Prepaid Energy Meter using GSM and Arduino

The Prepaid Energy Meter system operates by combining microcontroller processing, cellular communication, and power control hardware centered around an Arduino Uno. An analogue electricity energy meter measures the mains power consumption of a connected load, such as a bulb and holder. To interface the mechanical or digital pulses of the energy meter with the low-voltage digital domain of the Arduino, a 4N35 optocoupler is utilized, which provides safe electrical isolation while transmitting pulse signals corresponding to the consumed units. The Arduino Uno tracks these incoming pulses to continuously calculate and decrement the user's available energy balance. Users can top up their credit remotely by sending a specific message via a SIM card installed in the GSM module. The GSM module communicates with the Arduino to receive and process these recharge SMS commands, updating the stored balance accordingly. Real-time system information, including current consumption metrics and remaining balance, is rendered visually on a 16x2 LCD display, adjusted for contrast using a potentiometer (POT). When the calculated balance drops to zero, the Arduino triggers a control signal through a ULN2003 relay driver, which safely switches a mechanical relay to cut off power to the load. Conversely, a positive balance keeps the relay closed, maintaining uninterrupted power flow to the appliance.