A decade counter is a binary counter which counts from zero to nine, then back to zero, so it counts 10 numbers.

This is a simplified version that does not ensures a self-reset if a unknown status is in it's outputs at any given time. While this makes the truth table shorter and the circuit simpler, it may misbehave if it reaches a unknown status.

The design was thought as a two stages logic: and gates from the output then or gates to the inputs. This particular design does not needs not gates to be used as the Flip-Flop has Q and Q' as outputs.

This design was restricted to use only 2 input AND, 2 input OR and NOT gates.

If you want to build this circuit, you will need:

• D-Type Flip-Flop with clear and negated output* (4) [74174 (1) or any equivalent]
• Two input AND (10) [7408 (3) or any equivalent]
• Two input OR (4) [7432 (1) or any equivalent]
• Power source
• A clock signal
• Optional a push button for reset
• Optional 4 leds and 4 resistor for the outputs

The Logic equations that describes the circuit are:

s0 = S1*S2*S3 + S0*S3'

s1 = S1'*S2*S3 + S1*S2' +S1*S3'

s2 = S0'*S2'*S3 + S2*S3'

s3 = S3'

Where: s0, s1, s2 and s3 are the Flip-Flop D inputs of it's respective S0, S1, S2 and S3 outputs, and S0', S1', S2' and S3' are the negated outputs.

* If you can't find a D-Type Flip-Flop with negated outputs, you can use NOT gates like 7404 to get S0', S1', S2' and S3'.