Breadboard Simple LED Circuit

A simple LED with a corresponding resistor (10kΩ) on a breadboard powered by a 9V block battery:

Setup on a breadboard

List of quantities, and their units as reference:

Quantity	Symbol	Unit	Abbr.	Description
Voltage	V	Volt	V	Potential difference across an electric component
Current	I	Ampere	A	Current going through an electric object
Resistance	R	Ohms	Ω	Resistance of an electric object
Capacity	C	Amp-hour	Ah	Battery capacity

Ohm’s Law

Ohm’s law describes the relation between voltage, current, and resistance:

V = I × R        V (volts) = I (amps) × R (ohms)

Q1. How would you arrange Ohm’s law to solve for current intensity?

I = V ÷ R        I (amps)  = V (volts) ÷ R (ohms)

Q2. How would you arrange Ohm’s law to solve for resistance?

R = V ÷ I        R (ohms)  = V (volts) ÷ I (amps)

Q3. If voltage increases while the resistance remains constant what would happen to the current intensity?

The current intensity increases proportional to the voltage increase.

R = 3.3V ÷   560Ω ≃ 0.0058A
      5V ÷   560Ω ≃ 0.0089A
      9V ÷   560Ω ≃ 0.0160A

Q4. If the resistance in a circuit is increased without changing the potential difference, what would happen to the current intensity?

The current intensity decreases proportional to the increase in resistance.

I = 9V ÷   560Ω ≃ 0.0160A
    9V ÷  1000Ω = 0.0090A
    9V ÷ 10000Ω = 0.0009A

Light Emitting Diode

Light Emitting Diodes (LEDs) are electronic semiconductor components producing light when current is applied.

Q1. Explain why LEDs can only be connected in a specific direction within a circuit.

LEDs are polarized which means that current needs to flow from the anode (positive) to the cathode (negative). The longer terminal (anode) goes to the positive voltage. A flat spot on the LED indicates the cathode terminal required to be connected to ground.

Q2. What is the voltage drop and current rating for standard LEDs?

Standard LEDs are rated for currents up to 20mA with a voltage of:

Color	Volt
red, orange, yellow	1.9-2.2V
green, blue	3.0-3.4V
white, violet, purple	2.9-4.2V

Resistor

The formula to calculate the correct resistance is:

R = (Vₛ - Vₗ) ÷ I

Where Vₛ is the voltage source and Vₗ is the voltage drop of the LED.

Q1. Given a standard through-hole red LED with a voltage drop of 2V, and a 9V battery. What resistance is required in the circuit to limit the current at the LED to 20mA?

R = (9V - 2V) ÷ (20mA × 1000) = 350Ω 

Q2. Through-hole resistors use color bands to indicate their resistive value and tolerance. Explain the 4- and 5-color-band systems.

The band-system uses colored rings that encircle the body of the resistor.

• The 4-band system uses four colored rings: digit, digit, multiplier, and tolerance
• The 5-band system uses five colored rings: digit, digit, digit, multiplier, and tolerance

Color	Digit	Multiplier	Tolerance
Black	0	1
Brown	1	10	± 1%
Red	2	100	± 2%
Orange	3	1,000
Yellow	4	10,000
Green	5	100,000	± 0.5%
Blue	6	1,000,000	± 0.25%
Violet	7	10,000,000	± 0.1%
Grey	8
White	9
Gold		0.1	± 5%
Silver		0.01	± 10%
None			± 20%

Typically the space between the multiplier and the tolerance is bigger. Hence the color band first digit is on the opposite side.

Q3. What 5-band colors have resistors with 560Ω, 220Ω, 10kΩ and a tolerance of ±1%?

560Ω = Green, Blue, Black, Black, Brown
220Ω = Red, Red, Black, Black, Brown
10kΩ = Brown, Black, Black, Red, Brown

Battery

Batteries have a electric specification i.e. an Alkaline 9V battery has a 9V nominal voltage, and a capacity of 565mAh.

Battery capacity is simply the product of current (I) multiplied by time (T) in hours:

C = I × T        C (amp-hours) = I (amps) × T (hours)

Q1. How would you arrange the equation to solve for time?

T = C ÷ I        T (hours) = C (amp-hours) ÷ I (amps)

Q2. Given a 9V battery with a capacity of 500mAh, and a load of 0.012A. How long can the battery support this load?

T = (500mAh ÷ 1000) ÷ 0.012A ≃ 41h