Problem w/ j/k flip flops?

[xerxes]

I built a circuit with a 555timer driving a 4-bit syncronous counter.
after I constructed the first two bits using an ls7476 it worket fine. When I added the 3rd and 4th bits the 1st two bits would not register on my logic probe but the 3rd and 4th did. If anyone has some insight it would be greatly welcomed.

Thank you

[audioguru]

Hi Xerxes,
Welcome to our forum. Your circuit has a few problems:
1) I hope that your "counter" ICs are not SN7476, which operate at 5V, and you are using 9V. An SN74C76 is fine with 9V.
2) You don't have a syncronous counter. Your circuit is called a ripple counter since the clock signal ripples through all stages. A syncronous counter applies the master clock (your 555 output) to the clock input of all flip-flops so that they are in sync, then the output of the 1st flip-flop drives the parallel-connected JK inputs of the next, then its output drives the JK of the next, etc.
3) Your schematic has an error where the Q-NOT output of the 3rd flip-flop is connected to ground.
4) Your circuit has the flip-flop outputs as logic signals and as LED drivers. A flip-flop output can't do both, since the LED will load down the output so much (and maybe even destroy it without a current-limiting resistor) that it won't function properly as a logic output. You should use a separate buffer circuit (transistor) to drive each element of the LED display.

[xerxes]

My goal is to use the counter through a 4-16 decoder to drive 16 led's in sequence at around 5-7Hz. I cannot seem to figure it out anyone who knows how to do this, I really could use your help.

Thanks :-\

[audioguru]

Hi again, Xerxes,
I lost my 7400-series text book about 25 years ago but I will still try to help you.
There are many ways to sequentially light 16 LEDs at only 5Hz, using a 9V battery for power. I don't know why you selected 74LS76 dual JK flip-flops to make your counter when they need a regulated 5V supply, use a lot of current to operate and are made to operate at up to 30MHz. For your application, I would use the 4000 CMOS family, which operates directly from a 9V battery and uses very little current. An oscillator/counter IC is available but also counter/decoder ICs may be used.
If you wish to continue using your 74LS76 flip-flops, the best place to start is with their datasheet, here:
ttp://www.ece.mcmaster.ca/~djones/2di4/datasheet/TI/sn74ls76.pdf

Important specifications and ratings that apply to your 74LS76 circuit are as follows:
1) Page 6 states that the maximum recommended supply voltage is 5.75V, and page 4 states that the absolute maximum supply voltage is 7V. So if you connected them directly to a 9V battery then they may be destroyed!
2) They are made to provide fast logic output voltages, not for directly driving LEDs or short-circuits. Page 6 states that the maximum recommended output current is 0.4mA for a logic 1, and 8mA for a logic 0, and the note for Ios on page 6 states that "only one output should be shorted at a time, and the duration of the short circuit should not exceed one second". So your short on the output and direct LED connections again may have destroyed them!

If you replace your destroyed ICs, build a regulated 5V supply for them, disconnect the shorted output and use current-limiting resistors for the LEDs, I will continue this discussion with you. If you decide to use the CMOS family which is better for your application, I will help you with that, too.

[Kevin Weddle]

What kind of sequence are you trying to create?I don't see anything meaningful to the sequence. Although I looked at the site "How Stuff Works" and they had the same circuit. I don't see the sequence. It may be a repeatable sequence that has sixteen states but not incrementally. I would be interested and I will continue to find out for you.

I found it. The sequence starts with all flip flops preset. The first flip flop is the LSB because it changes more often. The count actually goes down instead of up. There appears to be a clock cycle error in one of the help sites.

[audioguru]

Hi Kevin,
Xerxes's circuit doesn't have a decoder connected yet. So his LEDs will simply flash, on-off-on, etc., with each divider flashing at half the speed of the one before it.
With a decoder, he can make any sequence that he wants, but I think that he will probably make a sequential chaser.
Later on, when his sequence starts all jumbled, we will discuss adding "power-up reset".

[Kevin Weddle]

I would recommend using an integrated form of the counter. Using discretes is just a step in the wrong direction. However, discretes do work very good. The one thing I notice is that integrated circuits often come with things that are not wanted. But I think with any luck you could located a simple integrated counter. Simple integrated circuits are my goal too.

[audioguru]

(slightly off-topic)
Man, those were the days. I remember them well.
Long ago when I was designing very complicated circuits with transistors, SCRs and relays, then 7400 series, then 74LS00, then CMOS and 74HC00, we still had an ozone layer and no terrorism (but had to watch-out for those russians).
I even had cute drafts-ladies with mini-skirts. (Yeah, man.)
When the stinky (ammonia?) blueprints came back, they were huge and had to be kept away from sunlight.
Been there, done that.
Anybody else remember those days?

[xerxes]

Thank you , I have new ic's. what would a 5.1V zener do if i put it series with the battery?
I took a look At the How stuff works and they tied the set/clear to high. I tried it and it worked !. I have all 4 bits counting.this is what I would like to do. can the 7404 drive one or two LED's. Also could you please explain what you meant by current limiting resistors. are they needed to protect the LED or The output of my IC, and how is the size calculated.

Thanks for all your help

[audioguru]

Hi Xerxes,
I'm glad that you got you counter working. Now you must make a 5V power supply for it. Your 7400-series ICs use too much current for a zener diode and for a 9V battery. For a suitable 5V power supply, see:
How Stuff Works, How Electronic Gates Work, The Power Supply.

A 7404 is a logic inverter, not an LED driver. Also, a low output current 7404 is not the same as a high output current 7406 inverting buffer. The 7406 is designed to drive LEDs through current-limiting resistors.

An LED (and a zener diode) need a current-limiting resistor in series to reduce current to be within their ratings and to be within the ratings of the LED driver. The resistor is calculated with Ohm's Law. In your complete circuit, the JPG schematic is not clear enough for me to see how the LEDs are wired to the 74154 and to the 7406s.
I couldn't find your circuit at How Stuff Works.

[GPG]

Still have my 74xx book (TI, '76)
04, 16 mA sink at Vout .4V max
L and LS only 4mA
Quite capable of driving a LED

[audioguru]

Hi GPG,
Sure, a 7404 TTL IC can sink 16mA through an LED, but does its output voltage go high enough to turn it off? If not, then why not just add a diode or two in series with the current-limiting resistor?
The outputs of the 74154 can also sink 16mA, so why aren't the LEDs connected to it directly, with a single current-limiting resistor to +5V (and maybe a diode or two)?
Maybe a long time ago when this circuit was developed, LEDs were so dim at 16mA that the 7406s were added to get more drive. Remember how dim those old LEDs were? We used to joke that they work by making their junction glow red-hot! ;D ;D

[xerxes]

I got it working, Thanks for all your help, I used the 7404 because the outputs of the 74154 are inverted. anyway I think I got it......?
thanks again ;D

[audioguru]

Xerxes,
Good, you did a fine job.