Touchless Doorbell That Rings Only When Hand Is Sanitised

This touchless doorbell system operates by combining gas-sensing technology with microcontroller logic to enforce hand sanitization before a visitor can sound the alert. At the heart of the setup is an Arduino Board R3, which continuously monitors the analog output from an MQ3 alcohol sensor connected via a 4-pin connector (CON3). The MQ3 sensor is specifically sensitive to alcohol vapors, which are commonly present in hand sanitizers. When a visitor places their freshly sanitized hand near the sensor, the volatile alcohol compounds evaporate into the air and alter the conductivity of the MQ3 sensor, causing its output voltage to rise. The Arduino reads this voltage change through its analog-to-digital converter. Once the signal crosses a predefined threshold indicating the presence of alcohol, the Arduino sends a control signal from one of its digital pins to the IC1 ULN2003 relay driver. Because microcontroller pins cannot supply enough current to directly drive an electromagnetic relay, the ULN2003 uses Darlington transistor arrays to safely switch the higher current required by the 5V relay (RL1). When RL1 activates, it closes its internal switch contacts to complete the circuit for the alerting peripherals, which include a piezo-buzzer (BZ2) to provide an audible chime and a 5mm LED (LED1) paired with a 220-ohm resistor (R1) for visual indication. A 1N4007 rectifier diode (D1) is placed across the relay coil to safely dissipate back-EMF voltage spikes when the relay turns off, protecting the driver IC and microcontroller from inductive kickback. Power is supplied uniformly to the circuit via a stable 5V regulated power supply connected through 2-pin terminal blocks (CON1 and CON2).