As part of the solar hot water system, we have been installing in our home we wanted to be able to keep track of the mains electricity being used by the house. Based on the open-source OpenEnergyMontor website we developed our own version of their mains logging system to log the mains voltage and current consumption. The circuit comprises two circuits on the same printed circuit board.
The current logging is using the OpenEnergyMonitor code and circuit using CT Sensors. Visit the CT sensors - Interfacing with an Arduino page to view the circuit and code.
The image below shows the back of the PCB
The second part of this circuit is to monitor the mains voltage. The circuit is a basic power supply using a small 9v transformer which a bridge rectifier, smoothing capacitor and a voltage divider with one resistor using a multi-turn pot (the yellow one in the image below) to adjust the output to be 2.4V when the mains input is 240V.
This is read via one of the analogue ports on the Arduino board.