Voltaire was born in 1694 and died in 1778. He came from a noble family and was clearly the opposite of Rousseau: he was a posh, educated man who would only be associated with noble people. He was very wealthy, liked luxury, nice food and was a bit of a materialist. He was also very disdainful towards poor people and thought that happiness cannot exist without luxury. He believed in science and reason and also contributed to the Encyclopedia. Contrary to many of his fellows, Voltaire was a deist, which means that he believed an upper spirit created the Universe, however he didn't believe in any religion.
As far as I'm aware, Voltaire took part in four trials trying to denounce corruption and to prove convicted people innocent. His first commitment was the Calas affair.
Marc-Antoine Calas was born in a protestant family in the South of France in 1732. At the age of 29, he was found hung in his parents' shop. To avoid a shameful funeral, his parents decided to tell the police he was murdered. A rumour in the neighbourhood started to spread saying that Marc Antoine Calas wanted to convert to Catholicism. His father Jean Calas would have disagreed with his decision and killed him. The police officers were convinced by the neighbours' saying. Jean Calas was thrown in jail and tortured to admit his crime. Although he always claimed he was innocent, on the 10th, March 1762, he was executed. A few days after his death, his other son Pierre Calas was forced to go abroad, and went to Geneva where he met Voltaire. Pierre told him his story and asked him to help. Voltaire managed to convince the authorities to retry the Calas family thanks to his book Treatise on Tolerance, published in 1763. This final trial was more successful as Jean Calas was finally proved innocent. The King Louis XV gave the family 36,000 pounds in compensation.
Thus, Voltaire was the first French author who got involved in a miscarriage of justice. He also took part in a similar trial; the Sirven affair where a protestant couple was accused of having killed their disabled daughter who apparently wanted to become catholic. They were condemned to the death penalty, but Voltaire succeeded in proving they were innocent in time.