Trouble with 25 LED Counter Setup - Need Advice

[ricker24]

Hello, I have constructed a 25 Led counter and am having a little trouble. I have cascaded 3 4017's with a 4011 according to how the data sheet shows. I have added a pushbutton along with a 4001 to debounce for a clean count. I get a good clean count with one push of the button from LED 1 to LED 17 but then have to push the button 2 times to get LED 18 to light. Does anyone know if this is normal or if it might have a fix because I do not see anything anywhere that tells me this is normal. I am powering this with 6 volts DC. I have a rough draft of my project that I can post if I am allowed. Thanks for any help I can get...

 

[vk6kro]

Sure, post the schematic if you can.

A 4017 has 10 outputs and one of them has to be "high" at a time. So, to get 27 different outputs, they might stop each chip counting at "9" but not use this output to drive a LED.

This may be why your counter is taking an extra pulse to get past 18.

It might do it at 9 as well.

 

[ricker24]

Thanks for your reply. The 4017's according to the data sheet will have 9 outputs on the 1st one and 8 outputs on the second and third when cascaded but I did not see anything that said there would be a dead spot on any of them. I guess it may not really be what you could call a dead spot because all 8 outputs do light an LED it is just that you have to push the button twice to get output one of the 3rd 4017 to light Led 18. Thanks...

 

[uart]

Make sure you haven't left any inputs floating

Make sure you haven't connected to the wrong clock phase (inverted clock) on last stage.

 

[vk6kro]

The problem with 4017's is that at least one of the outputs of the chip has to be "high".

So, this would result in 3 LEDs being lit if they had all the output pins connected to a LED.

But, presumably you have only one LED lit at anyone time.

So, the 10th output of each chip, "9", must go high and stay high on pulse 10 but not be connected to a LED.
This would be physical pin 11 of each chip, so you could check this pin with a multimeter or a spare LED to see how it operates.

That would be why you need an extra clocking pulse. You have to clock the first chip from "8" to "9" and then clock the second chip from "9" to "0".

Just guessing without a circuit, though. You attach it by clicking the paper-clip thing at the top of the editing screen.

It should be fairly easy to do this with shift registers instead of 4017s.

 

[ricker24]

Here is a rough drawing of what I have done and yes one Led is lit at a time. Thanks...

 

[ricker24]

Thanks to everyone for all of your help. The problem is solved and it is working perfectly now. I am a member of other forums and someone in one of the other forums saw that I was using the wrong gate. I switched the gates and it is working exactly the way I wanted. Thanks again everyone...

 

[vk6kro]

That's good. Which gate was wrong?

There is a peculiar arrangement on pin 11 of the right hand 4017. Pin 11 is an output, but the switch pulls it high regardless of its logic state. Since it would be low most of the time, pushing this switch would possibly destroy the chip.

 

[ricker24]

I have pushed the switch many times and it does not seem to have affected the chip yet. Cascading ties pin 11 of the 4017 to the right back to the reset 15 pin of the 1st 4017 and I just added the switch so I could reset the count any time I wanted. Is their a better way to do it? Oh ye and I was using a NAND gate for the cascading instead of an AND gate.

That's good. Which gate was wrong?

There is a peculiar arrangement on pin 11 of the right hand 4017. Pin 11 is an output, but the switch pulls it high regardless of its logic state. Since it would be low most of the time, pushing this switch would possibly destroy the chip.

 

[vk6kro]

Maybe the output is protected, but it is better not to do this sort of thing.

If you lift the connection to pin 11 and put a diode in there, with its anode connected to pin 11, then the diode should be reverse biased if the reset button is pushed while pin 11 is low.

There will be a small drop across the diode, so a small Schottky diode like a BAT85 or similar could be used to minimise this.

 

[ricker24]

Sounds good to me. I will give it a shot. I should be able to use a 1N914 shouldn't I? Thanks again for all your help.

Maybe the output is protected, but it is better not to do this sort of thing.

If you lift the connection to pin 11 and put a diode in there, with its anode connected to pin 11, then the diode should be reverse biased if the reset button is pushed while pin 11 is low.

There will be a small drop across the diode, so a small Schottky diode like a BAT85 or similar could be used to minimise this.