Automatic Headlight Dimmer for Vehicles using LDR and Relay

High-beam glare from oncoming vehicles is one of the most common night-driving complaints on Indian highways and rural roads, and much of it is accidental — drivers simply forget to dip their beams in time. Manual dimming depends on reaction and discipline, so it fails exactly when it matters most. This project automates that one decision: an LDR light sensor watches the light arriving at the vehicle, an Arduino Nano decides whether an oncoming headlight is present, and a relay switches the headlamp between high and low beam without the driver touching anything. A hysteresis band in the firmware keeps the relay from chattering at the threshold, and a manual override switch lets the driver take control at any moment. The bench prototype demonstrates the complete sense–decide–actuate loop with a model headlamp, and the report documents how the same circuit maps onto a real vehicle's headlamp wiring.

1. The LDR forms a voltage divider with a fixed resistor; the Arduino Nano reads the divider voltage on an analog pin, so brighter incoming light gives a higher ADC value.
2. Every loop, the firmware compares the reading against two thresholds: if it rises above the high threshold, the relay is energized and the headlamp drops to low beam; it returns to high beam only when the reading falls below the lower threshold (hysteresis).
3. A second, longer-term reading tracks ambient light; when the ambient level indicates daytime, the dimming logic is held off so daylight flicker never triggers a dip.
4. The manual override toggle is read on a digital pin with debouncing; when forced high by the driver, it bypasses the automatic logic and holds high beam until released.
5. The Arduino drives the relay module through a transistor or the module's built-in driver stage; an LED on the module shows the energized state for the demo.
6. For calibration, the student shines a flashlight at the LDR while watching serial-monitor readings, then sets the two thresholds to bracket typical oncoming-headlight intensity versus night ambient.

Tech stack:

• Arduino Nano (ATmega328P)
• LDR sensor module (CdS photoresistor · LM393 comparator)
• 5V single-channel relay module
• Arduino IDE (C/C++ firmware)
• Voltage divider + threshold logic
• Toggle switch (manual override)
• Serial Monitor (calibration readouts)
• LED headlamp model (bench demonstration)

• [Automatic beam dimming] When the LDR detects bright incoming light above the set threshold, the Arduino drives the relay to switch the headlamp from high beam to low beam automatically, restoring high beam once the light level falls back.
• [Hysteresis band in firmware] Separate switch-on and switch-off thresholds with an adjustable dead band stop the relay from flickering when the light level hovers at the trip point — the classic fix for sensor chatter, implemented in code.
• [Manual override switch] A toggle switch wired in parallel lets the driver force high beam at any time; the override path is independent of the sensing logic so it works even if the sensor misbehaves.
• [Day/night awareness] Ambient light level is read continuously; the dimming logic only engages after dark so passing shadows or daylight driving don't trigger false dips.
• [Relay-based load switching] A 5V single-channel relay module switches the headlamp circuit with proper NO/NC contact wiring, opto-isolated from the Arduino's 5V logic side.
• [Threshold calibration in code] Trip levels are named constants in the firmware with a commented calibration procedure, so the student can tune sensitivity during the demo using a flashlight as a stand-in for oncoming traffic.
• [Bench headlamp demonstration model] The prototype drives a model headlamp bulb so the full sensing-to-switching behaviour is visible on the bench without touching a real vehicle.

• Working bench prototype: Arduino Nano, LDR sensor module, relay module, model headlamp, manual override switch
• Complete firmware (Arduino sketch) with hysteresis, day/night logic and debounced override
• Wiring diagram and breadboard layout
• Calibration procedure using a flashlight and the serial monitor
• Component list with sourcing notes
• Project report PDF (background, design, methodology, observations)
• PPT presentation for final review
• Viva Q&A preparation document (sensors, hysteresis, relays, isolation)