Circuit Basics

Date Created: October 20, 2026 Author: Shani Mensing Tools & Skills: electronics Status: In Progress

Let’s build a circuit! We’ll start by lighting a bulb with a battery and alligator clips, then use a multimeter to measure what’s happening as we add a switch, more batteries, and more bulbs. We’ll compare series and parallel circuits, and finish by building LED circuits on a breadboard. Along the way, we’ll go over voltage, current, and resistance, and practice Problem Solving by testing and troubleshooting our own circuits.

💡 Prerequisites: None! You don’t need any electronics experience to get started. Risks: Connecting a battery’s + and – sides directly creates a short circuit. This can make the battery heat up quickly, so check your connections before adding power. Light bulbs get hot when they’re on. • References • Arduino To Go, Chapter 2: The Multimeter • Arduino To Go, Chapter 4: Ohm’s Law • Arduino To Go, Chapter 4 Review • Electrical Classroom: Series and Parallel Circuits • What Is Electricity and How Does It Work? | Constellation • Common circuit symbols (ggebooks) • What is a circuit? (YouTube)

• • References
• • Arduino To Go, Chapter 2: The Multimeter
• • Arduino To Go, Chapter 4: Ohm’s Law
• • Arduino To Go, Chapter 4 Review
• • Electrical Classroom: Series and Parallel Circuits
• • What Is Electricity and How Does It Work? | Constellation
• • Common circuit symbols (ggebooks)
• • What is a circuit? (YouTube)

Material List

Safety First Battery & Electronics Safetysafety-1-dont-short.png Don’t short the battery A wire, an alligator clip, or even a metal tool can connect the + and – sides of a battery. Don’t let this happen! With very little resistance in the path, too much current can flow and the battery can heat up quickly. If anything starts to feel warm, stop and get the facilitator. Disconnect power if you can do so safely; don’t handle a hot battery.safety-2-sharp-ends.png Handle parts carefully Watch those little ends! LED legs, jumper wires, and alligator clip teeth can be sharp. Point the ends away from yourself. Bulbs get hot! An incandescent bulb makes light by getting hot. Let it cool before you touch it or unscrew it, and disconnect your circuit when you’re not using it.safety-3-return-batteries.png Return every battery Keep coin batteries in the supply bin until you need them, and return them at the end of the workshop. They are very dangerous if swallowed, so don’t leave them lying around the space.safety-4-no-wall-outlets.png Multimeter use We’re only testing battery-powered circuits in this workshop. Never put the probes in a wall outlet! Before touching the probes to your circuit, check that the dial is on the setting you want to use.

What is a Circuit? A circuit is a closed path in which an electric current can flow. What’s moving in our wires? Tiny particles called electrons! Electrons have a negative charge. In a simple battery circuit, they move through the wires and parts from the negative (–) side of the battery toward the positive (+) side. This movement of charge is an electric current, which transfers energy to the parts in our circuit. But what happens if there’s a gap? The circuit is open, so no current flows. We need a complete loop!doc-open-closed-circuit.png Image Source: https://www.youtube.com/watch?v=iZYedWOERN0 What Is Electricity and How Does It Work? | Constellation Voltage, Current, and Resistance Three words help us describe electricity in a circuit: • Voltage (V): the “push” that causes current to flow. Measured in volts (V). • Current (I): the flow of electrons in a circuit. Measured in amps (A). • Resistance (R): opposes (slows down) current flow in a circuit. Measured in ohms (Ω).doc-water-analogy.png Image Source: https://blog.sonicelectronix.com/what-is-impedance-and-why-does-it-matter/ Ohm’s Law Voltage, current, and resistance are related through a formula named Ohm’s Law: V = I × R. Think of voltage as the “push” and resistance as what restricts the flow. No matter how much push there is, if there is resistance, the current will be restricted. For a given voltage, more resistance means less current.doc-ohms-law.pngdoc-ohms-law-forms.png Source: https://arduinotogo.com/2016/10/07/chapter-4-ohms-law/ Short Circuit So what happens if there is too little or no resistance in your circuit? Current can become too high, causing your battery to heat up and drain quickly. Imagine a dam suddenly breaking open: the water rushes out without anything to regulate it. To avoid this, never directly connect the POSITIVE AND NEGATIVE sides of your battery!doc-short-circuit.png

