Schematic Diagrams

  
   
When building a circuit, it's good to have a design first. A schematic diagram is a drawing of the circuit, with wires represented by lines and special symbols used for the various electronic components. There are many different components that can be part of a circuit, and we'll become familiar with many of them later. For now, here are a few of the most basic. Activities & Practice
to do as you read
   

Wire  A wire is represented by a solid line. Simple enough. ________________________

 

  

Switch  A switch turns a circuit on or off. You can think of it as a drawbridge for electrons: when the drawbridge is up (open), no charge can get through; when the bridge is down (closed), the current can flow. Here's the symbol used for a basic switch:

                           

 

  

Junction  Where two wires join is called a junction. In a diagram a dot may be drawn where they connect, for emphasis, although this really isn't necessary. If there are wires that don't connect to each other, try not to draw them crossing on the diagram. If it's unavoidable, make a little semi-circular loop in one of the wires to indicate that the two wires are not electrically connected.
                                 

 

  

Battery  A battery provides current, plus the voltage needed to push the current through a circuit. Batteries come in a variety of sizes and shapes, but they all use either of these schematic symbols:

                                     

To be nit-picky, the top symbol is not a battery, but rather a cell. The most common chemical reaction used produces a voltage of 1.5 V, and that's why the standard AA, AAA, C and D "batteries" are all 1.5 volts. Cells can be connected end-to-end ("in series") to produce higher voltages. Such a stack of cells is truly and correctly called a battery, and would use the second of the two symbols above. Car batteries have six 2-volt cells in series, for a total of 12 volts. Nine-volt batteries have a stack of six 1.5-volt cells inside that rectangular case.

Despite the distinction made between cells and batteries in the previous paragraph, the word "battery" has, in recent years, come to refer to both cells and true batteries. In circuit diagrams, which of the above symbols is used nowadays is really a matter of preference.

The short-line end of the battery symbol is always the negative end, regardless of whether the - and + labels are present.

 

  

Resistor A resistor hinders the flow of current by a specific amount. The symbol is a zigzag line:

 

  

Example  Here is an example of a simple circuit schematic. It has a battery, a switch, and a lightbulb (the resistor symbol with the circle around it.)

1. Draw schematics for the following circuits:

(a) One battery, two light bulbs, and one switch that turns them both on and off.

(b) One battery, two light bulbs, and two switches, with each switch controlling one bulb.

(c) One battery, one light bulb, and two switches. The light bulb is ON if either switch is on (closed).

(d) One battery, one light bulb, and two switches. The light bulb is ON only if both switches are on (closed).

You can use the Circuit Construction Kit to verify your answers.

 

  

 

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