The science idea
Every working electrical device is a circuit: a complete loop through which charge can flow. A circuit needs a source of energy (a cell or battery), a path made of conductors, and a load that uses the energy, such as a bulb. If the loop is broken anywhere, by an open switch, a loose wire or a blown bulb, the current stops everywhere in that loop. This project shows the two basic ways of joining things together, and what each way does to brightness and reliability.
In a series circuit the parts are joined one after another in a single loop. There is only one path, so the same current passes through every part. The cell’s voltage is shared out among the parts: with two identical bulbs, each gets about half. Their resistances add up (R total = R1 + R2), so the current is smaller than with one bulb, and both glow dimly. If one bulb is removed or blown, the loop is broken and the other bulb goes out too.
In a parallel circuit each part is connected straight across the cells, on its own branch. The voltage across each branch equals the full voltage of the cells, so each bulb is as bright as it would be if it were alone. The currents in the branches add up, so the cells deliver more current in total, and they run down sooner. If one bulb is removed, the other branch is untouched and stays lit. This is why homes are wired in parallel: every appliance gets the full supply voltage, and switching one off does not turn off the others.
The board also introduces the LED, or light-emitting diode. An LED is a one-way device: it lights only when current enters its anode, the longer leg, and leaves by the cathode, the shorter leg and the flat side of the case. It also needs a resistor in series. Once an LED has started to conduct, the current through it can rise very sharply with a small rise in voltage, so without a resistor to limit the current an LED can burn out in a moment. The resistor takes the rest of the voltage: R = (V supply − V LED) ÷ I. A red LED drops about 2 V and a current of about 10 mA is plenty for a bright, safe glow.
Voltage, current and resistance are linked by Ohm’s law: V = I × R. If you have a multimeter you can check the sharing of voltage in the two circuits; the readings are an excellent set of results for your report.
Materials and tools
Cost: Under Rs 100
The cardboard, wire, paper clips, split pins and used cells are free. Two torch bulbs with holders, one red LED and one 100 Ω resistor together cost roughly Rs 40 to Rs 80 at an electronics shop (bulbs and holders from old torches and toys are free).
| What you need | How many | Free substitute or where to find it |
|---|---|---|
| Stiff cardboard for the base | 1 piece, 30 cm × 25 cm | A flat side of a carton. Use two layers glued together if it bends. |
| AA cells (1.5 V each) | 2, in a 2-cell holder | Two cells from a remote control. Without a holder, tape two cells end to end (the + button of one against the flat − end of the other) in a card tube. |
| Torch bulbs, 2.5 V (or 3 V), with screw-type bulb holders | 2 bulbs and 2 holders | Take them from an old torch or a toy. A holder is not essential: you can twist the wire ends round the metal tip and side of the bulb. |
| Red LED (5 mm) | 1 | Salvage one from an old electronic toy, or buy it (a few rupees). |
| Resistor, 100 Ω (brown, black, brown) or 220 Ω (red, red, brown) | 1 | From any broken electronic item, or buy one (about Re 1). |
| Thin insulated wire | About 2 metres in pieces of 10 to 15 cm | Cut up from an old phone-charger cable (an adult strips the outer cover) or telephone wire. |
| Metal paper clips and split pins (paper fasteners) | 1 clip, 2 pins for the switch | Stationery box. |
| Tape, glue, marker pens | 1 roll, 1 tube | From the house. |
Tools
- Scissors
- Pen or marker to label the board
- Ruler
- Small screwdriver if your bulb holders have screws
- Multimeter (optional) set to the DC voltage range only
Safety first
Read this before you start
- Use only two AA cells (3 V). Never connect any part of this board to a mains socket, a phone charger, a power bank or a car battery.
- Never join the two ends of the cells with a wire on its own (a short circuit). The wire gets hot and the cells are used up quickly. Always have a bulb, LED or resistor in the loop.
- Never connect an LED directly to the cells. It must have a resistor in series, or it will burn out and may snap.
- Torch bulbs and wires get warm when they have been on for some time. Switch them off between observations and do not touch the glass of a bulb that has been on for a while.
- An adult should strip the wire ends if you need a blade or a wire stripper. Keep small parts and cells away from young children.
- If you use a multimeter, set it to the DC volts range and touch the probes only across a bulb. Do not use the current range on this board.
Parts and layout
These drawings show the parts and how they fit together. Keep them beside you while you build.
Scroll sideways to see the whole diagram.
Scroll sideways to see the whole diagram.
Step-by-step build
13 steps. Do them in order, and test each part before you go on to the next.
Plan the layout on paper
Draw the board, then the two bulb holders, the switch, the cells, the LED and the resistor. Draw the series circuit and the parallel circuit as they will be wired. Decide where each part will sit.
Tip: Leave about 8 cm between the two bulb holders so you can handle them easily, and put the cells and switch near the edge.
Cut and mark the base
Cut a 30 cm × 25 cm piece of stiff cardboard and mark the positions of the parts. Label each with a marker: Cells, Switch, Bulb 1, Bulb 2, LED, Resistor.
