# What Is A Circuit

A very common term in electronics is the word circuit. In general, you will hear the word used to describe what a schematic shows. Most projects are a circuit and most schematics are a circuit.

The literal definition of a circuit comes from "circle", and is used to refer to the path that electricity flows in a project. Lets look at a simple circuit.

 Schematic Symbol Actual Image

Above is a circuit consisting of a battery and a lightbulb. Notice how it makes a circle. The circuit is complete, so the lightbulb is lit. This is called a closed circuit.

Open Circuit

If you do not have an closed circuit, then you have an open circuit. Lets look at some open circuits.

 Schematic Symbol Actual Image

Open Circuit Example 1: missing top wire.

In Example 1 we have left off the top wire. This is an open circuit. Notice that the light bulb is off. Without a complete circuit, the light bulb will not be lit. In this example the light bulb is said to be missing voltage, because ground is hooked up but the wire providing +9 volts is not.

 Schematic Symbol Actual Image

Open Circuit Example 2: missing bottom wire.

In Example 2 we have left off the bottom wire. This is also an open circuit and the light bulb is off. In this example, the light bulb is said to be missing ground because the light bulb has +9 volts but doesn’t have a ground connection.

Note that these are still circuits, they just happen to be open circuits.

Switches

Switches are a way of manually opening or closing a circuit. Switches come in many varieties, including push button, slide and toggle. The buttons on a telephone keypad and the buttons on your remote are also switches. Lets look at a couple of switch examples.

 Schematic Symbol Actual Image

Switch Example 1: Open Switch = Open Circuit

In Example 1 the switch is open, so the circuit is open and the light bulb is off. Opening a switch has the same effect on a circuit as disconnecting a wire.

 Schematic Symbol Actual Image

Switch Example 2: Closed Switch = Closed Circuit

In Example 2 the switch is closed, so the circuit is closed and the light bulb is on. Closing a switch has the same effect on a circuit as connecting a wire.

Not All Circuits Look Complete

In a previous guide we talked about using V+ and GND instead of a battery symbol in a schematic. Keep in mind that when you do that, your closed circuits will no longer make circles, but they are still closed circuits. Lets take a look at an example.

 Schematic Symbol Actual Image

Above is a circuit consisting of a battery and a lightbulb. See how the schematic doesn’t make a circle any more, but the real life example does? This is still a closed circuit and the light bulb is still lit.

Summary

In general, you can tell if you have a closed circuit using the following rule:

If the circuit works, then it’s a closed circuit.

If you are using a motor, and the motor is turning then the circuit is closed. If you are using a buzzer and the buzzer is buzzing then your circuit is closed. When using a light bulb, the glowing light means you have a closed circuit. In all of these cases the part in question (motor, buzzer, light bulb) is doing what it is supposed to do, so your circuit is closed.

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