Simple Flip Flop simulation. Blinking LED problem

[Lawrey]

Hi!

I've got a problem about simulating a simple flip flop circuit. The output is connected to an LED for indication of the output. However, the LEDs just keep blinking which gives me the impression that the output is not stable (which from my analysis, the output should be stable). I'll post a picture and the multisim file for you to see the circuit. Thanks! 🙂

[Lawrey]

Sorry for double posting. I don't know and can't find how to edit my post. I just want to add that my prof successfully simulated this circuit although I haven't seen fully how he did it. I will attach below the picture of my simulated circuit in multisim. 🙂

[Lawrey]

Aaah sorry now for triple posting! I would just like to say that I found out what's the problem. It's just that the voltage rating of the gates I used is 15V while I use 5V for input.. So careless of me. -.-

[johnsold · Accepted solution]

I do not have Multisim so I cannot comment on the simulation issues directly.

I was about to mention the 5 V/15 V issue.  Also, a typical LED operates at 1.6 to 3 V depending on the color (and thus the semiconductor composition). Attempting to drive LEDs directly from the output of a 4011 will cause problems (in a physical circuit). The Ioh source current for the 4011 is 8.8 mA typical and 3.4 mA minimum at 15 V, under the assumption the Voh = 13.5 V.  The current at a typical LED voltage might be as high as 32 mA (dissipating ~0.5 W!) until the device goes up in smoke! But with the voltage that low, the feedback logic will not work because the output voltages will always be below the threshold of the other gate inputs.

The simulation may not accurately reflect the current and voltage limits of the outputs.

When I put together a quick simulation of the logic in LabVIEW, I find that there are unstable states. This logic includes a propagation time effect but assumes ideal logic levels.

Lynn

[Lawrey]

Thank you for your response!
I can simulate my circuit properly now although some combinations gives out unstable outputs. Since I don't always simulate at multisim, I never considered the component's specification for these logic gate combinations. Thanks to you I will now put into consider those things when simulating 🙂

[johnsold]

Simulations are no better than the models used. Many times the limitations of the model (especially commercial models) are not explicitly stated.  

It is often necessary to simulate some very simple test circuits to validate the models or to find their limits.  For example a circuit with one NAND gate with both inputs tied high and the output connected to the LED and a second gate with both inputs tied low would tell you a lot about how the models for the gate and for the LED handle an out-of-specification condition for each device.

Another interesting test might be to look at the 15 V logic with 5 V inputs and check the simulated voltages and currents at input and output.

Lynn

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