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Design Idea
Cascade two decade counters to obtain 19-step sequential counter
Interleaving the outputs of a pair of Johnson counters yields a 19-step, sequential count.
Jeff Tregre, Dallas, TX; Edited by Charles H Small and Fran Granville -- EDN, 12/14/2007
This Design Idea offers a practical approach to cascading two or more Johnson counters together with a bare minimum of parts. The CD4017 Johnson decade counter finds use in simple circuits ranging from sound effects to LED displays. The counter’s outputs are normally low and go high only at their respective decoded time slot. Each decoded output remains high for one full clock cycle. The dc-supply voltage can range from approximately 3 to 18V dc. The dc-current drain per each output pin (Q0 to Q9) is 10 mA. The circuit has passed tests at 12V dc at 0 to 150°F without anomalies.
The circuit in Figure 1 uses only four ICs and yields a 19-step sequential count. You cannot get 20 outputs without adding more hardware because of the fact that, upon powering up, each CD4017 counter displays output Q0 as being on. Therefore, the circuit does not use output Q0 of IC3 and can use only 19 of the 20 outputs.
At first blush, you might think that you could simply cascade two counters together using the carry-out pin, Pin 12, from one counter to feed the clock-input pin, Pin 14, of a second counter. But the problem with this configuration is that it does not provide sequential count from 1 to 20 because the first counter begins to count over again once it has reached 10. Such a configuration is a zero-to-99 counter because every 10 counts on the first IC counter causes one count on the second IC counter.
By hooking together two counters, you can obtain a sequential count from 1 to 19. The circuit uses IC4, a CD4069 inverter, as a reset-delay enable to cause a few milliseconds of delay before each counter can begin to count. A high signal on the Pin 15 Reset clears the counter to its zero count.
Without the delay time, each counter powers up with a random output count such that several LEDs may be on. The circuit uses IC1, a 555 timer, as the clock to generate a 1.5-Hz square wave. You can change the frequency by changing the RC time constant comprising R1, R2, and C1. Keep in mind that, to obtain a 50% output duty cycle, make R2 much larger than R1. Pin 14 of IC2 has a positive-edge clock trigger. Pin 13 of IC3 has a negative-edge clock trigger. Therefore, when the clock goes high, IC2 produces an output count. When the clock goes low, IC3 produces an output count. By interleaving the outputs, you obtain a sequential count from 1 to 19. Because each clock cycle has both a high and a low state, after the first clock pulse, two LEDs will always be on—that is, LED 1, LED 1 and 2, LED 2 and 3, LED 3 and 4, and so on. See below for two short video clips of the finished circuit in action.
