Use an LED with light arrows and current limiting
Identify outgoing light arrows and build a forward-connected indication circuit with a supplied operating model.
Section P4.3.1 · Circuit diagrams and circuit components
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Read diode anode and cathode orientation.
- Use V = IR with supplied component values.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Where is the diode cathode marker?
What you will learn
- Draw a light-emitting diode symbol.
- Interpret forward emission and a series limiting resistor.
Distinguish emitted and incoming light
A light-emitting diode, LED, is a diode that emits light when operating with suitable forward current. Its symbol adds arrows pointing outward from a diode symbol. An LDR has inward arrows because it receives light; the two components have different roles and different base symbols.
Read the cathode bar exactly as for a diode. With ordinary intended use, reverse connection does not produce the desired emission and may damage a device beyond its rated reverse limit. This is an operating-range statement, not a claim of harmless reverse operation at any voltage.
Use a stated current-limiting model
A series resistor limits LED forward current. For a supplied constant-forward-drop approximation, subtract the given LED drop from the source p.d. to get the resistor p.d., then use R = Vresistor/I. This is a calculation using supplied data, not a universal forward voltage or a universal component-selection recipe.
For a 5 V ideal source, a stated 2 V LED drop and target 0.010 A current, the resistor p.d. is 3 V and the ideal resistance is 300 Ω. Real selections also require source tolerance, device ratings and resistor power considerations. Written examples use isolated low-voltage contexts; teacher-supervised construction remains separate.
Sources for this lesson
Pause and explain
With 5 V supply, supplied 2 V LED drop and 10 mA, what resistance fits the ideal model?
Worked example
A 5 V source feeds an LED with a supplied 2 V drop and 0.010 A current. Find the series resistance in this model.
Show the worked solution
- The resistor p.d. is 5 − 2 = 3 V.
- Use R = 3/0.010.
- The model gives 300 Ω.
Answer 300 Ω in the supplied ideal constant-drop model.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Draw a forward-connected LED and resistor in a complete cell circuit, with light arrows.
Give me a hint
Retain the diode bar and add outward arrows.
Compare my reasoning
- Draw the source, series resistor and diode body.
- Orient the LED anode toward positive and complete the return.
- Add two arrows directed away from the diode to show emitted light.
Forward-connected LED with outward light arrows and a series limiting resistor.
Look for these in your work
- Retain polarity.
- Show emitted-light direction.
A supplied model has a 9 V source, 3 V LED drop and 0.020 A target current. Find the limiting resistance.
Give me a hint
Subtract the LED drop before dividing by current.
Compare my reasoning
- The resistor p.d. is 9 − 3 = 6 V.
- R = 6/0.020.
- R = 300 Ω.
300 Ω in the supplied model.
Look for these in your work
- Use the given drop.
- State ohms and model conditions.
A learner removes the resistor because an LED is a diode and labels its outward arrows as incoming room light. Correct both errors.
Give me a hint
Separate one-way behaviour, current magnitude and optical direction.
Compare my reasoning
- Forward diode conduction does not guarantee suitably limited current.
- Keep suitable series current limiting and the device ratings.
- Outward arrows show emission; inward arrows belong to an LDR input.
Retain suitable current limiting; the LED outward arrows represent emitted light.
Look for these in your work
- Correct both claims.
- Do not infer real ratings from the drawing.
Common mistakes
- LED arrows point outward.
- Forward-drop values must be supplied.
- A symbol does not prove component ratings or practical suitability.
What can you explain now?
How do LED light arrows differ from LDR light arrows?
Compare with the explanation
LED arrows point outward; LDR arrows inward.
LED emits light while the LDR receives light as a sensing input.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Draw an LED with outward arrows and use only supplied voltage-drop and current models.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.