The importance of controlling the speed of DC motors in manufacturing industries such as plastics, textiles, chemicals and pharmaceuticals hardly needs to be emphasized, since it ensures efficient and consistent production. In this article, the authors present implementation of a hardware circuit that is designed for remote speed control of a DC motor by using dual tone multi-frequency (DTMF) tone mobile phone. The hardware circuit includes the use of the DTMF decoder IC MT8870 and the driver driver ULN2003. The keys of the mobile keyboard have been assigned to the speeds of 'High Speed', 'Medium Speed', 'Low Speed' and 'Stop' to regulate the speed of the DC motor.
A DC motor generates a torque directly from the direct current supplied to the motor by internal switching, permanent stationary magnets and by rotating electric electromagnets. It operates on the principle of the Lorentz force according to which any conductor carrying current within the external magnetic field experiences a torque or force known as a Lorentz force. The latest development in dc drives are brushless DC drives, the advantages of a brushed dc motor include low initial cost, increased reliability and simple motor speed control. DC motors can be used as DC drives invariable applications that require adjustable speed as well as good regulation of speed and frequency of start-braking-reverse. Some of the important industrial applications of DCdrives are rolling mills (paper, steel), mine winders, forklifts, machine tools, traction, printing presses, textile mills, excavators and cranes. There are a number of methods that can be used to control the speed of DC motors. If a DC motor works with the remote control by the mobile phone it is obviously advantageous for the industry. In this project, the speed control of a DC motor is guaranteed by the dual frequency tone (DTMF) tone of mobile phone. DTMF is a method to represent digits with tone frequencies in order to transmit them through an analog communication network . The tone received by the mobile headset is processed by the DTMF decoder IC MT8870 which decodes in equivalent binary. This binary signal is also sent to the relay controller IC ULN2003 to ON / OFF of the relays according to the user-pressed key which is responsible for controlling the speed of the DC motor.