The Johnson digital counter or Twisted Ring Counter is a synchronous shift register with feedback from the inverted output (Q’) of the last flip-flop. Q’ of the last flip-flop is connected back to the input D of the first flip-flop. This inversion of Q before being fed back to input D causes the counter to “count” in a special way.
Animation/ simulation of this Johnson counter circuit is also given in this article. You can create simple dancing LED effects using the synchronous Johnson digital counter we have discussed here.
The main benefit of this type of counter is that it only needs half the number of flip flops compared to that of the standard ring counter to represent multiple states. So an n-stage Johnson counter gives a sequence of 2n different states. And thus it can, therefore, be treated as a “Mod 2n counter”. Whereas an n-stage ring counter has only n states; that is “Mod n counter”.
Johnson Digital Counter Circuit Diagram
Components Required for Johnson Digital Counter with Dancing LEDs
- 7474 D flip flop x 2
- Resistors (100Ω -1/4 watt x 4)
- Astable multivibrator (3 KHz)
- LEDs x 3
Simulation | Animation | Working of Twisted Ring Counter
- Above circuit diagram represents a 3 bit Johnson counter using 7474 D flip flop. You can easily extent this circuit upto 4 bit, 5 bit, etc. by adding flip flops after the 3rd flip flop.
- A single 7474 IC consists of 2 flip flops. So you need two 7474 ICs for implementing Johnson ring counter.
- Initially all the flip flops are cleared. So the time inverted output (Q’) of 3rd flip flop is high or logic 1. This logic 1 is appears at the input of 1st flip flop.
- During the first clock pulse this logic 1 is transferred to the output of 1st flip flop. Thus the total output of Johnson counter is 100.
- Then input of 1st and 2nd flip flop is logic 1. And after the second clock pulse these inputs appear at the outputs of 1st and 2nd flip flop. So the total output is 110.
- Similarly for the next clock pulse, the output will be 111.
- During this state (111) the time inverted output (Q’) is logic 0. This 0 is fed to the 1st flip flop. Then the 0 will circulate through the flip flops as 011, 001, 000.
- In the above circuit either you can connect the Set pin (S pin) to Vcc or you can leave it free.
- Feed a 3Hz (or any) 555 astable oscillator to the clock input to start counting.
3-bit Johnson Digital Counter Truth Table
After state 5, the circuit goes to state 0 and this will repeat.
4-bit Johnson Digital Counter Truth Table
Similar to the 3-bit Johnson counter, the 4-bit one will also repeat to state 0 after state 7.
Among the synchronous digital counters, ring counters are well known and well used. And among the ring counters Johnson ring counter has a special space for its applications. The effect on output can be expected by a simple analysis of the Johnson digital counter circuit diagram. Its twisted tail output of the last D flip-flop into the input of the first one makes all the difference.