BCD and decade counters, the seven-segment display with its decoder/driver to show decimal digits, the 4017 decade counter as a sequencer with one output high at a time, and resetting a counter early to make a custom count length.

What this topic is asking

WJEC Eduqas wants you to know BCD and decade counters, how a seven-segment display with its decoder/driver shows decimal digits, how the 4017 decade counter works as a sequencer (one output high at a time), and how to reset a counter early to make a custom count length. This is the applied end of sequential logic.

BCD and decade counters

A plain binary counter counts in powers of two; a decade counter is arranged to count in tens to suit decimal displays. BCD is the natural output: each decimal digit becomes its own 4-bit binary code, so 7 is 0111 and 9 is 1001. After 9 (1001) the counter rolls back to 0. Decade counters are used wherever you count in decimal, such as a digital tally or a clock.

Seven-segment displays and the decoder/driver

The display cannot use the BCD code directly - it needs to know which of its seven segments to switch on for each digit. The decoder/driver does this translation (for example, BCD 0010 lights the segments that form a "2") and provides enough current for the LEDs. So the chain is decade counter then decoder/driver then seven-segment display. Leaving out the decoder/driver is a common design error, because the raw count would not form readable digits.

The 4017 as a sequencer

Unlike a BCD counter that outputs a 4-bit code, the 4017 already decodes the count to ten separate output lines, lighting each in turn. This is ideal for sequences: running lights, a stepper of stages, or triggering events in order. Each clock pulse advances the single high output by one position, and after the tenth it wraps around.

Resetting for a custom count length

This is the standard way to set a custom modulus. You decide how many steps you want, identify the output one beyond the last wanted step, and wire it to the reset. The instant that output goes high, the counter clears to 0, so that step is skipped and the sequence length is fixed. The same idea shortens a BCD decade counter (using logic on its outputs to reset) to count, say, 0 to 5 for a minutes-units display.

Try this

Q1. State what a decade counter counts up to before it resets. [1 mark]

• Cue. It counts 0 to 9 (ten states) then resets to 0.

Q2. State which output of a 4017 you would connect to reset to make it count 0 to 7. [1 mark]

• Cue. The "8" output (one beyond the last wanted step), so it resets after 7.