This document discusses Johnson counters, which are a modification of ring counters. In a Johnson counter, the output from the last flip-flop is inverted and fed back to the input of the first flip-flop, rather than directly connecting the last output to the first input as in a ring counter. This allows a Johnson counter to use half as many flip-flops as a standard ring counter of the same modulus. The document provides the circuit diagram, truth table, and timing diagram of a 4-bit Johnson counter, and lists some example uses and components needed to build a 4-bit Johnson counter.

1. Johnson Counters
Jatinder Kumar chaurasia(11508173)
M.Revanth(11503559)
O.Purnachandra(11504239)

2. Introduction to counters
• Counters –count no of times a particular event occurred
• A counter used two or more flip-flop depending upon the no. of bits required
• There are two types of counters:
1.Asynchronous counter 2. Synchronous counter
• Asynchronous counter used a single clock at the first stage
• Synchronous counter ,all flip flops are clocked together with a common clock
pulse

3. Johnson counter
• Johnson counters also called twisted ring counters
• It is a modification of ring counter
• The only difference is that instead of last output(Qd) back to the input(Qa)
the not of (^Qd) is connected to the first input.

4. Circuit diagram of 4 bit Johnson counter

5. Truth table of Johnson counter

6. Timing diagram of 4 bit Johnson counter

7. Uses of Johnson counter
• Used in walking LEDs
• Used to convert square waves to sine waves

8. Conclusion
• Johnson counter is a modified ring counter where the output from the last flip-
flop is inverted and feed back to the input of first flip-flop
• The main advantage of Johnson counter is that it only needs half the no. of flip
flops compared to the standard ring counter of same mod
• If we use n flip-flops then it is known as 2n bit Johnson counter or mod-2n
Johnson counter

9. Components used for making
4 bit Johnson counter
• 7474-IC(D-flipflop):2
• 5 resistors(100 ohm)
• 5 LED bulbs
• One battery(9v)
• Wires(20m)
• One 555 timer(bistable multivibrator used as a clock)