Back in college we had this subject on Programmable Logic Controllers – which is used to control industrial motors, actuators and indicators while collecting input data from sensors? Why use a PLC? Well PLCs are compact, off the shelf, easy to program using Ladder Logic, its much lesser in cost and less power hungry compared to the other means of control while able to control high power instruments and machines.
Did I say much less in the previous paragraph? Well it cost less but not cheap – here is a home brew PLC. It doesn’t go in a black box [like the PLCs from Siemens] but it sure do high power control. The main controller of this PLC project is an ATMega8A-P which is rigged to an external EEPROM using I2C bus to be able to hold on more command sequences. The microcontroller is isolated from the high power devices. Inputs are made out of opto couplers – this would enable the microcontroller to sense from sensors that outputs voltages higher/lower than the limits of the AVR. For the output side, relays are used to switch on and off resources – the relays is driven through an isolating resistor that supplied the current that it needs to excite the coils which will in return activate the relay. Both output and input lines have their own LED indicators – for the user to see which relay is active and which input is at logic high – very useful when debugging your program. And more? The schematics is made in eagle thus you can edit and add stuff to it!