Some machines need to run for long periods and therefore may finish their work in the middle of the night or during the weekend. For the time remaining, until the operator returns, the machines stay idle, uselessly consuming power. This Design Idea allows a machine to completely shut itself down after finishing its work. In addition, the method allows for informing the machine operator by phone. You insert the circuit into the area that Figure 1 indicates as a dashed line into the main supply line of the machine. The relay, K_STOP, connects to a free output of the programmable controller of the machine. You must program the controller in such a way that relay K_STOP is energized as long as the process is running. In normal operation, switch S₁ stays in manual position; thus, the power contactor, K₁, is on, and the machine receives power. When an operator starts the process, relay K_STOP energizes, and the indicator, H₁, lights, signaling the operator that switch S₁ is ready for operation. The timer relay, K_2T, is also on, closing its contact 18-15. Switching S₁ to automatic now has no effect.

At the end of the process, relay K_STOP and indicator H₁ turn off. Because contact 18-15 of relay K_2T incurs a delay before opening, because K_2T is a time-delay relay, the machine stays on during the delay time. This delay allows a second contact of K_STOP to control an automatic telephone dialer (not shown) to inform the remotely located operator and allows the process to finish supplementary tasks, such as cooling down, removing chips, allowing coolant to flow back into the tanks, for example.

Once the delay time expires, the contacts of K_2T open, K₁ turns off, and the machine completely turns off. The varistors, V_R1, suppresses voltage spikes. You must select V_R1, K₁, K_2T, and H₁ in accordance with the power-mains voltage and the power rating of the machine. You select K_STOP according to the controller's output (the relay coil) and the power-mains voltage (the relay contacts). The circuit has worked satisfactorily in hundreds of machines over a five-year period.