• • Voltage (V): the “push” that causes current to flow. Measured in volts (V).
• • Current (I): the flow of electrons in a circuit. Measured in amps (A).
• • Resistance (R): opposes (slows down) current flow in a circuit. Measured in ohms (Ω).

Let’s Build a Circuit: Parts To get started, we need a power source, a path for current to follow, and a load, something that uses electrical energy, like a light bulb. Circuits are often described with symbols in a diagram. Let’s look at the parts and the symbols used to show them. More commonly used symbols can be found here.doc-picture-vs-diagram.png💡 Battery This is our power source! The battery provides the voltage, or “push,” that causes current to flow. Each AA battery gives about 1.5V. In the battery symbol, the longer line marks the + side.parts-battery.png💡 Wire Connects the parts so current has a path to follow. About wire colors: In this workshop, use red for + and black for –. The wires themselves work the same way. Stick with consistent colors for good design practice… and save yourself some confusion! • Alligator clips • Jumper wires • Other conductive material!parts-wire.png💡 Light Bulb A light bulb turns electrical energy into light. Today we’ll use two kinds: • Incandescent bulb: turns electrical energy into heat, and heat into light. Current heats a tiny wire inside called a filament until it glows. These bulbs work either way around, and they get hot! • LED [Light Emitting Diode]: converts electrical energy into light with much less heat. But there’s a catch: an LED only lights in one direction! We’ll test this in Part IV.parts-bulb.png💡 Switch A switch opens and closes the path in our circuit. Close the switch and current can flow. Open it and you’ve made a gap, so the light turns off. Closed = connected = ON. Open = gap = OFF.parts-switch.jpg

• • Alligator clips
• • Jumper wires
• • Other conductive material!

• • Incandescent bulb: turns electrical energy into heat, and heat into light. Current heats a tiny wire inside called a filament until it glows. These bulbs work either way around, and they get hot!
• • LED [Light Emitting Diode]: converts electrical energy into light with much less heat. But there’s a catch: an LED only lights in one direction! We’ll test this in Part IV.

Part I: Your First Circuit (Battery, Alligator Clips & Bulb) Overview The most basic circuit is a light and a battery. We’ll use one AA battery in a holder, two alligator clips, and an incandescent bulb in a holder to make a complete loop. Step 1: Light the Bulb 1. Get one AA battery holder with a battery in it. Find the + end of the battery. The red lead connects to + and the black lead connects to –.p1-battery-holder.jpg 2. Get a bulb in a holder. Notice the two metal tabs with screws. These are where we connect our wires. 3. Clip a black alligator clip from the holder’s black (–) lead to one tab on the bulb holder. • Is the bulb on? Not yet! There’s a gap, so the circuit is open.p1-open-circuit.jpg 1. Clip a red alligator clip from the holder’s red (+) lead to the other tab on the bulb holder. You’ve closed the loop, so your bulb should light up!p1-bulb-lit.jpg 2. Now swap the clips so red goes to the tab black was on. Does the bulb still light? Incandescent bulbs work either way around. (Remember this when we get to LEDs!) 3. Disconnect one clip when you’re done. Bulbs get warm, and an open circuit saves the battery.💡 But… what if your bulb doesn’t light? Is the bulb screwed all the way into its holder? Are the alligator clips biting onto bare metal, not the plastic? Is the battery facing the right way in the holder? Still nothing? Try a fresh battery or bulb.

1. 1.

Get one AA battery holder with a battery in it. Find the + end of the battery. The red lead connects to + and the black lead connects to –.

p1-battery-holder.jpg

1. 2.

