So I'm trying to learn some of the basics in electrical engineering such that I can better understand how computers work and I'm currently reading about circuits with switches, relays, different gates and how to produce different outputs based on inputs. I don't have a background in electrical engineering and this is the first book I've read related to this stuff so bear with me.

What I'm struggling with is this RS-latch. I understand the logic behind how the output is the input inverted and all that, and how to "activate" the different cords. I think that's basically what the circuit is meant to illustrate. However, what I don't understand is how the lightbulb is lit in this circuit. From my understanding there needs to be a closed circuit with a power source (battery) connected for the electrons to travel through the lightbulb to turn it on. Here, none of the batteries are connected to the circuit and the lightbulb is still lit.

What am I missing here? Is this just an illustration of how the concept of logic gates work and not a complete circuit?

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  • 2
    \$\begingroup\$ the illustration is not a schematic diagram ... it is a logic diagram ... it is a kind of a block diagram ... the power connections are implied ... they are not shown because they would only clutter up the diagram and they are have nothing to do with logic anyway \$\endgroup\$ – jsotola Aug 13 '19 at 16:28
  • \$\begingroup\$ I don’t see any batteries. \$\endgroup\$ – Andy aka Aug 13 '19 at 16:39
  • \$\begingroup\$ @jsotola: that is the answer. Please paste it into an answer and it'll be good. \$\endgroup\$ – TimWescott Aug 13 '19 at 16:55

This is a very conceptual cartoon diagram of how a logic circuit works. The aspect that is most confusing for you, I think, is that the diagram omits the connection of the power supply and ground to each of the two NOR gates. So, when the output of the bottom NOR gate is high, current will flow from the power supply, through the transistors inside the NOR gate, through the lamp, and then back to the common ground.

The diagram also doesn't show proper connections to the switches and the NOR gate inputs. If the switches connect the inputs to the power supply voltage then there should also be a resistor from the NOR gate input to ground, to pull the input low when the switch is open.

Finally, simple logic gates can not normally drive enough current to light an incandescent bulb. You shouldn't expect more than a few milliamperes from the gate output pin, which might be enough to illuminate a small LED.

  • \$\begingroup\$ Thank you for your answer! Indeed, the missing connection of the power supply and ground to the NOR gates made it very confusing. Looking at this as a potentially complete circuit made it very hard to grasp, but now it makes more sense! \$\endgroup\$ – whatshappening Aug 13 '19 at 18:53

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