Tutorial 2
 

- moving HDD light -

  
 

Introduction

The Idea

  

In this tutorial I will describe how to build a LED moving light which is triggered by the HDD LED signal. The idea and the circuit scheme I have found on Arne Rossius' electronics website, which I had accidentally hit while surfing.

My first variant, which was built on a hole grid card, was a complete failure. There were just too many wires, and I had no clue of electronics. So I first interviewed a electronics savvy working colleague, who helped my mightily (thank you, Niels! :)

The current version is built on an etched board, making it look much nicer and saves a lot of soldering work. The circuit I had planned with a layout program. Eagle from Cadsoft is the first choice here - the reduced feature set of the freeware version is totally sufficient for small hobby projects.

I have decided to add both a PNP and a NPN transistor for controlling the counter; you can switch between them by setting a jumper. Strangely enough, my board only worked after I had connected the HDD LED ground line with the plus line and connected them with the NPN control transistor ...

Step 1

Material

  

Here is the material:

  • soldering iron and tin
  • edge cutter
  • needle-nosed pliers
  • sharp knife (cheap replacement for insulation stripping tongs)
  • Dremel + cutting disks
  • hot glue gun
  • hobby etching set (find at a well assorted electronics store)
  • acrylic glass
  • hole grid card
  • contact pin bar (40 pins)
  • 1xIC HEF4017C (Johnson counter)
  • 10 super bright LEDs (3mm, 3.6V, 20mA)
  • transistors (1xBC557C, 1xBC550C, 10xBC33740)
  • resistors (10x39, 11x1K8, 1x33K)
  • 10 contact plugs with cable
  • 5m braided wire (two-core)
  • Acetone
  • permanent marker
  • HSS drills (0,8 - 1 mm)

The material has cost about 45 Euros, with the LEDs being be far the most expensive lot.

The series resistors for the LEDs (here 39 Ohm) are calculated according to the fall of voltage at the LEDs + control transistor (which usally has about 0.6V), so for me (R=U/I) 5V - (3.6 + 0.6V)/ 0.02 = 40 Ohm. A circuit scheme für Eagle 4.11 be found here. It must be printed mirrored two times enlarged on a copy foil (see Eagle's print dialog) and can then be used as a light exposure mask. The surrounding closed copper line is meant as cutting edge.

Step 2

Creating the board

  

There are several ways to do this. Of course, creating an etching mask with a layout program, UV lamp, development and etching bath are more elegant, but also more expensive and time-consuming. Therefore I have used a simple etching set, where the circuit paths and pads are marked with etching-resistant ink.

First of all I attached the hole grid card to my board with some sticky tape. Then I marked all pad positions on the hole grid card with the permanent marker as I had laid them out on the connection scheme. After that, I drilled the holes for the mounting, using the hole grid card as a template. Finally I marked the circuit paths with etching-resistant ink. After half an hour in the warm etching bath the circuit board was done, and after rinsing it ink, grease and other residues were removed with acetone.

Step 3

Assembly

  

Now the components are soldered to the circuit board. Small gaps in the circuit paths can be bridged with soldering tin (ok, ok, the UV exposure method is better :P). Power, control line and LEDs are connected to the board with contact pins. For these, pairs of two pins are cut off the pin bar with the edge cutter. The contact plugs are soldered to the LEDs, the cables being elongated by 40cm with the braided wire.

Step 4

Mounting

  

The only thing missing is a cool display added to my case. The LED displays you can usually see on the various case modding sites imho are rather putting me off. Often the LEDs are not neatly built into the case, but look naked and irregularly placed in bland holes drilled into some plastic bezel. After some pondering on this, I got the idea to build a back-lit acrylic display into the small "roof" protruding from my case front's top (that's why I need the long LED cables ;).

I believe the result of my work speaks for itself.

Here are two small sample videos showing the moving light in operation. During the first clip the HDD is accessed only occasionally. The apparently irregular jumps happen because some accesses follow each other so rapidly that the LEDs are too slow. During the second clip huge amounts of data are being read causing a very regular disk access pattern, hence the regular, fast operation of the moving light.