Most industrial processes require rotation of the motors in forward and reverse directions for desired periods. One good example is the automated bottle filling plant. Here the bottles move on a conveyor belt. When the bottles come under the filler, the filler comes down (the motor attached with the mechanism rotates forward) and fills the bottle (the motor stops), then it goes up (the motor rotates in reverse direction) and stops until the next bottle arrives. For moving the filler up and down, the time of rotating the motor forward and reverse is calibrated and fixed. Also, the stop time of the motor is calibrated based on the time required to fill the bottle and the time before arrival of the next bottle.
A good domestic application is in washing machines. Once the timer is set to wash clothes, the motor automatically rotates forward and then backward for fixed periods (10 to 15 seconds) with small pauses in between.
As this is a sequential process, a sequential timer can be used to implement it. Sequential timer is a widely used circuit in industrial plants because most industrial processes are chain reaction type. That means as one process ends, it triggers the next. The ending of the last process triggers the first process. Thus the cycle continues.
Generally, such sequence timers are microcontroller-based, multifunctional and programmable. But a very simple sequential timer can be developed using NE555 ICs wired in monostable mode. Cascading a number of these monostable stages forms a sequential timer. The output of one stage is applied as the trigger to the next stage. So when the output of a stage drops, it triggers the next stage and the output of the next stage goes high, and likewise the chain reaction starts. Because here the process involves four steps (forward→stop→reverse→stop), four stages of NE555 ICs connected in monostable multivibrator mode are used to form a four-stage sequential timer. The first stage rotates the motor forward. The second stage stops the motor. The third stage rotates the motor in reverse. The fourth stage stops the motor.
These stages actually energise or de-energise the relays that connect the motor to the supply.