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I have been working on a simple circuit which performs XOR operation.

The problem is that I am seeing a noise-like graph when both inputs are set to 1 (5V) and I have been doing a research on what it could be but I have failed to find any explanation.

At first I was curious to see whether if the simulation program (Falstad simulator) added computer generated noise for some kind of "close to real life testing" purposes(?) but I figured that it would be unnecessary and there was no source to prove such point.

What could this graph possibly be indicating?

Note: Although the transistor on the schematic is displayed as BC547, it is not chosen to be this exact transistor in the simulator, instead the simulator uses default NPN transistor characteristics, not staying loyal to characteristics of this spesific transistor.

Here is the circuit diagram:

enter image description here

Here is the graph of Vout (which is V_LED):

enter image description here

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    \$\begingroup\$ It claims to be at the microvolt level. Probably rounding effects in the sort of matrix calculations simulators do. Changing the time step size may improve things, but why bother? \$\endgroup\$
    – user16324
    Commented Jun 24, 2020 at 17:48
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    \$\begingroup\$ You can eliminate the bridge, R1, and Q1 from your circuit and still have a working "XOR" behavior. See Wikipedia. \$\endgroup\$
    – The Photon
    Commented Jun 24, 2020 at 17:59

1 Answer 1

Reset to default
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You may be seeing numerical error in the computations...the voltage you see is very, very small. I would suggest using a more typical circuit simulator, such as LTspice, and a better transistor model.

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