Fig. 7 shows a circuit used in an electronic night light. The circuit turns on an LED when the light level falls below a set value. The item shown is common...

(a) The components labelled in the circuit are:

• A - Light Dependent Resistor (LDR) [1]: The circuit symbol shows a component enclosed in a circle with arrows pointing inward, indicating sensitivity to light. An LDR is a component whose resistance changes with light level: in bright light, its resistance is low (typically a few hundred ohms), and in darkness, its resistance is high (up to several megaohms). In this night light circuit, the LDR acts as the sensor that detects when the ambient light drops.
• B - Fixed resistor (10 k-ohm) [1]: This rectangular symbol with "10k" marked inside represents a resistor with a fixed value of 10,000 ohms. Together with the LDR, it forms a potential divider (voltage divider). As the light level changes, the ratio of the LDR's resistance to the fixed resistor's resistance changes, altering the voltage at the junction point between them. This voltage is what triggers the transistor.
• C - Transistor (NPN transistor) [1]: The three-terminal component acts as an electronic switch in this circuit. When the voltage at its base (from the potential divider junction) exceeds approximately 0.7 V, the transistor switches on, allowing current to flow from the collector to the emitter and through the LED circuit. As light falls, the LDR's resistance increases, raising the base voltage until the transistor turns on.
• E - Light Emitting Diode (LED) [1]: The triangle-and-bar symbol with two outward-pointing arrows represents a diode that emits light when current flows through it in the forward direction. The arrows indicate light emission. The LED is the output of the circuit, providing the night light illumination when the transistor switches on. It is connected in series with resistor D (330 ohms), which limits the current to a safe level to prevent the LED from burning out.