The basic idea of multiplexing is to transmit two or more analog messages or digital signals concurrently over a single communication channel, thus sharing what might be an expensive resource. As an example in the telephone industry, a number of phone calls can be carried on a single wire. Another example is a home stereo system remote control that allows one to select between a CD player, a DVD player, or cable TV. Sound systems having digital output can carry several channels over a single fiber optic cable. In electronics, a multiplexer, or MUX for short, is a device that performs this multiplexing function—forwarding the selected input into a single channel.
Students will be able to:
– Build digital multiplexers in which the number of inputs is a power of two (2, 4, 8, 16, …) and there is one output
– Understand how to select one of the inputs and send it down the single output channel
– Communicate their learning to others
Observe students working with multiplexers. After the hands-on activity, consider extending learning by asking students to research real-life uses for multiplexers and present their findings to their peers.
NGSS Disciplinary Core Ideas:
– ETS2: Links among engineering, technology, science, and society
– NGSS Science and Engineering Practices:
– SEP5. Using mathematics and computational thinking
– SEP8. Obtaining, evaluating, and communicating information
Duration: 1 45-minute class
High School (ages 14-17)
Middle School (ages 11-13)
MODULES & ACCESSORIES USED (19)
toggle switch (1)
slide switch (1)
double AND (4)
double OR (1)
synth speaker (1)
mounting boards (1)
battery + cable (1)
STEP 1 : Introduce multiplexers
Use the background information in the included PDF to give students an overview of how multiplexers are used to transmit multiple messages over a single communication channel.
Explain that in the next steps, students will follow instructions to build and explore different types of multiplexers.
Because each activity builds on the previous activity, it is important for students to complete them in sequence. Decide how to best divide your students into groups to give each student hands-on experience working with the multiplexer circuits. You may want to complete the lesson all in one-day or over a series of days.
STEP 2 : Construct a 2-to-1 MUX without Enable
Guide students through the following instructions listed in Activity 1 on the activity PDF:
Let’s begin by constructing a 2-to-1 MUX without an enable input, as shown in the lesson photo. The inputs “I0” and “I1” are oscillator modules, each set to a different frequency, so they can be easily distinguished by their sound. The output “Out” is a synth speaker module. The select input “S” is a slide switch module. This MUX is similar to a MUX on a remote control that allows you to switch between, say, a DVD player and a CD player. When the slide switch is in the OFF (0) position, then oscillator “I0” is the output. When the slide switch is in the ON (1) position, then oscillator “I1” is the output.
Each student group will NEED:
-1 power module
-1 fork module
-1 branch module
-2 AND modules
-1 OR module
-1 inverter (NOT) module
-2 oscillator modules
-1 synth speaker module
-1 slide switch module
-1 pulse module
-1 delay module
STEP 3 : Convert the 2-to-1 MUX without Enable to a Time Division Multiplexer (TDM)
Guide students through the following instructions listed in Activity 2 on the activity PDF:
Replace the slide switch with a pulse module. You will then have what is commonly known as Time Division Multiplexing or TDM for short. In TDM the input lines are alternately allowed to transmit for a period of time in a cyclic fashion. By adjusting the speed of the pulse module, you can change the rate at which the two lines alternate transmission. (You can also produce some very interesting sound effects by adjusting the two oscillator frequencies and even more interesting sounds by adding a delay module just before the synth speaker.)
STEP 4 : Add an Enable Input to the MUX
Guide students through the following instructions listed in Activity 3 on the activity PDF:
Beginning with your multiplexer from activity 2, add the logic and other components required for an Enable input. When you have completed this, you should notice that setting the enable switch to OFF (0) will stop all transmission (except some echoing from a delay, if present).
ADDITIONALLY, each group will NEED:
-1 toggle switch (or a slide switch or button)
STEP 5 : Construct a 4-to-1 MUX
Guide students through the following instructions listed in Activity 4 on the activity PDF:
If you are in a classroom setting, and each lab group of students has constructed a 2-to-1 MUX, you might find it interesting, challenging, and fun to connect three lab group 2-to-1 MUXs together into a 4-to-1 MUX. To keep the number of components required to a minimum, it is suggested that you use 2-to-1 MUXs without enable lines. Also, use slide or toggle switches for the two enable inputs on your 4-to- 1 MUX, and use oscillators for the four inputs I00, I01, I10, and I11. Set each oscillator at a different frequency, so you can more easily make sure that the circuit is working correctly when you test it with different combinations of enable inputs S0 and S1. You will probably need a few more wires and a split to complete your class’s 4-to-1 MUX. Also, note that there is nothing wrong with having more than one power source on a given project—so you can leave the power modules as they are on your two 2-to-1 MUXs with the four input lines if you wish.
STEP 6 : Discuss and extend
Now that students have a contextual understanding of how multiplexers use a single communication line to send multiple digital signals at the same time, discuss implications of this technology with students.
What are some real-life examples of multiplexers?
What are the advantages of using multiplexers?
Now may also be a good time to introduce demultiplexers, or electronic devices that separate multiplex signals back into their separate components.
Consider extending student learning, by asking students to research an actual system that uses multiplexers (i.e. the telephone industry) and then report their findings back to their peers.