So I am designing a 3-bit asynchronous binary counter which counts up to 11 then repeats using D flip-flops. That much I have done, however I am stuck when it comes to decoding the outputs to stop the counting sequence. I am only supposed to use one logic gate, though. I was thinking of how I could do this using an actual decoder, but I am completely lost when it comes to using just one gate. Any tips to get going? I want to actually figure this out.
Electronic – How to terminate binary counting using a logic gate
counterdecoderdigital-logicflipflopripple-counter
Related Solutions
The circuit you cited is a ripple counter, not a synchronous counter. It actually has eleven states, 0000 through 1010, but as soon as the last state is reached, the NAND gate immediately (asychronously) resets the flip-flops to the 0000 state.
In a synchronous counter, all of the flip-flops would share a common clock, and you'd control the sequence of states by driving their J and K inputs. This would require a few additional gates, which would include decoding the 1001 state so that the counter goes directly to the 0000 state on the next clock edge.
A simple circuit can be built using a 4017 counter.
The circuit normally counts 10 input pulses but by connecting the next output to the reset input via a small signal diode (eg. 1N4148) the 4th count resets the circuit to 0. It also allows you to expand the circuit very easily.to more outputs if required.
Edit (additional)
As OQ requires change when switch or signal input goes low then the clock input needs to be inverted. This can be easily accomplished with a single PNP transistor. A 100R resistor has been added in series with the switch to prevent it getting 'sticky' when discharging the capacitor (see Spehro's comment).
Also added the 10k between D1 and C2/R3 to the original circuit.
Best Answer
Can't count to 11 on 3 bits, so I presume you're actually using 4 bits. What you need to do is AND the two highest bits together, and feed that into the async reset of all 4 flip flops. This will reset the counter back to 0 very shortly after it reaches 12.