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CD017 to use as multiplexer

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bannier

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Could someone please help me with the following cct?
I am using a CD4017 clocked by a 555 one shot monostable oscillator. I need the 4017 to trigger 8 LED's independently by push buttons. Each LED,depending on which of the 8 push buttons are used,must have an ON time of approx 2 seconds.The outputs of the CD4017 must not be sequential.
Best regards....
 

betwixt

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Thats a very confusing description.

A 'one shot monostable oscillator' is a contradiction of terms, is it a 'one shot' (monostable) or an oscillator (astable).
The CD4017 is a sequential decade counter but you want non-sequential outputs.

My best guess is you want exactly the opposite, something that senses 8 random inputs and when an input is 'live' it should light the corresponding LED for 2 seconds. Am I correct?

Brian.
 

bannier

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Thats a very confusing description.

A 'one shot monostable oscillator' is a contradiction of terms, is it a 'one shot' (monostable) or an oscillator (astable).
The CD4017 is a sequential decade counter but you want non-sequential outputs.

My best guess is you want exactly the opposite, something that senses 8 random inputs and when an input is 'live' it should light the corresponding LED for 2 seconds. Am I correct?

Brian.

Hi Brian
Thanks for your reply. You are correct. This is exactly what I'm trying to achieve. Do you have a schematic for this function?
Best regards.
 

bannier

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Hi Brian
Thanks for your reply. You are correct.This is exactly what I'm trying to achieve.Would I be using an astable or a monostable oscillator to clock the CD4017? Do you have a schematic for the complete circuit?
Best regards.
 

betwixt

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I'm not sure you are going the right way about it. The CD4017 is a sequential counter, it has 10 outputs (plus a carry output) but only one input. Each time you pulse the input the current output turns off and the next turns on. That would make it difficult to use when you have 8 inputs and the output is not sequential.

To generate a fixed time delay you would use a 555 in monostable mode. Monostable means "stable in only one state" so it will return to stable condition and stay there until re-triggered.

Before going further, can I ask why you chose those parts because they are not the best ones to use. I also need to know what you want to happen if more than one button is pressed, for example if you press button 1 and LED 1 comes on for two seconds but before the two seconds has finished you press button 3, what should happen? Should LED 1 go out immediately or should it continue until it's 2 seconds is up and LED3 start it's own 2 second period?

Brian.
 

KlausST

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Hi,

On each button place a capacitor with pullup resistor. Feed this to a comparator or a schmitt trigger gate. Control the LED wite the output of the gate.
With the correct values of R and C.... Press a button and the corresponding LED is on for 2 seconds


Is this what you need

Hope this helps

Klaus
 

bannier

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Hi Brian
Preferably I would like LED 1 to time out after 2 seconds without LED 2 being able to light up even if it is activated accidently.If two buttons are mistakenly pressed,the first LED which was activated should light up and LED 2 should not light up until LED 1 has timed out after 2 seconds. LED 2 would have to be activated again and also time out after 2 seconds.This would have to be the same operation for all 8 LED's.Each of the 8 LED's would have to be activated independantly without affecting the rest of the LED's.
Best regards.

- - - Updated - - -

Thanks for your reply Klaus
I'll check out this theory in the morning.
Best regards.
 

betwixt

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So what you want is each to be exclusive of the others. It can be done with discrete logic but not with a CD4017, you need a fairly complicated circuit in which any active output causes the other inputs to be inhibited. My first thought is to use an 8-bit "priority encoder" followed by a binary decoder but it is far more complicated than a 555 and a counter!

Being honest, this is a job for a simple microcontroller, although it can be done other ways, a micro can do it in one device but you will need to write some simple software.

Brian.
 

bannier

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Thanks for your suggestions Brian,I'll check into it.
Best regards.
 

bannier

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OK Brian,I see what you're saying. (I was going ass about face)!
I going to try using a HCF4532 encoder to a 74C138 decoder for dedicated switching outputs to the LED's.. I'm a little in the dark about the timing of 2 second output to each of the LED's though.
Best regards.
 

betwixt

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If only one LED is on at any time, use the 555 to provide the power to one side of the LEDs and the 74C138 to the other side, when a button is pushed you trigger the 555 so it only produces power for the two seconds before removing it again. Basically you use the 555 like a power supply that only turns on for two seconds before turning itself off again.

I'm not sure such a thing as a 74C138 exists, if it did, the current it could output would be very low and probably not enough to light LEDs brightly. Try 74HC138 instead but bear in mind it still has low output capability.

Brian.
 

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