Common mistake: Labelling only when you finish. Label as you go, so you can follow your own wiring.
Mount the cells
Glue or tape the cell holder near one edge so that the + end is easy to see and reach. Mark + and − clearly. If you have no holder, tape two cells end to end in a card cradle with the + button of one touching the − end of the next.
Common mistake: Putting the cells in with two + ends touching. Cells must be end to end, so the + of one touches the − of the other.
Prepare the wires
Cut about eight pieces of wire, each 10 to 15 cm long. Strip about 1 cm of insulation from both ends (an adult can help), and twist the strands of each end tight.
Tip: Keep the bare ends short. Long bare ends touch each other and cause short circuits.
Make the paper-clip switch
Push one split pin through the board and put a straightened paper clip under its head so the clip can swing like a lever. Push a second split pin in where the free end of the clip touches when you press it down. Wrap a wire end around the leg of each split pin underneath the board.
Tip: Press the clip down to make contact and let it spring up to break the circuit. If it stays down, bend it so it springs up.
Mount the bulb holders
Screw each bulb into its holder, and glue or tape each holder to the board. Check that the bulbs are screwed in all the way so the tip touches the contact at the bottom.
Common mistake: A bulb that is only half screwed in. It looks fine but does not light because its tip does not touch the contact.
Build circuit A: two bulbs in series
Connect the + end of the cells to one switch pin. Connect the other switch pin to one terminal of bulb 1. Connect the other terminal of bulb 1 to a terminal of bulb 2. Connect the other terminal of bulb 2 to the − end of the cells. Close the switch and watch both bulbs.
Tip: Switch off between observations, so that the bulbs and cells do not get warm.
Test the series circuit
Note how bright each bulb is. Unscrew bulb 1 while the switch is closed and watch bulb 2. Screw it back in. Write what you see in the table.
Common mistake: Not checking that both bulbs are the same. Different bulbs share the voltage unequally, so use two identical bulbs.
Build circuit B: two bulbs in parallel
Disconnect the wires between the bulbs. Connect one switch pin to a terminal of bulb 1 and the same switch pin to a terminal of bulb 2 (use a pair of wires on the same pin). Then connect the other terminal of each bulb to the − end of the cells. Close the switch.
Tip: It helps to use two coloured wires: one colour for the rail from the + side and another colour for the rail from the − side.
Test the parallel circuit
Compare the brightness with the series circuit. Unscrew one bulb while the switch is closed and look at the other. Note what happens, and how soon the cells feel warm if you keep the switch closed.
Tip: Keep the switch closed for no more than 15 seconds at a time. Two bulbs in parallel draw a larger current, so the cells run down faster.
Measure the voltage (optional)
With a multimeter on the DC voltage range, touch the probes across the cells and write the voltage, then across bulb 1 and bulb 2 in each circuit. In series the two bulb voltages add up to about the cell voltage; in parallel each is close to the cell voltage.
Common mistake: Turning the dial to the current (A) range and touching the probes across a bulb or the cells. Keep it on the DC volts range.
Add the LED circuit
Connect + of the cells to the switch, then the switch to the 100 Ω resistor, the resistor to the long leg of the LED, and the short leg of the LED back to the − end of the cells. Close the switch. If it does not light, reverse the LED, which may be the wrong way round. Do not connect the LED without the resistor.
Common mistake: Joining the LED straight to the cells for a moment to “just check”. A single try without a resistor can destroy it.
Label and present
Draw the symbols for a cell, switch, bulb, resistor and LED on a card, and label the board with the names of the circuits. Add a short note under each: what you saw and why.
Tip: Ask someone else to build the series and parallel circuits from your labelled board. If they can, your labelling is clear.
How it should look when it is finished
A neat cardboard board about the size of a school notebook, labelled in marker, with a cell holder at one edge, a paper-clip switch, two bulb holders side by side, and a small LED with a resistor. Insulated wires run between the parts, and you can reconnect them to turn the same two bulbs into a series or a parallel circuit.
This is a description, not a photograph. Compare your own model with it and with the figures above.
How it works
The cells pull charge round the loop. Chemical energy in the cells is turned into electrical energy, which is carried by the moving charges and turned into light and heat in the bulb. Current flows only in a closed loop, and the switch opens and closes the loop.
In the series circuit the current has one route, so it is the same in both bulbs, and the cell’s voltage is divided between them. Two identical bulbs each get about half the voltage, so each glows dimly, and when you remove one the loop is broken, so the other goes out.
In the parallel circuit the two bulbs are joined between the same two rails, so each has the full voltage of the cells and glows as brightly as it would alone. The current splits at the junction and joins again, so the cells supply the total of the two currents. Removing one bulb breaks only its own branch.
The LED conducts only one way. The resistor in series limits the current by taking up the difference between the cell voltage and the LED’s own voltage drop, so the LED gets about 10 mA and glows safely.
Scroll sideways to see the whole diagram.