Get a bulb in a holder. Notice the two metal tabs with screws. These are where we connect our wires.

2. 3.

Clip a black alligator clip from the holder’s black (–) lead to one tab on the bulb holder.

• •

Is the bulb on? Not yet! There’s a gap, so the circuit is open.

p1-open-circuit.jpg

1. 1.

Clip a red alligator clip from the holder’s red (+) lead to the other tab on the bulb holder. You’ve closed the loop, so your bulb should light up!

p1-bulb-lit.jpg

1. 2.

Now swap the clips so red goes to the tab black was on. Does the bulb still light? Incandescent bulbs work either way around. (Remember this when we get to LEDs!)

2. 3.

Disconnect one clip when you’re done. Bulbs get warm, and an open circuit saves the battery.

Part II: Measure It (Multimeter, Switch & More Batteries) Overview Multimeters are used to find out more information about your circuit. The main parts of a multimeter are the display, dial, ports, and probes. The dial is marked with what the meter can measure, like DC and AC voltage, resistance, current, and continuity.doc-multimeter-parts.png Image Source: https://arduinotogo.com/2016/08/22/chapter-2-the-multimeter/p2-multimeter.jpg Step 1: Set Up Your Multimeter 1. Plug the black probe into COM and the red probe into VΩ. Leave the A and mA ports alone for today.doc-probes.pngdoc-ports.png 2. Turn the dial to DC voltage (V⎓). Our batteries will give us less than 20V, so choose 20 in the V⎓ section. • Never use a current (A) setting across the battery! That can create a short circuit.doc-multimeter-dial.pngp2-dial.jpg Step 2: Measure One Battery 1. Touch the red probe to the battery’s + side and the black probe to –. 2. What does the display say? A fresh AA battery reads about 1.5V. • Try switching the probes. What changes on the display?p2-1-battery.jpg Step 3: Add a Switch 1. Open the switch (handle up). 2. Rebuild your Part I circuit with the switch in the loop: battery + → switch → bulb → battery –. Use one alligator clip for each connection. 3. Close the switch. Your bulb should light! Open it again. The bulb turns off because you’ve made a gap. • Closed = connected = ON. Open = gap = OFF. 📷 Image: battery, switch, and bulb circuit Step 4: Add More Batteries Our battery holders snap together end to end, so the + of one battery connects to the – of the next. When batteries are connected this way, their voltages add up. 1. Open the switch. Predict: what will the meter read with 2 batteries? With 3? 2. Snap a second battery holder onto the first. Measure across the whole battery pack: red probe on the + end, black probe on the – end.p2-2-batteries.jpg 3. Add a third battery and measure again.p2-3-batteries.jpg 4. Connect your bulb to the battery pack each time and close the switch. What happens to the brightness as you add batteries? More voltage = more push = more current through the bulb = brighter light!p2-bulb-2-batteries.jpgp2-bulb-3-batteries.jpg 5. Record what you found: • 1 battery: predicted ___ V, measured ___ V, brightness ___ • 2 batteries: predicted ___ V, measured ___ V, brightness ___ • 3 batteries: predicted ___ V, measured ___ V, brightness ___💡 Facilitator note: Check the voltage rating on your bulbs before the workshop. If 3 AAs (about 4.5V) is more than a bulb is rated for, it will glow very brightly and may burn out quickly. Keep the 3-battery test short, or stop at 2 batteries.

1. 1.

Plug the black probe into COM and the red probe into VΩ. Leave the A and mA ports alone for today.

doc-probes.png



doc-ports.png

1. 2.

Turn the dial to DC voltage (V⎓). Our batteries will give us less than 20V, so choose 20 in the V⎓ section.

• • Never use a current (A) setting across the battery! That can create a short circuit.

1. 1. Touch the red probe to the battery’s + side and the black probe to –.
2. 2. What does the display say? A fresh AA battery reads about 1.5V.

