untitled

You’ll also find symbols for commonly used components, such as resistors,

diodes, capacitors, and transistors, in schematics (see Figure 4-4). Chapter 3

explains what most of these components do in a circuit.

Switching gears with switches............................................................

You use switchesto turn power to a circuit on or off or to connect/disconnect

a pin on a component to +V or ground. However, switches don’t use just a

single symbol. Rather, they come in several varieties, indicating

How many wires they control

Whether they stay in the position you set them at or return to a normal

position after you release them

An SPST (single-pole, single-throw) switchhas one incoming wire and one out-

going wire that you use to open or close a connection in a circuit. For exam-

ple, if a wire runs from the negative pole of a battery to an SPST switch and

another wire runs from the SPST switch to a circuit, current can flow through

the circuit when you have the switch in the closed position. When you flip

the switch to the open position, no current can flow through the circuit.

SPST switches also come in the momentary switch variety; these can be nor-

mally open (NO) or normally closed (NC) and are generally controlled by

pushbuttons or relays. A normally open switch conducts current only when

the button is pressed and returns to its open position when it’s released. A

normally closed switch won’t conduct current when you press the button but

returns to its normal position and conducts current when you release it.

An SPDT (single-pole double-throw) switchhas one incoming wire and two out-

going wires that you use to control which of two components is connected.

Suppose that the incoming wire is connected to power, one outgoing wire is

connected to a green LED, and the other outgoing wire is connected to a red

LED. When you have the switch in one position, the green LED lights up; when

you flip the switch, the green LED goes dark, and the red LED lights up.

Resistor

Capacitor

Diode

Transistor

Figure 4-4:

Common

component

symbols.

62 Part I: Project Prep