If your LCD has the back light LED, you can connect these pins to the power supply terminals with a 39 Ohm resistor in series. The pins 15 and 16 are the anode and the cathode of the LED. These pins are available only in those LCDs that have a back light illumination LED. In the circuit diagram, the pins 15 and 16 of the LCD are shown open. The input to the regulator is given from a 9V DC wall adapter. The + 5V power supply for the circuit is derived from a LM7805 regulator IC. It also beeps when the device is turned on or off. A piezoelectric buzzer provides audible tone when the timer is started and stopped. The LCD is operated in 4-bit mode, therefore, only 6 I/O pins of PIC16F628A are required to drive it. A standard 16×2 character LCD is used in the project to display the device status, program menu and time. The functions of these push buttons are discussed in the operation section below. Digital inputs from the 4 push buttons are read through port pins RA2, RA3, RA4, and RB0. A 5V relay is driven by a PN2222 transistor that is controlled by RB3 pin of PIC16F628A. ![]() The circuit diagram of this project is shown below. Programmable digital timer Note: (June 30, 2016) A revised version of this project with added new features is posted here.
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