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Project Notes

#227 555Timer/KnightRider

The familiar “Knight-Rider” LED circuit using a 555 and 4017


This is a well-known LED effects circuit using the 555 timer and 4017 decade counter.

This project is a variation of the circuit published on - I’ve added steering diodes rather than allowing paired outputs to sink each other.

Clock Generator

The 555 timer is configured as an astable multivibrator, with a frequancy of 10.5 Hz at ~50% duty cycle.

Decoding the LED Display

The clock signal from the 555 drives the 4017 decade counter, providing 10 signal lines. The 10 outputs of the 4017 (Q0 to Q9) are wired to the appropriate LEDs per the table below to generate the “Knight-Rider” pattern:

Q0 off off off off off ON
Q1 off off off off ON off
Q2 off off off ON off off
Q3 off off ON off off off
Q4 off ON off off off off
Q5 ON off off off off off
Q6 off ON off off off off
Q7 off off ON off off off
Q8 off off off ON off off
Q9 off off off off ON off

Steering diodes are used to prevent 4017 outputs sinking current back to another pin connected to the same LED.

NB: diodes are omitted for LED1 and LED6 since they are only driven by one output. This does mean LED1 and LED6 are driven with a slightly higher current than other LEDs, but the difference is not noticeable; to even up the current, redundant steering diodes could be added, or the current-limiting resistor increased propoertionally.



The Schematic

The Build

Credits and References

About LEAP#227 555 TimerCMOSLED
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This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is my personal collection of electronics projects, usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting (IMHO!).

The projects are usually inspired by things found wild on the net, or ideas from the sources such as:

Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.