41099 Chap.3 Resistors, LEDs, breadboards and schematics
Resistors, LEDs, breadboards and schematics
What this chapter covers
- 01
Why a light emitting part is not a resistor, and what the steep characteristic means
- 02
Polarity, and the two ways to tell one end from the other
- 03
Designing the limiting resistor from the supply, the forward voltage and the target current
- 04
Reading four band and five band colour codes, tolerance band first
- 05
Breadboard continuity: vertical rails, horizontal banks and the gutter between them
- 06
Debugging against the schematic rather than against a photograph
Design a limiting resistor from published part figures and justify the rounding
- 1Subtract the forward voltage from the supply to find the resistor's share.
- 3Divide by the target current and compare the result with stock values.
- 2Round toward safety and confirm the result against the published maximum.
Key terms
- Forward voltage
- The voltage a light emitting part drops once it is conducting, set by its material and colour.
- Anode
- The terminal a light emitting part's current enters by.
- Cathode
- The terminal a light emitting part's current leaves by.
- Limiting resistor
- The series resistor whose value decides the operating current of a light emitting part.
- Schematic
- A drawing of which components exist and how they are connected, independent of physical layout.
- Wiring diagram
- A drawing of one physical arrangement that satisfies a schematic.
Resistors, LEDs, breadboards and schematics FAQ
Why can a light emitting part not simply be connected across the supply?
Its characteristic is almost vertical above the forward voltage, so a tiny change in applied voltage produces an enormous change in current. Without a series resistor nothing in the circuit decides the current except the internal resistance of the supply and the part, and the result is normally far past what the part can survive.
What does the colour of the part tell you about the design?
Colour is a clue to forward voltage. Red parts begin to conduct near 1.2 V and blue parts near 3.6 V according to the course materials, so a blue part on a 5 V rail leaves the resistor far less voltage to work with. That tighter headroom is why blue and white parts vary noticeably in brightness between otherwise identical boards.
How do you read a resistor whose bands are ambiguous?
Find the band sitting slightly apart from the rest, hold the part with that band on the right, and read left to right. Two digits then a multiplier on a four band part, three digits then a multiplier on a five band part. If the bands remain unclear, measure the part with a meter while it is disconnected from any circuit.
Why does a circuit that looks identical to the picture still not work?
Because the picture shows layout and the schematic states connection, and only connection determines behaviour. Two legs that happen to land in the same five hole bank are the same node, which shorts the component between them, and no photograph reveals that. Check continuity node by node against the schematic instead.
When should a design move off the breadboard?
As soon as it is finalised. Breadboards exist for prototyping because they assemble and disassemble easily, and the course materials say a completed design should move to a printed circuit board. A device that has to be carried to a demonstration is exactly the case where an intermittent jumper costs marks a working design had already earned.
Assessment move
For every component question, ask three things in order: which way does current flow through this part, how much current should flow, and what decides that amount. Practise reading colour bands on parts you then measure, so the reading is confirmed rather than assumed. Debug with a meter in continuity mode against the schematic.
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