Automatic control of street lights – suitable sensor component Which component is commonly used to sense ambient light for automatic switching of street lights (dusk-to-dawn control)?

Introduction / Context:
Automatic street lighting requires a reliable, inexpensive way to detect ambient illumination so that lamps switch on at dusk and off at dawn. Several passive and active components exist, but not all are suited to sensing light directly. The most common low-cost choice is an LDR (light-dependent resistor), also called a photoconductor cell.

Given Data / Assumptions:

• Goal: detect ambient light level with a simple threshold circuit.
• Cost and ruggedness matter for outdoor installations.
• We need a component whose resistance changes strongly with light intensity.

Concept / Approach:

Photoconductors (e.g., CdS or CdSe LDRs) have resistance that drops dramatically when illuminated and rises in darkness. A simple voltage divider feeding a comparator or transistor can switch a relay or a triac to control the lamp. Thermistors respond to temperature, not light. Transistors require a preceding photosensor or phototransistor/photodiode variant to detect light. Varistors respond to voltage surges and are used for protection, not sensing, while Zener diodes provide references or clamps, not light sensing.

Step-by-Step Solution:

Verification / Alternative check:

Commercial dusk-to-dawn switches widely use LDRs or photodiodes; LDR-based designs are ubiquitous for cost-sensitive applications.

Why Other Options Are Wrong:

Thermistors sense temperature; varistors clamp surges; a plain transistor needs a light-sensitive front end; Zeners regulate voltage.

Common Pitfalls:

Ignoring ambient temperature effects on LDR resistance; proper hysteresis and weatherproofing improve reliability.

Final Answer:

Photoconductor / LDR (light-dependent resistor)