A reverse LED chaser circuit

Hobbit 77 · Post by **Hobbit 77** » Tue Mar 20, 2012 10:51 pm

I'm somewhat familiar with electronics. I've put together a few simple project kits over the years. That's pretty much the extence of my electronic skills. But I,m trying to find schematics or curcuit for, what I believe would be called, a reverse LED chaser. A normal LED chaser would have 5 or 10 LEDs lined up in a row and they would individually lightup and then turn off moving down the row. What I'm looking for is the opposite. All the LEDs would be on and each light would turn off and then come back on going down the row. Hope I'm explaining myself clear enough. I would appreciate any help on this curcuit. If one exist.

Thanks

Mr. Engineer · Post by **Mr. Engineer** » Wed Mar 21, 2012 7:18 pm

Well, I am not sure but if you have seen a normal chaser circuit (555 & 4017) which gives the single LED output, my theory is to use the same circuit again, but with PNP transistors. I have not tried this but the theory is, when there is no voltage, the LED would light up.

If again, you are wanting to put them in a model where space is limited, then the only recourse would be to use a single programmable micro controller chip which will do what you want. I like this method because unlike the circuit above, one single chip does the same function.

Hobbit 77 · Post by **Hobbit 77** » Wed Mar 21, 2012 7:41 pm

thanks for responding, Mr. Engineer. Believe I have space for the chip you described. Don't know much about the chip you are describing. This is all new to me. Would like to give it a try though Where does one get a programmable micro chip controller and how do you program it to possibly get the effect that I'm trying to achieve?

Mr. Engineer · Post by **Mr. Engineer** » Wed Mar 21, 2012 8:38 pm

Hi,

The chip (the ones I personally use and its the only one available where I am) is called the PIC16F628A is similar in size to that of the 4017 IC but slightly longer. This chip needs to be programmed first. You will need a programmer and the know-how to go to do this, which, to a beginner is expensive and daunting.

Or, you can get a friend to help you.

en'til Zog · Post by **en'til Zog** » Wed Mar 21, 2012 10:41 pm

If you can make a chaser with a 4017 chip - reverse the connections on the LEDs.

Outputs on a CMOS chip like the 4017 go POSITIVE and then NEGATIVE.

So connecting the LEDs "backward" gives you a moving Dark spot rather than a moving Lighted spot.

Don't forget to connect the common connection of the LEDs to POSITIVE rather than the usual NEGATIVE.

tetsujin · Post by **tetsujin** » Wed Mar 21, 2012 11:04 pm

en'til Zog wrote:If you can make a chaser with a 4017 chip - reverse the connections on the LEDs.

Outputs on a CMOS chip like the 4017 go POSITIVE and then NEGATIVE.

So connecting the LEDs "backward" gives you a moving Dark spot rather than a moving Lighted spot.

Close, but not quite.

The 4017 doesn't take a negative voltage, and it's not equipped to produce one. The voltage outputs of the 4017 alternate between 0V (ground) and VCC (usually +5V) - "low" and "high" states, respectively. ("ground" is usually a negative battery terminal in these kinds of circuits - but by convention, throughout the circuit that voltage level is referred to as "zero volts" and everything is measured relative to it. So "negative voltage" would be a voltage lower than that.)

The "normal" situation, then, is when the LED connects the 4017 output to ground. When the output goes high (+5V), you have current flowing through the LED, and it lights. When it goes low, both sides of the LED have approximately the same voltage, so there's nothing to drive current through it.

So to invert the display, you turn the LEDs around as you said, but you also need to connect the LED to VCC rather than ground:

(4017)----K|----/\/\/\----(+5V)

So when the 4017 output goes low, you have this:

(4017 -> 0V)----K|----/\/\/\----(+5V)

So the LED and resistor have a voltage drop of 5V, in the right direction, and current flows. Then when the output goes high:

(4017 -> +5V)----K|----/\/\/\----(+5V)

The voltage drop of the LED and resistor is approximately zero - the voltage on each end is the same so there's no potential, and no current is driven through the LED.

If you just reversed the LED and didn't change the other connection, you'd get this situation:

(4017 -> 0V)----K|----/\/\/\----(0V) (No potential, LED doesn't light)
(4017 -> +5V)----K|----/\/\/\----(0V) (5V potential, but in the wrong direction; LED doesn't light.)

Hobbit 77 · Post by **Hobbit 77** » Thu Mar 22, 2012 9:17 pm

Thanks for the replies. I was looking on youtube and found something pretty close to what I'm trying to make. But I would like to make the LEDs go just in one direction with just 8 LEDs instead of 12.

http://www.youtube.com/watch?v=DHsjQSv8zMo

en'til Zog · Post by **en'til Zog** » Fri Mar 23, 2012 11:22 pm

For me - the way I see things - the outputs effectively go POSITIVE and NEGATIVE. Your explanation is accurate, if a bit over complicated. I think we both mean effectively the same thing.

