I need help with 3-bit counter on multisim!

[Elec_you]

hi everyone i am a beginner in electronics, i want to make a 3_bit binary up counter using JK-FF ( 7476) after constructing it in multisim i can't simulate it because it displayed an error.
I am a new user of Multisim so i can't resolve this problem, anyone can help?

I attached the file containing my circuit called circuit1 so please give a look!
I copied the text displaying the error :

( i copied a part where i think there is a problem , sorry for that because i don't know exactly where the error is dispayed)

Output from instrument analysis
| | TRAN: Timestep too small; initial timepoint:trouble with (led_mod:xled3) d1:xled3
| | doAnalyses: timestep too small

 

[MikeMl]

I know nothing about Multisim, but I have the following observations: You need current limiting resistors in series with the LEDs, otherwise they will short the outputs of the flip-flops. The clock source must be a pulse signal which never goes more negative than zero V, and never more positive than 5V.

 

[Elec_you]

I know nothing about Multisim, but I have the following observations: You need current limiting resistors in series with the LEDs, otherwise they will short the outputs of the flip-flops. The clock source must be a pulse signal which never goes more negative than zero V, and never more positive than 5V.

Thank you Mike for your reply, i added the resistors in series to the LEDS and now i can simulate my circuit but there is still a problem ( i think in the part of the clock) the LEDs go ON and stay in that state and don't scan through the counter states.

 

[crutschow]

We can't help you with that since you haven't shown the clock waveform. So what is the clock signal amplitude, frequency, and type (shape)?

 

[Elec_you]

We can't help you with that since you haven't shown the clock waveform. So what is the clock signal amplitude, frequency, and type (shape)?

Thanks for your reply , i've set the function generator to 2.5 Vp square wave , 2.5 V offset to get 5 v not negative square wave

 

[crutschow]

And the frequency?

 

[Elec_you]

I tried 1Hz then 1 Hz

 

[crutschow]

I simulated your circuit in Electronics Workbench, which is the predecessor to Multisim, and it counted ok, so your circuit diagram appears to be correct.

What resistor value do you have in series with the LEDs? Have you looked at the output with the oscilloscope? Try disconnecting the LEDs and looking at the output with the probe indicators or the oscilloscope.

TTL outputs are better at sinking current than sourcing, so it would be preferable to connect the LED anodes through resistors (try 300Ω) to +5V and connect the cathodes to the -Q outputs instead of the Q outputs (to still light the LEDs when the respective Q output is high).

 

[Elec_you]

Thank you for the explanation, but still i don't see the leds turning on and off to doing the counting even the oscilloscope shows that everything is ok, i think the problem is with these leds i have chosen