Most electronic circuits are combinations of series and parallel connections. How you arrange components in a circuit depends on what you're trying to do. Take a look at the series-parallel circuit shown here.
Note the three parallel branches, each containing a switch in series with a resistor and an LED. The switches are represented by the symbols at the top of each branch.
If only one switch is in a closed position, as shown in the following figure, all the supply current flows through just one LED, which lights, and the other LEDs are off.
If all three switches are closed, the supply current travels through the resistor and then splits three different ways — with some current passing through each of the three LEDs. If all three switches are open, the current does not have a complete path to follow, so no current flows out of the battery, as shown here.
By alternating which switch is open at any time, you can control which LED is lit. You can imagine such a circuit controlling the operation of a three-stage traffic light (with additional parts to control the timing and sequencing of the switching action).
To analyze combination circuits, you apply voltage and current rules one step at a time, using series rules for components in series and parallel rules for components in parallel. At this point, you don't quite have enough information to calculate all the currents and voltages in the LED circuits shown here. You need to know about a rule called Ohm's Law and about how voltage is dropped across diodes. Then you'll have everything you need to analyze simple circuits.
To build the three-LED circuit described here, you need the following parts:
Four 1.5-volt AA batteries
One four-battery holder (for AA batteries)
One battery clip
(yellow-violet-brown stripes and then a gold or silver stripe)
Three LEDs (any size, any color; I used one red, one yellow, and one green)
Three single-pole, double-throw (SPDT) slide switches designed for solderless breadboard use
One solderless breadboard and assorted jumper wires
Each SPDT switch has three terminals for making connections, but for the three-LED circuit, you need to use only two of the terminals. (See the following figure.) The slider button controls which end terminal gets connected to the center terminal.
With the slider in one position, the center terminal is connected to the terminal at the end where the slider is positioned. Move the slider to the other position, and the center terminal is connected to the other end terminal. This type of switch is also known as an on/on switch because it can switch between two circuits, closing one while opening the other.
For the three-LED circuit, you need the switch to function as an on/off switch, so you connect two of the three SPDT terminals in your circuit. Leave the unused end terminal in a breadboard hole but not connected to anything in your circuit, as shown here. With the slider positioned towards the unused terminal, the switch is off. With the slider positioned towards the other end of the switch, the switch is on.
The simplest type of on/off switch is a two-terminal, single-pole, single-throw (SPST) switch, which simply connects or disconnects the two terminals when you move the slider. But it's hard to find such a switch with terminals designed to fit into solderless breadboards.