Tired of the old boring Hard Drive indicator LED, one puney LED.
Then one choice is to add a HDD ( hard drive ) Tachometer.
This tach will show the HDD activity on a desktop computer. Not for automotive use.

Built with the 555 and 4017 combination. But the " Interface " is what makes this work.
Tricky thing is the HDD LED has full V+ all the time and it is the ground that is varied to make the LED flash. 
That means the HDD LED ground becomes the input signal for the tachometer, or pin2 of the 555.
There is more descriptions in the pictures.

I did not go in-depth of the chip operations and theory, just a down and dirty hack.
Actually i do not know exactly why / how the interface works or why it works.
Somebody with a physics or electronics background could probably understand and explain how this works. 
For me,   IT WORKS.

My regular camera is not here so the pictures are, well not good.
<p>Without flipflops next to the output of 4017,how did you get the bar graph like effect?</p>
<p>I dont really understand the question but if you study how a 4017 works maybe that would answer your question. </p>
<p>Oh,ok,I thought the circle is working by counting the hdd led pulse,per unit time,then reset,thus providing a display of counts per interval.</p><p>But this one doesn't reset.so it just lights up at varied speed in a sequence.</p><p>(I didn't observe the circuit at first look carefully)</p><p>It's more like novelty pattern creator then a tachometer.</p>
Ahaaa a Hard Drive. <br> <br>Sorry for my ignorance.
No problem. <br>I changed the title and an HDD explanation in the content.
HOW do you know, when you drive at 15 mph ? <br>What does your tach show ?
15 mph have to be shown of coarse on a speedometer in a car. <br>This shows the HDD activity of a desktop PC. <br>Did i write it to seem that is was a car tach?

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Bio: Why would i buy something ready made when i can make it myself with half the features for twice the money? DIY!
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