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I am pretty new in Proteus and made a simple flip flop circuit as shown below.

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When I run the simulation, the circuit does not work as intended. Both of the LEDs light up at the same time. (LEDs should light up one by one)

Also, after I stop the simulation I get this error code.

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Why am I having this issue? Is it about the circuit itself that I am doing wrong or is it about the error code I am getting?

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  • \$\begingroup\$ I tried with two different NPN transistors like BC547 and BC548. And it works !! \$\endgroup\$ Dec 25, 2020 at 15:41

4 Answers 4

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I had exactly the same problem.

Normally, Proteus triggers (energizes) both transistors at "exactly" the same time when the simulation starts. This type of initiation leaves the simulated circuit in a certain "equal" situation. For an A-stable circuit, it is always assumed that the circuit starts with one transistor in the "off" state and the other in the "on" state.

To do this simply add a "switch" to one part of your design. This will be left on the "off" state before you start your simulation. Then turn the switch "on". Everything will go as expected.

The working Astable circuit in Proteus

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Two suggestions....

Place a small valued capacitor from one of the transistor bases to GND. This will imbalance the circuit so that it will start to oscillate at startup.

Simulations are often too prefect in behavior and the two sides of the vibrator will come up balanced. In a real world this would be much less likely becasue the two transistors, two capacitors and the resistors would have a natural variation of characteristics that cause the circuit to imbalance at startup.

Secondly do consider moving your LEDs to be in the collector circuits instead of the emitter circuits.

Edit: I would also suggest changing R3 and R4 to 10K to start with. I found that with the 2.2nF capacitor on one BASE and with the above changes that the simulation stated to toggle. Note I also made the 9V supply switch in with a rise time.

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  • \$\begingroup\$ I did all of them but it did not work any way. I think the problem is about Proteus itself. I tried the circuit in Lt-spice and I get a square wave graph as expected. I assume you also used the same program. I suppose I can not get a real time simulation in LT-spice. \$\endgroup\$
    – Orkun
    Dec 7, 2014 at 14:34
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I tried this circuit in proteus and exactly the same results as you said. the resistor and cap values are too low, and oscillation is too fast. change 10K resistors to 100K and cap to 10uF and it works as expected.

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All same components in these simulators are exact clones of each other, contrary to the real world where there isn't any same component exactly equal to each other. Only in the logic world of simulators you have component cloning and so your simulation fails. Try to explore the MC (Monte Carlo) function in LT Spice to add some randomness to the circuit.

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  • \$\begingroup\$ There are also no parasitics in a spice simulation (although certian components in lt spice do include parasitics) which could make or break simulating a design like a DC switcher \$\endgroup\$
    – Voltage Spike
    Aug 18, 2016 at 21:21

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