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I am trying to understand SR flip-flops from a book which uses the above circuit to describe different states that this circuit can get in. Initially, both the switches are open and the current flows from the output of left NOR gate( highlighted in red in the image). However, I am struggling to understand as to why both the inputs to the left NOR gate are zero and the right NOR gate aren't since their inputs are dependent on each other's output. In other words, why can't we assume that since in the beginning we have no input voltages to right NOR gate the output should be 1 which will mean that the current will be between the wire connecting output of the right NOR and the input of the left NOR?

  • \$\begingroup\$ i think that the book author is assuming that an incandescent bulb is being used ... the filament resistance is very low when cold ... at powerup, that would pull the output of the right gate low for a brief moment, just long enough for the left gate to supply a high to the right gate input \$\endgroup\$
    – jsotola
    Jul 12, 2020 at 16:50

1 Answer 1


When an SR latch like this "wakes up" as power is first applied, it will (in practice) be in either the state where the output is high or the output is low. We can't look at a circuit like this and know what its initial state will be. My hope is that the book says somewhere "suppose that the output is initially..."

I have a big problem with this example because when the switches are open the input voltage for the NOR gates will be unknown. A proper schematic would have an explicit pulldown resistor from each input to ground.

I think you also have a misunderstanding about the flow of current. In general, no current (actually, negligible current) flows into the input pin of a gate. So, the left gate never supplies the current that makes the bulb light up. This current always comes from the right NOR gate. How is this possible? Each gate has implied connections to a power supply and ground, so when the bulb is lit the current is coming from the power supply through the right hand NOR gate.

  • \$\begingroup\$ unfortunately, the book fails to assume a wakeup state. I guess my confusion stems from that. The name of the book is CODE by Charles Petzold written to understand computer hardware and software, which definitely targets layman and therefore glosses over many aspects of electrical circuits. Actually, what I meant was that the only current that flows in this circuit as per book is from the output of the left NOR gate which acts as an input to the right NOR and therefore the bulb doesn't glow. \$\endgroup\$
    – yash471994
    Jul 12, 2020 at 15:12
  • \$\begingroup\$ Now when I notice carefully, Don't you think that the asymmetrical nature of the circuit might have an influence on the initial state? \$\endgroup\$
    – yash471994
    Jul 12, 2020 at 15:26
  • \$\begingroup\$ Find a different book. And, yes, if there is asymmetry in the circuit it can affect how it will wake up, but the presence or absence of a load is not the only factor. The figure is a cartoon, and a poor one at that. \$\endgroup\$ Jul 12, 2020 at 15:46

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