Fig 16 shows a circuit diagram for a model traffic light system built by a student as part of a design and technology project. The circuit uses three LEDs i...
Question 1 Report
Fig 16 shows a circuit diagram for a model traffic light system built by a student as part of a design and technology project. The circuit uses three LEDs in the colours red, amber and green, each controlled by a separate output pin of a microcontroller. Each LED is connected through its own current-limiting resistor to prevent damage from excessive current. A push-button switch is wired to an input pin and resets the traffic light sequence to the starting position when pressed. The fig shows the standard symbols used for each component in the circuit, including the microcontroller, LEDs, resistors and push-button switch. The complete circuit is assembled on a small printed circuit board and powered by a 5V regulated supply.
(a) Give one reason why each LED in the circuit requires its own series current-limiting resistor. [1]
(b) The red LED has a forward voltage of 1.8V and requires 20 mA of current. The supply voltage is 5V. Calculate the value of the series resistor needed. Show your working. [2]
Answer Details
(a) Each LED requires its own series resistor because different coloured LEDs have different forward voltage drops. [1]
Red, amber and green LEDs each have slightly different forward voltages (typically 1.8V, 2.0V and 2.1V respectively). Without individual resistors, the current through each LED cannot be controlled independently. A single shared resistor would cause unequal current distribution: the LED with the lowest forward voltage would draw the most current and could be damaged, while others would be dimmer. Individual resistors ensure each LED operates at its correct safe current.
(b) Calculating the series resistor for the red LED: [2]
Given: supply voltage = 5V, LED forward voltage = 1.8V, required current = 20 mA = 0.020 A.
Voltage across the resistor:
\[ V_R = V_{\text{supply}} - V_{\text{LED}} = 5 - 1.8 = 3.2\text{ V} \]
Using Ohm's law:
\[ R = \frac{V_R}{I} = \frac{3.2}{0.020} = 160\text{ }\Omega \]
The nearest standard preferred resistor value is 180 ohms. Using 180 ohms rather than exactly 160 ohms slightly reduces the current (to about 17.8 mA), which is safely within the LED's operating range and extends its lifespan.
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