• • Try switching the probes. What changes on the display?

1. 1. Open the switch (handle up).
2. 2. Rebuild your Part I circuit with the switch in the loop: battery + → switch → bulb → battery –. Use one alligator clip for each connection.
3. 3. Close the switch. Your bulb should light! Open it again. The bulb turns off because you’ve made a gap.

• • Closed = connected = ON. Open = gap = OFF.

1. 1.

Open the switch. Predict: what will the meter read with 2 batteries? With 3?

2. 2.

Snap a second battery holder onto the first. Measure across the whole battery pack: red probe on the + end, black probe on the – end.

p2-2-batteries.jpg

1. 3.

Add a third battery and measure again.

p2-3-batteries.jpg

1. 4.

Connect your bulb to the battery pack each time and close the switch. What happens to the brightness as you add batteries? More voltage = more push = more current through the bulb = brighter light!

p2-bulb-2-batteries.jpg



p2-bulb-3-batteries.jpg

1. 5.

Record what you found:

• • 1 battery: predicted ___ V, measured ___ V, brightness ___
• • 2 batteries: predicted ___ V, measured ___ V, brightness ___
• • 3 batteries: predicted ___ V, measured ___ V, brightness ___

Part III: Series and Parallel Overview If we wanted to add more components or lights to our circuit, how could we do it? Parallel and series are two ways of completing a circuit. Use a 2-battery pack for this part.doc-series-parallel.png Image Source: https://www.electricalclassroom.com/series-and-parallel-circuits/ Series In a series circuit, current flows through the first bulb and then onwards to the next, and so on until it makes its way back to the battery. Parts connect one after another in a single loop. If one bulb breaks or is removed, the flow of current is broken. This will shut off all the bulbs in the circuit. Step 1: Build a Series Circuit 1. Open the switch. Connect: battery + → switch → bulb A → bulb B → battery –. 2. Close the switch. How bright are your bulbs compared to one bulb on its own? 3. Use your multimeter on V⎓ 20 to measure across the battery pack, then across bulb A (one probe on each tab), then across bulb B. What do you notice when you add the two bulb readings together? 4. Carefully unscrew bulb A (let it cool first!). What happens to bulb B? 📷 Image: two bulbs in series Parallel In a parallel circuit, current flows through separate branches and then back to the battery. Each bulb has its own connection to the battery. If one bulb breaks, current can still flow through the other branches and the other bulbs will stay on. Step 2: Build a Parallel Circuit 1. Open the switch. Connect bulb A across the battery: battery + (through the switch) → bulb A → battery –. 2. Now give bulb B its own clips to the same two points: one clip from the + side to bulb B, and one from bulb B back to the battery –. 3. Close the switch. How bright are your bulbs now? 4. Measure across the battery pack, then across bulb A, then across bulb B. How do these readings compare to your series circuit? 5. Unscrew bulb A. What happens to bulb B? 📷 Image: two bulbs in parallel What did you notice? Bulbs in a parallel circuit are brighter than bulbs in series. Why? In a series circuit, the bulbs share the battery’s voltage. Some of the voltage drops across each bulb, so each one gets less push. In a parallel circuit, each bulb has a direct connection to the battery, so each one gets the full battery voltage. The voltage across each part is called… yep, you guessed it, voltage drop! You can use a multimeter to measure voltage drop anywhere in your circuit. This is important in larger circuits, where you want to make sure you have enough voltage to run every part.💡 Tip: Parallel bulbs are brighter, but they also use up the battery faster, because the battery is powering two full branches at once.

