Closed Circuit

A closed circuit is one which allows electricity to flow and, in this case, to light the lightbulb.

A battery symbol resembles a stack of cells, the broad line indicating +ve and the short line indicating -ve).

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AND and OR Using Switches

In an AND circuit, both switches must be closed before electricity flows in the circuit.

In an OR circuit, either switch must be closed before electricity flows in the circuit.

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AND and OR Using Transistors

In digital circuits, transistors are used like switches.

[Aside: transistors are used differently in analog circuits.]

The squiggly line is a resistor. Electricity flows in a circuit like water flows in a hose. A resistor limits the amount of electricity that can flow, like a pinch in a hose.

Transistors can’t handle as much electricity flow as switches can, so we insert a resistor to limit the electricity. [Aside: in fact, the lamp used in transistor circuits is usually a LED (light emitting diode) which needs less electricity than an incandescant lamp does.]

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See Also