Fig 6 shows one of the standard circuit symbols used in electronic sensing circuits. This component is part of an automatic outdoor security light that dete...

(a) The component shown in Fig 6 is a variable resistor (also called a potentiometer). [1]

The symbol shows a standard resistor rectangle with a diagonal arrow passing through it. The arrow represents the sliding contact (wiper) that can be moved to change the resistance. Unlike a fixed resistor, the user can adjust the resistance value continuously between zero and its maximum rated value.

(b) In a sensing circuit, a variable resistor is used to set the threshold at which the circuit triggers. [1]

For example, in the outdoor security light described, the variable resistor allows the user to adjust the light level at which the lamp switches on. By turning the control, the user decides whether the light activates at dusk, at full darkness, or somewhere in between.

(c) How the variable resistor adjusts sensitivity in a potential divider with an LDR: [2]

The variable resistor is connected in series with the LDR to form a potential divider across the power supply. The voltage at the junction between the two components depends on the ratio of their resistances.

1. Higher variable resistance setting: The variable resistor takes a larger share of the supply voltage. The LDR must reach a lower resistance (brighter light conditions) before the junction voltage changes enough to trigger the circuit. The system is less sensitive, requiring darker conditions before switching on.
2. Lower variable resistance setting: The variable resistor drops less voltage, so even a relatively high LDR resistance (moderate light) produces enough voltage change at the junction to trigger the circuit. The system becomes more sensitive, switching on at a higher ambient light level.

This allows the user to calibrate precisely how dark it must be before the security light activates.