1. 1. Open the switch. Connect: battery + → switch → bulb A → bulb B → battery –.
2. 2. Close the switch. How bright are your bulbs compared to one bulb on its own?
3. 3. Use your multimeter on V⎓ 20 to measure across the battery pack, then across bulb A (one probe on each tab), then across bulb B. What do you notice when you add the two bulb readings together?
4. 4. Carefully unscrew bulb A (let it cool first!). What happens to bulb B?
5. 1. Open the switch. Connect bulb A across the battery: battery + (through the switch) → bulb A → battery –.
6. 2. Now give bulb B its own clips to the same two points: one clip from the + side to bulb B, and one from bulb B back to the battery –.
7. 3. Close the switch. How bright are your bulbs now?
8. 4. Measure across the battery pack, then across bulb A, then across bulb B. How do these readings compare to your series circuit?
9. 5. Unscrew bulb A. What happens to bulb B?

Part IV: Breadboards and LEDs Overview An LED is a Light Emitting Diode. This type of diode converts electrical energy into light. Diodes only allow current to flow in one direction, so if you connect an LED backwards, it won’t light. A breadboard, despite its name, is not actually used for bread! Its purpose is to help us prototype circuits, so we can try out connections and make changes before committing to a design. In the past, circuits were built on wooden boards used for cutting bread. Now breadboards are plastic, but the name still remains.doc-breadboard-history.png Step 1: Test an LED with a Coin Battery 1. Look at your LED. Notice that one leg is longer than the other. It matters which leg touches which side of the battery! • The longer leg is positive (+), called the anode. The shorter leg is negative (–), called the cathode. You can usually find a flat edge on the LED rim on the negative side, too.doc-led.png 1. Before touching its legs to a coin battery, check with the facilitator that your particular LED and battery are safe to use this way. 2. If the facilitator has approved the direct-contact test, touch the long leg to the battery’s + side and the short leg to –. Your LED should light up! Keep the test brief. If anything starts to warm up, stop and get the facilitator. 3. Now disconnect the LED and switch it around. Briefly test again. What happens? • Diodes only allow current to flow easily in one direction. Connect the LED backwards and it won’t light! How is this different from the bulb in Part I?✨ Tip: Give the long (+) leg a slight bend before building. This gives you a way to tell positive from negative once the legs are in the breadboard!doc-led-bent-leg.png Step 2: How Does a Breadboard Work? Breadboards are marked with letters and numbers to help you find the little holes, called pins. Letters label columns and numbers label rows.doc-breadboard-labels.png 1. Inside are metal strips that connect some of the pins. In the middle, the five pins A–E at one row number connect to each other. F–J make a separate group. Notice the gap? The connections don’t go across it! 2. The long + (red) and – (blue) rails along the edges usually connect lengthwise. These are used for power. Some boards have a break partway down. 3. Check your board with the multimeter’s continuity (beeper) setting before adding power. Touch the probes to two pins in the same strip, then to two separate strips. A beep means they’re connected. Never test continuity with a battery connected.doc-breadboard-inside.png Step 3: Light One LED on the Breadboard 1. Place your empty battery holder on the breadboard. Make sure the + and – pins go into separate strips that are not connected. It is important that these do not connect! Choose holes that fit your holder, and don’t force the pins.doc-coin-holder-poles.pngdoc-bb-holder.png 2. Now add an LED. Put it across the center gap. For example, put the long (+) leg in E15 and the short (–) leg in F15. These holes are on separate strips, even though they share a row number. 3. Take a red jumper wire and a resistor. We want this path: holder + → resistor → LED +. Connect the red jumper from the holder’s + strip to a spare strip. Put one resistor leg in that spare strip and the other in the strip with the LED’s long leg. Make sure the resistor’s legs are on different, unconnected strips, or current can bypass it! 4. Take a black jumper wire. Connect the strip with the LED’s short (–) leg to the holder’s – strip. 5. Have a partner check your connections, then put the battery in according to the holder’s markings. Your LED should light up!doc-bb-one-led.png Step 4: Light Multiple LEDs The outermost rails of the breadboard are used to connect to the + and – sides of a power source. This keeps our wiring organized and gives us room for more parts.doc-bb-rail.png 1. Take the battery out first! 2. Connect the holder’s + strip to the + rail with a red wire. 3. Connect the holder’s – strip to the – rail with a black wire. 4. Reconnect your LED branch so the path is + rail → resistor → LED +, then LED – → – rail. Check your work, then put the battery back in. Your LED should still light!doc-bb-rails-1.pngdoc-bb-rails-2.png 5. Remove the battery again. Add three more LED branches, each with its own resistor and connections to the + and – rails. Check your work, then put the battery back in. 6. Is this series or parallel? Predict what will happen if you remove one LED. Take the battery out, remove an LED, then reconnect power to test your idea.doc-bb-multi-led.png💡 But… what if your LED doesn’t light? First, take the battery out. Are the LED’s + and – legs the right way around? Does the battery face the right way in its holder? Check the LED, resistor, and jumper wire connections. Are they in holes that actually connect? Check for breaks in the power rails, too. Once the wiring is checked, put the battery back in and test again. Still no light? Measure the battery voltage with your multimeter or try a fresh battery.

