Desiccant dryers lower the dew point of compressed air by adsorbing the water vapor present in the compressed air onto the surface of the desiccant. Adsorption continues until equilibrium is reached between the partial pressure of the water vapor in the air and that on the surface of the desiccant. These dryers continuously process compressed air by using two identical towers, each containing a desiccant bed. While one tower is on-stream drying, the other tower is off-stream being regenerated (reactivated, i.e. dried out). The towers are alternated on and off-stream so that dry desiccant is always in contact with the wet compressed air. In this way a continuous supply of dry air down stream of the dryer is possible. The switching from one tower to the other is controlled by a solid-state controller on either a fixed time basis (standard) or a demand basis (optional).