It is full wave rectification without a filter capacitor.
Without any load resistor the sample and hold capacitor on the arduino input will act as a smoothing capacitor.
Put a 10K load ( resistor) between the analogue input and ground.
It is full wave rectification without a filter capacitor.
Without any load resistor the sample and hold capacitor on the arduino input will act as a smoothing capacitor.
Put a 10K load ( resistor) between the analogue input and ground.