Digital Timer can be built by using PIC16F876A micro-controller and a few additional components. The schematic is similar to Fan Speed Monitor Design. 3×4 Keypad and 5V Relay were added as you can see from figure below. Only three 7-Segments are used. The code for this design was also slightly modified. It still uses the same multiplexing and time base generation routines, however some changes were made. For example the circuit counts from 3 digit value entered via keypad down to 000. At this point a relay and LED will be turn ON by 2N3904 NPN transistor connected to pin 5 (RB5) of PORTA. Current Max setting is 999s. Timer will start counting immediately after receiving 3 digit number. Turn the power ON/OFF to reset the Timer.
A simple upgrade to current design can be made by increasing the timer range. It can be done for example by adding another function to generate 1 min counts instead of seconds or by enabling 4th digit to achieve 9999 max reading. You can download timer source code here. Additional information about interfacing 7-Segment Displays can be found in 7-Segment LED Display with I2C post and 7-Segment Display with SPI article. Design schematic diagram and a short video demonstrating circuit operation are available on the next page.