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Inputs on a 4017

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Wokyerdog

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I was digging through our electronics drawers and i found a 4017 chip. I looked up the pin assignments and found a simple circuit where a 4093 was set up to pulse the 4017 to make the LED's "chase".

In the circuit they had the reset and the enable going right to the neg and all the appropriate LED's/resistors going from the outputs to neg.

When I set up the circuit, I had everything hooked up as they did with the exception of the 4093. (We jsut didn't have one in the lab). I also used a 5v power source to pin 16 and 8 appropriately. For the clock input, I tried using a tactile button to "pulse" a signal to make the lights....well....do something at least but it didn't work. Why is this?

-wok
 
hi,
Please post the circuit diagram of the 4017 , showing your connections.
 
I think the most likely explanation is that in the high state the standard CMOS 4017 typical output current is only 0.36mA, as per the datasheet, i.e. not enough to drive a LED. You would need a 74HC4017, or a buffer stage, to drive a LED. I don't see how 'they' managed it otherwise.
 
A 4017 is perfectly capable of driving a LED, even at 6v.

Here's a clever little circuit for a 4017:
**broken link removed**
 
@colin55
We live and learn. The 0.36mA figure quoted seemed surprisingly low. Does it vary from one brand to another?
Your posted circuit nicely makes the point that only one resistor is needed as the current limiter for all 10 LEDs (since only one at a time is lit). Obviously 'they' didn't appreciate that point!
 
Do you have a pullup or pulldown resistor on the clock pin? as the switch will pull the pin one way (hi or lo) but it needs the resistor to pull the pin the other way or the clock pin will be floating when the switch is open.
 
At the time of reading this, im not sure what a pull-up or pull-down resistor is. I'll look it up.

As far as my circuit is concerned, It is wired exactly like colin's circuit with the exception of the input to pin 14. In mine i have the 5v going to the tactile switch, then to pin 14. and each led had their own resistor. (Yes, i now realize the inefficiency of using 10 seperate resistors.)

So why won't "Pulsing" the chip using a tactile switch work?

-wok
 
So why won't "Pulsing" the chip using a tactile switch work?
See post #6. The resistor in the corner of that circuit could act as a pull down for you with your switch. If it works, you should get multiple triggerings of the clock input due to switch bounce. There's another term for you to look up.
 
Texas Instruments have more detail on their CD4xxx Cmos datasheets than any other manufacturers. The minimum output low current with a 5V supply and only a 0.4V drop is 0.36mA (LS-TTL levels) when the IC is very hot (125 degrees C) and is a little higher (about 0.5mA) at room temperature.
But an LED is not only 0.4V, it is 2V to 3.5V so the graph on the Texas Instruments datasheet shows a typical output current into an LED when the supply is 5V and the IC is at room temperature of 3.5mA.

A CD4017B made by one manufacturer has the same spec's as a CD4017B made by another manufacturer. But some ICs are fakes or are defective "sweepings from the floor".
 
well i got the chip working. Originally i figured out what you guys mean by pull down/up (i think i used down) with the tactiel switch and the led's cycled properly. Although i figured out waht you guys meant by switch bounce. Is there anyway to prevent this?

Anyway, i looked up a flashing led circuit with the 555 chip and used that for the pulse and rearranged the leds to jsut go back and forth (like knight rider) and it's working fine. I even dug up a POT to adjust the rate. I know all this seems simple to you guys but im ecstatic that i figured it out :)

looking through the theory tab of your site, ive determined that i used the astable set up of the 555.

-wok
 
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