Here I made a digital counter using discrete transistors. Also use some resistors, capacitors, etc... The counter is an interesting machine which count numbers. Here it is a 4 BIT binary counter. So it count from 0000 binary number to 1111 binary number. In decimal it is from 0 to 15. After this I convert it into a BCD counter. The BCD counter is a counter which count upto 1001(9 decimal). So it reset to 0000 after counting 1001 number. For this function, I add some combination circuit to it. OK.For more details about this counter theory visit my BLOG : https://0creativeengineering0.blogspot.com/2019/03/4-bit-counter-using-discrete-transistor.htmlHere I made this circuit by using T flip-flops. IFor more details about T flip-flop please visit my Blog which is about it.