On my Prusa Mendal I have a heated bed and I use a Gen6

On my Prusa Mendal I have a heated bed and I use a Gen6 board which doesn’t have the hardware to control it. So it is either on or off, on it gets up to around 160 deg, powered by an ATX power supply. What would be the easiest way to add control to the heated bed?

PWM through an arduino?

Heated bed doesnt really need PWM but it sounds like temperature feedback is what you’re looking for. You could pick up a dedicated PID controller (relatively cheap off ebay) or hack one together with an Arduino, MOSFET, and thermistor.

A cheaper way (for fixed temperature only) is a NC temperature switch (direct if max current does not exceed heater amps or through a relay otherwise). Like this one: http://www.ebay.com/itm/KSD301-Gold-Tone-Terminals-125C-N-C-Temperature-Controlled-Switch-Thermostat-/310568164901?pt=LH_DefaultDomain_0&hash=item484f4e1e25

could you set up a voltage divider to control the voltage supplied to the heater?