In this paper, a three phase automatic changeover switch (ACS) having a timer circuit and over-current protector is developed to save industries and individuals of the drawbacks of using manual method of power supply changeover. This switch senses when the main supply is disrupted and/or when the generator set time is reached for operation and automatically starts the alternative power source (generator) and transfers the load unto the generator and monitors its circuit to know when the utility supply is restored in order to reconnect the load back to the former while powering off the generator. The ACS was developed based on a programmable logic controller (PLC) which coordinates the following components such as; contactor, transformer, relays, comparator, 555 timer, regulator, transistors, resistors, capacitors, diodes, sensor, etc. The simulation of the designed circuitry was done using Proteus 8 software and the microcontroller programmed with M-ICU studio that has emulator of emul8051 (TS control emulator 8051 v.1.0). After which the physical construction was done. The designed system worked satisfactorily and it takes approximately 15 seconds for the entire process of power supply changeover.