Playing with 555 timer and 4017 decade counter

I graduated long time ago as electronics engineer but eventually ended up in software engineering.  Recently I wanted to start playing again a bit with electronics and decided to build some mundane projects just to try to understand popular components.  In this post I show the popular Led Chaser, implemented using a 555 timer and 4017 decade counter.

Below you can find the fritzing schematic (which is rather complex due to the location of the decade counter’s output pins).

The 555 timer generates an output pulse (on pin 3) which is fed into the clock input (pin 14) of the 4017.  With each clock pulse on the 4017 the next output is enabled.

So, if you want to generate a nice led chaser where the LEDs light up in sequence you need to connect the right output pins to LED1 up to LED10. Concrete this means, pin 3,2,4,7,10,1,5,6,9,11 to LED1,2,3,4,5,6,7,8,9,10.

The 555 is configured in astable mode (see https://en.wikipedia.org/wiki/555_timer_IC).  In this mode it generates a stream of pulses with a frequency that depends on 2 resistors and a capacitor.  The formula you can find on the Wikipedia link.  In my case I wanted to have a frequency around 7Hz so I chose R1=1K, R2=10K and a capacitor of 10uF.  Feeding those values into the formula gives 1.44 / (21000 * 1e-5) = 6.86 Hz.

I checked the output with the Analog Discovery 2 and came to a very close result: 7.04 Hz which is within tolerance of the used components I assume…

See the result in action 🙂

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