The proposed constitution that open up the voting rights to all free adult men in 508 bc was written by

the proposed Constitution that open up the voting rights to all free adult men in 508 BC was written by Cleisthenes.

Cleisthenes (570 bce - 508 bce), "The founder of Athenian democracy", was a statesmen who allied with the popular Assembly against the nobles in 508 bce and created a series of reforms that aimed at breaking the power of the aristocratic families and preventing the rise of another tyrant by giving adult free men certain participation in politics matters.  

In order to reduce the influence of traditional groups or clans and promote democracy to a centain extent, Cleisthenes reorganized the people of Athens into 139 local units or "demes", in which all free adult male had to register to become a citizen and be able to participate in the boule, a new council of 500 where everyone had an equal right to speak, and elect its own official.