The Victim:

A surplus 1960's Civil Defense Radiation Meter. Shown shortly after I got it and started to disassemble it for modding.

The Result:

One kickass firewire case.

Firewire board is pulled from on old Teac firewire case. It wasn't originally able to handle hard drives, but I was able to modify the firmware and now it does. Unfortunately, the controller doesn't handle hard drives well. It's limited to 14MB/s read and write speeds while a proper firewire bridge board would allow 30MB/s or greater reads and writes. It's an issue, but not that bad considering this drive is used only for portable storage. It also currently has only a 6.4GB drive it, which pretty much maxes out at 15MB/s anyway.

As you can see in the last pic, a blue LED was add to the meter. It's being used as a power LED. Works quite well, a nice blue glow that's hard to miss.

Now we get to the part I like most, the hard drive activity indicator. I rigged the needle of the meter. When it's powered on, the needle rises to the "1" area. When there's activity, it goes all the way to the right. Depending on the type of files, the needle will twitch a bit.

I'm sure people are wondering how I made it work. Well, it wasn't that easy at first. The bridge board I used had no LED header for HD activity. Since the board was made for a CD-RW, there was no need. The CD-RW's LED would show activity. So I had to rig my own.

Normally, that's easy. Power to the LED with appropriate resistor with the ground line being pin 39 on an IDE cable. With a single drive this is easy, just plug it into the spare header on the cable. This is what I did initially to run a green LED until I figured out the meter.

While the above is easy for an LED, I had no spec sheet to use on the meter. Who knew what size resistor? Who knew what voltage to use? So I did some experimenting and found out that anything over ~1volt caused the needle to move all the way to the right. Okay, so I had to keep the activity voltage at or below 1 volt. The problem is, the only voltages available to me were 5 and 12 volts. I could also get 3 volts if I wanted to build a circuit into the firewire board.

Before I started that, I did some more experimentation to see what I really had to deal with. Some generic info:
- At idle, pin 39 on an IDE cable is at 1.5 volts.
- During activity, that pin drops to ground.
- In a "normal" HD activity circuit, at idle the voltage is 1.5 volts and jumps to 3 volts during activity. This is why some LEDs will be dimly lit even when nothing is going on.

With that, I looked at my options. If I used the 5 volt line available, I'd have 3.5 volts when idle and 5 to show activity. Using 12 volts I'd have 10.5 and 12 volts. In either case, I'd have to "soak" a lot of voltage up to keep the needle from maxing itself even when idle. Now I could have tried with a resistor, but as I said I had no spec sheet to go from. This was where I was stumped. I tried chaining LEDs, tried a few flashlight bulbs, but the needle would still go all the way to the right at idle. At this point I put it aside.

Flash forward a week or so, and I hit the idea. Zener diodes. Look it up if you don't know what they are, but as a summary, they only flow current above their rated value. Take a 5 volt Zener, give it 6 volts, and it lets 1 volt through. Give it 10 volts and the output is 5 volts. They make it very easy to build a simple voltage regulator.

Anyway, this was the key to make the needle move how I wanted it to move. In simplest terms, I chained two 5.1 volt Zeners, fed them with 12 volts, and plugged the ground side of the meter into pin 39 of the secondary IDE cable connector. As a result, at idle I get 0.3 volts out of the Zener. That's just enough to make the needle float up to "1" on the dial. With activity there's 1.8 volts, more than enough to make the needle jump all the way to the right. I also added a resistor to the chain, just as a preventative measure. Wanted to keep the current level down, no need to fry the hard drive or controller board.

Okay, you probably want to see it, huh? Here's a few stills and a link to download a 30 second video clip. Clip is about 450KB, WindowsMedia 9.

Off:

On, idle:

On, active:

Video shows power off, the power on "twitch", the "twitch" when the system recognizes it(which made me jump, visible in the clip), a short sequence of file transfer, and the needle dropping back to zero when power is removed.
Video Clip

And that's that.