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Problem with 555+4017 circuit (LEDs always on)

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madmazda86

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Hello - electronics n00b here. I'm trying to build an LED chaser to go on a costume I'm making. The LEDs need to do a single chase, then stay off for about 5-6 seconds, then do another.

Soooo, to change the duty cycle so it was off more than on, I put a N4148 diode in parallel with R2 on the 555 circuit. I'm using the 555 astable diagram from
555 and 556 Timer Circuits
with a 47uf decoupling capacitor and C1 as 10uf, plus the 0.01uf control.

This worked fine with a single LED - it came on for a second, then stayed off for five seconds.

But when I hooked it up to the 4017 using this diagram (4017 Decade Counter) and 270R resistors, all the LEDs came on dimly, and then each one would brighten in turn with 5 seconds between each LED. In addition the dim illumination of all the LEDs wasn't consistent, it varied.

I'm guessing that the 555 is generating a signal every five seconds, but instead of that triggering a cascade which was what I was after, it's just messaging the 4017 to switch the next output to high.

So what do I need to do to convert that signal every five seconds into a cascade of pulses to the chaser? And how can I make sure the LEDs stay off between chases, as opposed to being slightly lit?

Thanks in advance for your help - I've been on so many websites to try and figure out how to make this circuit, and now that it doesn't work I am at my wit's end!
 

Boncuk

New Member
To have the light sequence as desired (ten LEDs in fast sequence with a break of 5 seconds) you require one astable and a monostable circuit.

The astable will be controlled by the monostable when the last count (output "9" which will be the trigger input (inverted)) is reached. Reset the astable for the desired time. As soon as the monostable times out the astable will produce clock pulses for the counter.

That way you won't have a synchronization problem which you will have with different configurations.

Instead of using an IC for the inverters you might use NPN-transistors.

Boncuk
 

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ccurtis

Well-Known Member
The LEDs may be dim due to internal cross-coupling between the output stages of the 4017 resulting from excessive current through its outputs. The output high voltage of the 4017 drops greatly with only one milliamp of load current, making them hardly suitable for driving LEDs, except for perhaps the more efficient ones. The outputs of the 4017 are not designed for such current levels.
 
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madmazda86

New Member
@Boncuk: Okay, thankyou very much! I do have a second 555 timer chip I can use. So let me get this right - the resistors/capacitors I use in the monostable determine how long it is before the chaser sequence restarts? So by plugging in the capacitor and R2 I had selected for my original 555 circuit, I can produce that time delay there in the monostable, and then just use a standard set of resistor values for the astable circuit to generate the pulses I need to the 4017 counter?

@ccurtis: Really? That's not the right chip? Oh dear. I got the impression from all the circuit diagrams around that 4017s were just the thing for making LEDs light up in sequence :\ Should I be using smaller LEDs, then? Or is there an alternate chip I can use?
 
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ccurtis

Well-Known Member
@ccurtis: Really? That's not the right chip? Oh dear. I got the impression from all the circuit diagrams around that 4017s were just the thing for making LEDs light up in sequence :\ Should I be using smaller LEDs, then? Or is there an alternate chip I can use?

You can probably get by okay, but its not good design practice and not something I would sell. I noticed that the photo of the chaser in your link has diodes at each output to help cut off the LEDs when the 4017 outputs are low, but the schematic doesn't show them. Those diodes might help you, since they would tend to diminish the effects of cross-coupling.
 
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Boncuk

New Member
Minimize the load on the 4017 outputs by using high efficiency low current LEDs. Their forward current is just 2mA and the 4017 should be able to handle this (not quite the maximum brightness though).

Boncuk
 

colin55

Well-Known Member
I have just tested MC14017BCP IC and it delivered 15mA to a 330R resistor when the supply is 6v.

On 12v a CD 4017BE chip delivered over 25mA to 440R.

I don't know where you get the information that this chip will only deliver a very small current.
I have always used them to drive LEDs and had no problems and never had any LEDs illuminated when the output is LOW.
 

ccurtis

Well-Known Member
I have just tested MC14017BCP IC and it delivered 15mA to a 330R resistor when the supply is 6v. On 12v a CD 4017BE chip delivered over 25mA to 440R. I don't know where you get the information that this chip will only deliver a very small current.
I have always used them to drive LEDs and had no problems and never had any LEDs illuminated when the output is LOW.
Seeing no other associated reference, I presume you are referring to the post where I wrote that it was not designed to supply that amount of current. I got my information from the data sheet, which, for example, shows that at 1.1 mA source current, the output may drop from 5V to as low as 2.5V. It is a logic device, and a 2.5V output is an undefined logic state for a 5V supply. I am not inclined to design circuits by the seat of my pants method, but congratulations on your success. Perhaps you have an explaination why the OP's LEDs are dim when they are supposed to be out. I'll wait.
 

Mikebits

Well-Known Member
Attached is the IOL/IOH spec I found from the TI data sheet on the 4017BE. Does not seem to be a very beefy driver does it?
 

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colin55

Well-Known Member
That may be at 2MHz or may be minimum values or guaranteed values. The 4017 has always delivered 8 - 10mA in all the projects I have designed. (6v operation).


The values given in the data sheet are not maximum values. The same with Philips data sheet. The max value is about 10mA for 6 - 9v and over 20mA for 12 - 14v operation.
 
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Boncuk

New Member
Here is a circuit with a "beefy" :) driver. It uses only nine LEDs because you won't ever see the tenth lit due to the simultaneous RESET.

The driver IC (ULN2803) can supply 20mA LED current without any problems and won't mess up the decimal counter (CD4017).

See the rest of information in the attached circuit diagram.

Have fun.

Boncuk
 

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Mikebits

Well-Known Member
That may be at 2MHz or may be minimum values or guaranteed values. The 4017 has always delivered 8 - 10mA in all the projects I have designed. (6v operation).
Your historical data may prove the data sheet to be on the conservative side of the listed specs on the data sheet. The data sheet only list what the manufacturer guarantees. Usually one designs by specification offered in the data sheets. Before testing a design, one can only predict the design to meet certain specs based on data sheets for all parts in a design. If I were to design a board with above shown spec of the 4017, I would make the assumption that a higher current driver was needed for the led.
 

Mikebits

Well-Known Member
Here is a circuit with a "beefy" :) driver. It uses only nine LEDs because you won't ever see the tenth lit due to the simultaneous RESET.

The driver IC (ULN2803) can supply 20mA LED current without any problems and won't mess up the decimal counter (CD4017).

See the rest of information in the attached circuit diagram.

Have fun.

Boncuk
Gee Boncuk that was nice of you to make a schema...:)
 

Mikebits

Well-Known Member
Your a good guy Bon...:)
 

Mikebits

Well-Known Member
What else could I do than frying in the sun and stick a finger up my nose? :D
I dunno, but I have been to Pataya Beach and lots of nice looking ladies there...:)
 
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