Troubleshooting
| What you see | Likely cause | What to do |
|---|---|---|
| Neither bulb lights in the series circuit | A loose wire end, a bulb not fully screwed in, a dead cell, or a switch that does not touch. | Press each connection, screw the bulbs in fully, try fresh cells, and check the clip touches the second pin. |
| The bulbs are very dim in series | This is normal because each bulb gets about half the voltage. Weak cells also make it dimmer. | Compare with the parallel circuit. If both circuits are dim, change the cells. |
| In parallel, one bulb is lit and the other is not | The unlit bulb is loose, blown, or its wires are not on the right rails. | Screw the bulb in fully, test it by itself in a one-bulb circuit and check its two wires. |
| The LED does not light | It is the wrong way round, the resistor is too large, or a joint is loose. | Reverse the LED, check the resistor is 100 Ω to 220 Ω, and press the joints. |
| The wires or the cells get warm | A short circuit: bare wire ends touching each other. | Open the switch and remove the cells. Find where the bare ends touch and cover them with tape. |
| Both bulbs go out when I unscrew one in the parallel circuit | They are actually in series, because the wires are connected one after another. | Check the diagram: each bulb must connect directly to both rails. |
Test and record
Use the same two bulbs and fresh cells. Brightness can be written as very dim, dim, normal or bright. Voltages are optional, if you have a multimeter on the DC voltage range.
| Circuit | Brightness of bulb 1 | Brightness of bulb 2 | One bulb unscrewed: what happens to the other? | Voltage across bulb 1 (V) | Voltage across bulb 2 (V) |
|---|---|---|---|---|---|
| Series | |||||
| Parallel | |||||
| Voltage across the cells (V) |
Copy the table into your notebook and write each reading as you take it, in pencil, with the date. Repeat every test at least three times and use the average. Then draw a graph or chart of your results.
Viva questions and answers
A teacher or a judge will ask you how your project works. Practise these eight questions and answer them in your own words.
- What is the difference between a series and a parallel circuit?
In a series circuit the parts are joined in one loop, one after another, so the same current flows through all of them. In a parallel circuit the parts are on separate branches joined across the same two points, so each gets the full voltage.
- Why are the two bulbs dimmer in series?
The cells’ voltage is shared between them, so each gets only about half. Their resistances also add up, so the current is smaller.
- Why does one bulb stay on in a parallel circuit when you unscrew the other?
Each bulb has its own complete path to the cells, so breaking one branch does not affect the other.
- Why are the lights in homes wired in parallel?
So that every appliance gets the full supply voltage, and each can be switched on and off on its own.
- Why does an LED need a resistor?
An LED’s current rises very quickly with a small rise in voltage once it conducts. The resistor limits the current and so protects the LED from burning out.
- How did you choose the resistor?
R = (V supply − V LED) ÷ I = (3 − 2) ÷ 0.01 = 100 ohms for a red LED at about 10 mA from two cells.
- Which leg of an LED is positive?
The longer leg is the anode (+). The shorter leg, next to the flat edge of the case, is the cathode (−). Connect the long leg towards the + of the cells.
- State Ohm’s law.
The current through a conductor is proportional to the voltage across it at a constant temperature: V = I × R.
Written report outline
Use these headings in your project file, and write each part in your own words. Add a title page, your diagrams, the precautions you took and the books or sites you used.
- Aim
- To build series and parallel circuits with two bulbs and compare their brightness and behaviour, and to light an LED safely with a resistor.
- Apparatus
- Cardboard base, two AA cells in a holder, switch, two torch bulbs in holders, a red LED, a 100 Ω resistor, connecting wires, multimeter (optional).
- Procedure
- Describe how you built the board, wired circuit A (series) and circuit B (parallel), observed brightness and the effect of removing a bulb, measured voltages if you could, and built the LED circuit. Include the circuit diagrams.
- Observation
- The table of brightness, the behaviour when a bulb is removed and any voltage readings.
- Result
- In series the bulbs were dim and both went out when one was removed. In parallel each was bright and the other stayed on. Include your voltage readings.
- Conclusion
- In a series circuit the current is the same through each part and the voltage is shared; in a parallel circuit each branch has the full voltage and works independently. An LED needs a resistor in series to limit the current.
Take it further
- Add a third bulb and predict, then test, what happens to the brightness in series and in parallel.
- Make a mixed circuit: two bulbs in series, and a third in parallel with the pair. Predict which will be brightest.
- Use the multimeter to measure the cell voltage with no load, and again with one bulb and with two bulbs in parallel. Why does it drop slightly?
- Build a model of a house with three rooms, each lit by its own switch, in parallel, from the same cells.
- Find the best resistor for a green and a blue LED. (Blue and white LEDs need about 3 V, so two cells are not enough: look up the forward voltage.)
Free ebooks, kits and help
Free ebooks for this project
Science projects
Electricity and Magnetism Experiments
Hands-on experiments with cells, wires, bulbs and magnets
Robotics
Electronic Components Handbook
Resistors, capacitors, LEDs, diodes, transistors and more, with how to use each
Hardware & repair
Using a Digital Multimeter
Measure voltage, current, resistance and continuity safely
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