Our Professor Gave us this problem to design a 7-Segment display decoder from scratch, just using AND OR NOT gates.
After i finished doing all the truth tables and drawing the logic diagram,
i decided to simulate it first before getting into any hardware stuff.

so i used my decoder to display numbers from 0 to 9 in decimal and for this i needed a 4-bit binary counter
(the one that came in my mind was the 74ls93n, since I already had bought one from our local store), but i should test the counter first
before implementing it in any circuitry to make sure that whenever i encounter errors in the (0 to 9 counter), i should know then that
they are not from my counter.

i used Multisim simulator and looked in the 74ls93n datasheet from Texas Instruments for the appropriate pins configurations
to make it count from (0 to 9).

Now, after i found every thing i need, i used Multisim and started connecting the outputs of 74ls93n
to drive 4 LEDs instead of 7-Segment , just to test it first, and ran my design.
well, every thing just functioned the way i thought and the LEDs started counting from (0000)[0] to (1001)[9].

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the simulation file is in the attachments..
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Now, i took my counter and decided to test it for real this time on my breadboard and connected every thing just the same way i did it in the picture above with the simulator.

i checked my connections again to make sure that i connected everything right and pressed the switch ON and OFF to simulate the clock pulses for the counter (since i don't have timer or pulser) just the same way you see it in the design above, but the LEDs just went crazy and started counting in a sequence that i couldn't fully understand
and had nothing to do with any sequence in any number system.
sometimes it just repeats the same number and i don't know what exactly went wrong, i checked my connections again and again and again and they were the same ones i did in the simulator and worked there just fine !!!

so, anyone has any idea why did this happened or why it acted that way (just for the record, i used another counter from different manufacturer and it acted the same way)

 

Your circuit is suffering from the dreaded "switch bounce".

Read up on the problem and various solutions here:
http://www.ganssle.com/debouncing.htm

 

Yes, I have to agree. It sounds like switch or contact bouncing.

Also, I dont see any decoupling capacitors on the IC. That can lead to odd behavior also.

 

 

The 7400 series of ICs really do need a bypass cap from the VCC to ground as close to the IC as possible. Anything between 0.1 to 0.01 uF is usually fine.

 

Along with what Marshallf3 said (0.1uF/100nF poly metal film or ceramic preferred)
you must also have bypass capacitors as close as possible on the 7805 regulator from IN to GND and OUT to GND; 0.33uF on IN and 0.1uF on OUT, or oscillations are likely to occur.

 

By the way, "switch bounce" is not always "dreaded".
I used it as a random number generator for gaming.