Timer having a variable operation period

In a timer the combination comprising a frame, a clock motor, a drive train driven by the clock motor and having a slow cam and a fast cam, the slow cam being in the form of a disc having a plurality of notches equally spaced thereon, a slow cam follower movably mounted on the frame and responsive to the arrival of a notch in the cam, a shielding disc adjacent the slow cam and rotatable therewith, the shielding disc having a plurality of lobes of differing arcuate spacing and indexable with respect to the slow cam between a plurality of successive phase positions in which the lobes selectively shield thereby to activate the only desired ones of the notches for response by the slow cam follower, the fast cam having a notch, a fast cam follower movably mounted on the frame and responsive to arrival of such notch, control means operated jointly by the cam followers so that the control means is actuated to initiate a turn-on cycle only when both cam followers respond by dropping into respective notches, the slow cam being provided with a pointer, a time scale for cooperating with the pointer, the time scale being stationary but adjustably positionable on the frame to set the time that a turn-on cycle is initiated, the motor having a clutch to enable manual rotation of the drive train so that the pointer and associated cams are synchronized with the time scale.