LED sequencer

Posted on Feb 4, 2014

The model 4017 integrated circuit is a CMOS counter with ten output terminals. One of these ten terminals will be in a `high` state at any given time, with all others being `low, ` giving a `one-of-ten` output sequence. If low-to-high voltage pulses are applied to the `clock` (Clk) terminal of the 4017, it will increment its count, forcing the next

LED sequencer
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output into a "high" state. With a 555 timer connected as an astable multivibrator (oscillator) of low frequency, the 4017 will cycle through its ten-count sequence, lighting up each LED, one at a time, and "recycling" back to the first LED. The result is a visually pleasing sequence of flashing lights. Feel free to experiment with resistor and capacitor values on the 555 timer to create different flash rates. Try disconnecting the jumper wire leading from the 4017`s "Clock" terminal (pin #14) to the 555`s "Output" terminal (pin #3) where it connects to the 555 timer chip, and hold its end in your hand. If there is sufficient 60 Hz power-line "noise" around you, the 4017 will detect it as a fast clock signal, causing the LEDs to blink very rapidly. Two terminals on the 4017 chip, "Reset" and "Clock Enable, " are maintained in a "low" state by means of a connection to the negative side of the battery (ground). This is necessary if the chip is to count freely. If the "Reset" terminal is made "high, " the 4017`s output will be reset back to 0 (pin #3 "high, " all other output pins "low"). If the "Clock Enable" is made "high, " the chip will stop responding to the clock signal and pause in its counting sequence. If the 4017`s "Reset" terminal is connected to one of its ten output terminals, its counting sequence will be cut short, or truncated. You may experiment with this by disconnecting the "Reset" terminal from ground,...

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