Help!! 74ls47 & 90 Counting Problem

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Here is a schematic of the circuit I am working on.
My problem is this. When i push the button to advance the display it only goes 2, 3, 5, 6, 8 sometimes a 0. Why and what can I do to fix it.

Also I wanted to see it count by itself so I hooked up a 4047 set it in astable mode Shouldnt this provide the 7490 with a clock signal to advance the display or is that something different??? I have a picture of how the 4047 was wired up. although a fairly crude drawing. and my schematic of the circuit I have built
 

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Well, without looking into the time constants and such I would guess contact bounce...

D.
 
But I have a Debouncer worked in there. I dont think it is contact bounce. My results are always the same 2,3,5,6,8 execpt almost once every ten times a get a 0.

If it were contact bounce at least once out of all the times I have pushed the button you would think it would stop on some other number at random. Now if it is contact bounce how could i fix the problem. I am already using a 4049 here is a schematic of the Debouncer. I have used the one on the left.
 

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You are mixing antique old LS-TTL ICs that have a very high input current and Cmos which has a low output current. They don't mix very well.

The debounce circuit is supposed to use a Schmitt-trigger IC, not an ordinary inverter. The C14 is Schmitt-trigger but doesn't have enough output current to drive a TTL counter. The 4049 has enough output current but it is not a Schmitt-trigger.

Your circuit doesn't have supply bypass capacitors at each high current LS-TTL IC. The supply voltage will bounce all over the place each time an LS-TTL IC switches its high current if no supply bypass capacitor is used right at the IC.
 
ok then so either change my circuit over to cmos or use another type of switch bounce suppresion...
Thanks For The Info
 
And don't forget the bypass capacitors that Audio mentioned.

You may see strange results like those you described without them.
 
hi what,

Looking at the 4047 spec sheet, states that the min Tcap >100pF , you are showing 47pF, also you have Tres = 10K, which is the lowest recommended value.

The osc freq must be 'very' high ?.

Use a 2K2 pullup on the output of the 4047.

Where are you connecting the 4047 output, to the 74LS90 divider i/c ?



EricG
 
Last edited:
ericgibbs said:
Use a 2K2 pullup on the output of the 4047.
Click to expand...
No, then it won't work.
A Cmos output is push-pull with a minimum of 0.44mA available when it has a 5V supply. A 2.2k pullup resistor is not needed and would have an excessive current of (5V-0.4V)/2.2k= 2.1mA.

Actually, a Cmos output can drive a single 74LS input since its max input current is 0.4mA.
 

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Yes eric I did have it hooked up to the 74ls90 Clock Input. Thanks everyone for your help. I changed my debouncer to a RC type one and it works perfectly also added Bypass Capicitors. Now it works the way it should everytime. Now my next problem is finding a way to hook this circuit up to 12 volts so I can put it in my car. But that is a whole other question. I will do some research and see what I can figure Out
 
If you put it in a car, it will need to be protected from alternator "spikes".

About 2 or 3 years ago, an auto electics engineer wrote in this forum that these spikes can be up to 150 Volt.

So if you connected the circuit (assuming you use CMOS ICs which can work up to 15 Volt) directly to the 12 Volt car supply, it is likely to be blown by the spikes.
 
Ok heres something else thats interesting. I have this circuit on a bread board. Finally worked out all the kinks so I built the same circuit on another bread board. and hooked them both together. I uploaded a picture. Now the problem is this. The dispay advances at random. I have the clock input on 7490connected to pin 11 of the other 7490. Now Even when there not connected together. It will Advance witch would make me believe there is a problem with my power supply although the first one works no problem. or some intermittent connection on my bread board maybe. What do u guys think may be the problem, and how to solve it.
 

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Where are the bypass capacitors?

A circuit drawing would help us more than the breadboard photo.

Random results can be due to issues such as contact bounce, noise, a lack of bypass caps, etc.
 
Ok here is a schematic that shows only the 1 Display. the second display is hooked up the same way only there is no switch and RC debouncer off of the CP0 pin. this pin connects with Q3 witch is the D of 11 pin of the first 7490. Now circuit maker has the power and ground automatically ran so the Capacitor on the display is what they would look like if they were there. and also I forgot to draw in Pull Down Resistors. witch are on the clock inputs and it is a 470k Resistor.
 

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I'm attaching a schematic of my way to construct counters using 74LS90 and display drivers. I'm showing pin connections because I can think better that way.

Are you sure pin 12 of the '90 is connected to pin 1? Edit: I looked closer at your photo. This is right.

Also, pin 11 of the first '90 goes to pin 14 of the second counter. Edit: In a normally operating system, the "ten" display will be blank until it begins counting.
 

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The bounce suppressor is very crude.

I would use a Hex Schmitt 74LS14. Connect the RC & switch to the input of the 74LS14. The output of the 74LS14 goes to the input of the 74LS90.

This will make the rise and fall times small.

I don't understand what you mean by 470k pull down resistors.

A pull down resistor of 470k with TTL will do nothing. (470k would be acceptable with CMOS)

BTW you mean "which" not "witch". A witch is a lady with a pointy hat.
 
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ljcox said:
The bounce suppressor is very crude.

I would use a Hex Schmitt 74LS14. Connect the RC & switch to the input of the 74LS14. The output of the 74LS14 goes to the input of the 74LS90.
Click to expand...

Well It turns out I bought the wrong kind of Hex Schmitt so that at this point isnt an option. Now Crude maybe, but it works no problem on the one. It will count just fine with evey push of the button.

The problem is the second one. It is only connected by one wire to the first one. If I remove that wire it still counts at random. If I hook up my RC and my switch to the second 7490 when I push the button it acts even more fu*ked up. it changes twice once on the press, and another on the depress. Do you think maybe my 4.5v 500mA power supply isnt enough to feed my circuit.

ljcox said:
I don't understand what you mean by 470k pull down resistors.

A pull down resistor of 470k with TTL will do nothing. (470k would be acceptable with CMOS)
Click to expand...

I read on another post somewhere about the pulldown resistor. Why wouldnt it do anything with TTL? I thought it was there because TTL Doesnt like an open input. Or is it just too many Ohms.

ljcox said:
BTW you mean "which" not "witch". A witch is a lady with a pointy hat.
Click to expand...
Oops my bad, Didnt look it over before I submitted
 
A 74LSxx TTL input has a max current of 0.4mA. The highest acceptable voltage for a logic low is 0.8V. So the max value for a pull-down resistor is 0.8V/0.4mA= 2k ohms. It is preferred to have a low input voltage of 0.4V. Then the max value for a pull-down resistor is 1k ohms.
 
What said:
The problem is the second one. It is only connected by one wire to the first one. If I remove that wire it still counts at random.
Click to expand...
I assume that when you say "connected by one wire" you are not counting the gnd and +5 Volt lines.
It sounds like it is counting noise. Do you have enough bypass capacitors?

What said:
If I hook up my RC and my switch to the second 7490 when I push the button it acts even more fu*ked up. it changes twice once on the press, and another on the depress. Do you think maybe my 4.5v 500mA power supply isnt enough to feed my circuit.
Click to expand...
That sounds like contact bounce to me.

I doubt if the power suopply is the problem, 500 mA should be more than adequate.
What said:
I read on another post somewhere about the pulldown resistor. Why wouldnt it do anything with TTL? I thought it was there because TTL Doesnt like an open input. Or is it just too many Ohms.
Click to expand...
Audio answered this one quite well.
 
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