2-2 Parallel & Computational Circuits

Overview

Goals

• Students will be able to understand and construct parallel and series circuits.
• Students will be able to describe the voltage difference between a parallel and series circuit.
• Students will be able to explain how a single microcontroller can control the flow of multiple circuits.

Content Standards

Prep

Teachers should complete the following preparation for the lesson:

• Make sure computers are functional and MakeCode is not blocked.
• Prepare supplies as a kit per student (copper tape, 4 LEDs, 1 CPX, 4 alligator clips)
• Make copies of Parallel Circuit Diagram handout on card stock, and cut out the templates (1/pair).
• Have copies of Exploring Circuits handout (1/pair).
• Place students into pairs.
• Write warm up question on the board.

Lesson Guide

Introduction (5 minutes)

Think-Pair-Share

If you wanted to add a second LED to the simple circuit, how might you do it?

During discussion, emphasize the need to add more conductive material to connect a second LED. Lead into next activity by indicating that there are multiple ways we can connect the second LED.

Parallel Circuit Diagram (20 minutes)

Teacher hands out Parallel Circuit Diagram handout to pairs. Teacher explains that it is a parallel circuit.

When making a parallel circuit, we extend the negative line from one LED to the negative pole of the new LED. Likewise, we extend the positive line from one LED to the positive pole of the new LED. In this way, when we send current to the pin, since the positive lines of both LEDs are connected to the same pin, both LEDs will behave the same. LEDs in parallel cannot be controlled independently, as they are connected to the same pin, which is where the current is coming from.

Pairs add the copper tape and 2 LEDs to the parallel circuit diagram as written.

Pass out Exploring Circuits handout to each student. Direct students to Part 1 and Part 2. Students should talk in pairs and individually answer the two question:

1. Explain why both LEDs will turn on and off together when in parallel.
2. Assume current is flowing through the circuit and the LEDs are on. On the parallel circuit, if you were to remove a section of copper tape between the two LEDs, what do you think would happen? Explain your reasoning.

Bring students back for a discussion.

1. Both LEDs are attached to the same pin. When we send current through that pin, the current is flowing through both LEDs since their positive lines are connected, and current flows out of the power source through the positive pole.
2. The first LED is still connected to a closed loop, and current will flow through it; however, current will not flow through the second since the positive line is broken.

Computational Circuits (25 mins)

Tell students that they will now add a third LED which we will be able to control indpendently of the two in parallel using the same CPX.

Ask:

If we want to add a third LED that can be controlled separately from the other two,
how do you think we might achieve this? For example, if we want the two on the
paper to turn on and our third to turn off, should the third be attached to the
same pin or does it need to be connected to a different pin?

We want students to describe the need to send the new LED to a different pin. By connecting the LED to a different pin, we can use code to control the second pin separately from the pin where the two parallel LEDs are connected.

Invite students to draw a diagram for adding their third LED. They can add their third LED anywhere they like on the template. Inform students that the positive pole of their third LED needs to connect to a different pin, and the negative pole can either connect to a different GND or to the same negative line as the two in parallel, as shown:

In the computational circuit diagram above, notice that none of the lines are touching each other. Also notice that the third LED is attached to the same ground that the parallel LEDs are connected to. In computational circuits, only the positive line differentiates independence. This is because current flows out of the power source through the numbered pins (positive poles), and we control the current by manipulating the numbered pins. GND is just a way for the current to re-enter the power source.

It is best if students get their circuit diagram checked by the teacher before adding copper tape.

Things to look out for in student circuit diagrams: crossed lines, ensuring that they do not connect to VBATT or 3.3V.

Once students finish their circuit diagrams, invite them to construct the circuit. While students are adding the third LED, write the four functions students will need to code on the board:

Invite pairs of students to open MakeCode and create a new project. Give students the task of connecting the computational circuit to the CPX and programming the two LEDs in parallel and the new LED to blink alternatingly. Parallel on and single off for 1 second, then parallel off and single on for 1 second, and repeat forever. Remind students that they can use a different pin for each circuit.

Here is an example of what code might look like. In this example, we are using A7 for the parallel LEDs and A2 for the independent LED, as shown in the circuit diagram above. Students may choose to connect their third LED to a different pin.

Homework

Complete Part 3 of the Exploring Circuits Handout.

If you wanted to program the eyes of the cat to blink together  
and the nose to always be on, draw a circuit diagram that could  
align with your intention.

The easiest way to do this would be to make the eyes in parallel and have a separate line going to the nose. The solution is in the next lesson, 3-1.

Vocabulary

The following are terms used in this lesson.