I've been doing this so loing it seemed "obvious" to me that if you reversed the LEDs you changed the common connection so the LEDs would light. My apologies.

To use 8 LEDs on a 4017 I think you need to just connect the R (reset) input to the #8 output pin rather than to ground. When the 8 pin goes "high" it triggers the reset of the 4017 back to the "0" state - the 4017 counting UP from 0 to 7 (8 LEDs) and then resets.

Model ON!

Z

tetsujin · Post by **tetsujin** » Sat Mar 24, 2012 1:13 am

en'til Zog wrote:For me - the way I see things - the outputs effectively go POSITIVE and NEGATIVE. Your explanation is accurate, if a bit over complicated. I think we both mean effectively the same thing.

When terminology is imprecise or improperly used, the meaning gets lost. When I was starting out with electronics as a kid, I didn't really have any kind of grasp of the basic properties of voltage and current, and I got by - but I'd really prefer not to lead people to an incorrect understanding of this stuff. Everything is black magic until you at least understand Kirchhoff's laws (regardless of whether you know the name "Kirchhoff")

Everything is measured relative to ground, and no point in the circuit has lower potential than ground - so there is no negative voltage in the circuit. "High" and "Low" is very common terminology for describing the different output states of a logic circuit, that would be a better choice here.

I've been doing this so long it seemed "obvious" to me that if you reversed the LEDs you changed the common connection so the LEDs would light. My apologies.

To someone just learning this stuff, nothing's obvious...

en'til Zog · Post by **en'til Zog** » Sat Mar 24, 2012 3:49 am

CMOS outputs can SOURCE or SINK current relative to each other. Since I often connect things between outputs that's how I often think of CMOS connections. One valid way of thinking about using electronics.

I appreciate your way of thinking about electronics.

Over to you. I can now retire after teaching/showing/demonstrating this stuff for 40 + years. YAY!

en'til Zog · Post by **en'til Zog** » Sat Mar 24, 2012 12:30 pm

Sparky · Post by **Sparky** » Sat Mar 24, 2012 2:35 pm

When we used Zogs circuit in the nacelle lighting we used it to generate a moving dark spot. The logic chip will ground an output or drive it to Vcc (high), if the LEDs are hooked up so that their cathode (negative) is connected to the 4017 and their anodes (plus) to common Vcc (or a resistor going to Vcc) then you will get a moving 'off' LED.

the catch is in how much power the chip can pass. either in driving a single output (which could itself be a lot of LEDs) or in pulling down a lot of outputs (since grounding a connection implies it can take all the current your device is trying to pass to ground). So remember any resistor you use is stealing power form the total power that little chip can provide.

So if you have a lot of bright LEDs, that you need to be bright consider the total power the chip can handle and the max voltages it can take. I don't think the 4017 can put out a lot, they were meant to drive logic circuits not display lights. You may still need to boast the maximum by adding transistors to all the outputs.

Hobbit 77 · Post by **Hobbit 77** » Sat Mar 24, 2012 7:45 pm

Found this on youtube. Pretty much fits what I'm looking for. First part shows a normal chaser circuit and the last have shows the reverve of a it.

http://www.youtube.com/watch?v=9CCWvKmVIug

Went to the blog site but it's in another language. Also didn't see schematics for the circuit. Anyone have any ideas?
If possible, would like to try and make the non lit LED to be not just one but two or three non lit as it runs down the row.
Tthanks for the help so far!

Mr. Engineer · Post by **Mr. Engineer** » Sun Mar 25, 2012 12:52 pm

OK. this guy stays about two hour's drive from where I live. And the language is actually Bahasa Malaysia.

What the video shows is basically the 555 and 4017 with adjustable speed. The last 10 seconds is what you need and its actually what has been discussed by tetsujin and Zog earlier. Note the polarity of the LEDs have changed and the wiring remains the same (except for the power rails, methinks). You can see the difference between the first 10 seconds and the last 10 seconds of the video.

The reason why there is no circuit is because he wants you to buy his e-Books.

Hobbit 77 · Post by **Hobbit 77** » Mon Mar 26, 2012 11:18 pm

I have a 10 LED chaser circuit put together on a breadboard. I got it to work but just having some difficulty understanding rearranging the wires. Forgive me, but I'm rather new to this kind of stuff but not sure what VCC means and not sure where to rearrange the LED connection. Worried that I might over heat the 4017 and 555 chips because of my lack of knowledge in electronics.

I reversed the LEDs as informed but the LEDs don't light up. When I place them one at a time they flash together until I add the last LED then they all don't light up at all. Also noticed that the LEDs are not as bright when installing then in this arrangment.

Getting closer and closer to accomplishing this project. Thanks for all the help so far.

Hobbit 77 · Post by **Hobbit 77** » Tue Mar 27, 2012 10:04 pm

Played around with the circuit and got it to work! Thanks every one for your help.