by it and turned it off. the thing is tha even if it is limited to that kind of current if you power it at low voltage you can make a "super 4017"
and another thing, if you dont want to use a transistor for each led, you can use a buffer ic, it will be even cheaper maybe.

by the way, i looked in the datasheet and the max. Iout is 10mA

hmm so i got these counters... but there is no dot indicating pin 1 (like on a 555)

which is pin 1, and do they alternate?

ex.
1 2
3 4
5 6
etc

just like a 555? is this standard for ICs?

david

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