1. 1. Look at your LED. Notice that one leg is longer than the other. It matters which leg touches which side of the battery!

• •

The longer leg is positive (+), called the anode. The shorter leg is negative (–), called the cathode. You can usually find a flat edge on the LED rim on the negative side, too.

doc-led.png

1. 1. Before touching its legs to a coin battery, check with the facilitator that your particular LED and battery are safe to use this way.
2. 2. If the facilitator has approved the direct-contact test, touch the long leg to the battery’s + side and the short leg to –. Your LED should light up! Keep the test brief. If anything starts to warm up, stop and get the facilitator.
3. 3. Now disconnect the LED and switch it around. Briefly test again. What happens?

• • Diodes only allow current to flow easily in one direction. Connect the LED backwards and it won’t light! How is this different from the bulb in Part I?

1. 1. Inside are metal strips that connect some of the pins. In the middle, the five pins A–E at one row number connect to each other. F–J make a separate group. Notice the gap? The connections don’t go across it!
2. 2. The long + (red) and – (blue) rails along the edges usually connect lengthwise. These are used for power. Some boards have a break partway down.
3. 3. Check your board with the multimeter’s continuity (beeper) setting before adding power. Touch the probes to two pins in the same strip, then to two separate strips. A beep means they’re connected. Never test continuity with a battery connected.

1. 1.

Place your empty battery holder on the breadboard. Make sure the + and – pins go into separate strips that are not connected. It is important that these do not connect! Choose holes that fit your holder, and don’t force the pins.

doc-coin-holder-poles.png



doc-bb-holder.png

1. 2.

Now add an LED. Put it across the center gap. For example, put the long (+) leg in E15 and the short (–) leg in F15. These holes are on separate strips, even though they share a row number.

2. 3.

Take a red jumper wire and a resistor. We want this path: holder + → resistor → LED +. Connect the red jumper from the holder’s + strip to a spare strip. Put one resistor leg in that spare strip and the other in the strip with the LED’s long leg. Make sure the resistor’s legs are on different, unconnected strips, or current can bypass it!

3. 4.

Take a black jumper wire. Connect the strip with the LED’s short (–) leg to the holder’s – strip.

4. 5.

Have a partner check your connections, then put the battery in according to the holder’s markings. Your LED should light up!

1. 1.

Take the battery out first!

2. 2.

Connect the holder’s + strip to the + rail with a red wire.

3. 3.

Connect the holder’s – strip to the – rail with a black wire.

4. 4.

Reconnect your LED branch so the path is + rail → resistor → LED +, then LED – → – rail. Check your work, then put the battery back in. Your LED should still light!

doc-bb-rails-1.png



doc-bb-rails-2.png

1. 5.

Remove the battery again. Add three more LED branches, each with its own resistor and connections to the + and – rails. Check your work, then put the battery back in.

2. 6.

Is this series or parallel? Predict what will happen if you remove one LED. Take the battery out, remove an LED, then reconnect power to test your idea.