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help with a supposed simple circuit

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Big Sammy

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I'm trying to build a simple 10 led chaser circuit using a 555 ic and a 4017b ic. The diagram can be seen at (www.qrp.pops.net/LEDs.asp) Sorry I can't post the diagram here, but it is a fairly common circuit. I'm going to put the led's into a guitar neck. The problem I have is I'm only getting one of the led's to flash. I've checked them all individually, and they all work. The only one that flashes is the one connected to pin 2 of the 4017. The 4017 is brand new, so I don't think that is the problem. Anyone have any ideas? Thanks
 

crutschow

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Without seeing your circuit my guess is a circuit assembly error. Double check all your connections and polarity (such as the LEDs) as compared to the schematic.

What is the flash rate and duration of the one LED that flashes?
 

Big Sammy

New Member
It flashes at the appropriate rate. I tried changing the value of the resistor, and also the capacitor, and the flash rate changed accordingly. I have a picture of the breadboard circuit from the same site, and followed it exactly. Everything seems to work, except only the one LED lights and flashes. Although the circuit is for 10 led's, I'm only using 5, as I fried 5 yesterday and can't get more until tomorrow...would that cause the problem?
 

audioguru

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Every 555 circuit I have seen and used have pin 4 connected to the positive supply voltage. It is mentioned in the project's text.
Maybe most of your LEDs have their pins connected with backwards polarity. Some cheap LEDs sold on E-Bay have their case and their pins wrong.

Add a 100uF capacitor from the positive supply voltage to ground to keep the battery voltage from jumping all over the place.
 

ronsimpson

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How does the LED on pin-2 light? Is it on 50% of the time or 10% of the time? I am trying to find out if the counter (CD4017) is counting 0,1,2,3,4,5,6,7,8,9 or counting 0,1,0,1,0,1,

The LED on pin-3 should light first!

On the 555 pin 4 (reset) usually it is connected to pin 8 VCC. I looked at the details and the pin appears to pull its self high if left unconnected. I think it is OK to leave it NC but the new CMOS versions of the 555 will have problems with a NC on pin-4. Please connect it to supply just to be safe.

The CD4017 is very sensitive to unconnected pins. The reset pin might be floating. Double check the pins that connect to gnd and supply. (use ohm meter is you have one)
 

Big Sammy

New Member
I added the other 5 leds, and now they all light up and flash. The new problem (of course) is the first 4 led's light together, then flash and the rest of the led's light sequentially as they should. They the first 2 light together, then flash and the rest then operate as they should. Why would the first 4 (connected to pins 3, 2, 4, 7 of the 4017) light together?
 

Big Sammy

New Member
In looking more closeley, pins 3,2,4,7 flash together, then 3,2,4,7,10 flash together, then the rest follow sequentially except pins 1 and 11 come on, but 11 stays lit while 5, 6, 9 sequentially flash. Then it repeats. Really bizarre. Obviously, I'm new to all of this, and an getting a little frustrated!!!
 

audioguru

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Your CD4017 is not reset when the power is applied so its internal counters begin randomly scrambled then sort themselves after a few clock pulses.
Did you add the 100uF across the supply like I mentioned?
Did you try a new battery?
 

Big Sammy

New Member
Yes, I added the cap. across the positive and negative power...there was no noticable difference...I'm running two different kinds of led's...I think with marginally different forward voltages (5 are 2v-20 ma, and 5 are 1.9 v-20 ma)_and I've noticed that 5 of them (the 2 v) aren't sequentially flashing, and that the 1.9 v ones are flashing properly....could this be the issue?
 

ronsimpson

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I think all the diodes are in backwards. (or some of the diodes are in backwards)
I would use a ohm meter and check each LED.
If that does not work then cut free all the LEDs. Use one LED and a 1k resistor and touch each output. I bet each pin lights 1/10 of the time.
 

audioguru

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If one LED is connected backwards then it would cause the outputs of the CD4017 to fight with one output pulling the forward-biased but backwards LED low while an output is trying to drive its forward-biased LED high.
 

ronsimpson

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Power supply is 9V which is higher than the breakdown voltage on an LED. Pretend all LEDs are backwards.

Nine of the outputs are at 0 volts. One output is at 9 volts. This LED is backwards so no light and we loose 5 volts. The common connection of the LEDs is at 4 volts. All the rest of the LEDs have one connected to 0 volts (CD4017) and the other end connected to 4 volts. So current is going the right way for these 9 LEDs.

Why do the LEDs light in blocks, depending on their turn on voltage?
Remember: never connect LEDs in parallel. From pose #9 some of the LEDs turn on at 2 volts and some at 1.9 volts. When most of the LEDs are connected in parallel only the low voltage LEDs will turn on.
It may be hard to see that the LEDs are in parallel. One end is common. The other end goes to the CD4017 which connects them to ground. parallel.

To complicate thinks more, it is likely some of the LEDs are in the right direction.

The breakdown voltage is different on the two typed of LEDs used.
 

Big Sammy

New Member
Thanks to all of you that replied to my thread....I'll check the polarity of each led before soldering to make sure they are all aligned properly....I'll let you know how it ends up once I've put it together....I appreciate all the advice and suggestions...you are all making it easier to be a noob at this electronics stuff!!
 

ronsimpson

Well-Known Member
Most Helpful Member
Wire in one LED at a time and watch to see if it works right before adding another LED.
 
If there is no circuit assembly error, and the solderings are good, maybe the 4017 is damaged. Note that it's very susceptible to the electrostatic charges.
I always use a bracelet connected to the ground when I work with 4017.
 
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