Modulo 7 Counter Design and Circuit

A modulo 7 (MOD-7) counter circuit, known as divide-by-7 counter, can be made using three D-type flip-flops. The circuit design is such that the counter counts from 0 to 6, and on the count of seven, it automatically resets to begin the count again. On the seventh count, all the Q outputs will be 1's, and their complementary outputs (/Q) will be 0's. Hence, we feed the inputs of the NOR gate with complimantary outputs.

The trick is to start with a MOD-8 counter and then look for the binary sequence 000, which is when the output from the NOR gate is HIGH. This output is then used to control the RESET function on all three flip-flops.