Saturday, May 19, 2012

Tivo Expander Drive Internals

Everybody wants more space in their DVR right? And if you have TiVO you can purchase one of the ludicrously overpriced hard drives with the custom partitioning scheme called a “tivo expander” that western digital sells to pair with the drive in the TiVO. When I first decided to do this I was appalled that they should cost so much more than the cheapest drive from WD that I could find. They can’t be any different can they? It didn’t seem like online at the time people were having very good luck creating one for themselves with a regular disk and it turns out the plug on the TiVO is not a standard SATA plug but something slightly different, or at least the plastics are keyed differently meaning you can’t plug in the same cable you got with that el-cheapo drive.

So I gave in and bought one. It ran happily for years and then died. It’s been in the bottom of a drawer with dozens of other dead or now unneeded pieces of kit that I’ve been unable to part with. Recently I’ve been cleaning out my flotsam but rather than throwing them out I’ve been pulling them apart and hanging the boards up on my bathroom wall as a kind of geek wallpaper. So I tore into this drive to get the board out and the platters to hang up on the wall and was surprised to see that it actually is considerably overbuilt compared to other disk drives I’ve torn apart. The first thing that clued me in was the thickness of the platters themselves. They are twice as thick as a non-TiVO platter from WD.


when I picked it up I knew I should be taking a picture of this for posterity. Here it is compared to a disk from a more recent WD disk that failed:

By caliper measurement it’s actually twice as thick at 1.75mm!

the other WD platter at .82mm

and the TiVO expander at 1.75mm.

Additionally there were these odd spacers between each platter, just machined pieces of aluminum:



What purpose they could serve I don’t know but I’ve never seen them in any other drive I’ve torn open, though I’m far from an expert on disk drives.

So it seems to me that these drives are actually built to a higher spec than the cheaper drives we get for most other things. I dont know if that helps, this one still didn’t last as long as it should have, but it does explain why they are more expensive, it’s not just a branding issue with TiVO.

Wednesday, May 2, 2012

Metering The Smart Meter: part, the 4th

I’ve made some significant progress in measuring those pulses from the smart meter that I talked about in previous blog entries. The IR remote receiver was a total fail in reliably receiving them as they weren’t modulated that way, or in any way to make their reception reliable or easy to manage. It’s just on/off once for each watt hour used.

So i built a new sensor to velcro over the LED on the meter. This one has just a phototransistor and some resisters, the values of which I failed to record and cannot remember. So i’ll have to sort that back out again before I can make more of them. I did some math to decide what they should be but have cleaned up the lab since then and the notes are gone... Need a notebook rather than scratch paper I think.

I’ll post the software when it’s all working. The problem with a phototransistor is that you have to measure the output on an analog pin. You can’t just get a nice pulse of digital on/off. So that means sampling in a loop and comparing the values. Though that phototransistor is in a black casing which reduces it’s sensitivity to regular light it still changes it’s values significantly when the sun is shining on the meter.

This makes it difficult to just pick 2 readings and call one the ON threshold and the other the OFF threshold for the LED. What I finally did was to save each loops reading into a buffer of an arbitrary size calculated that even for the lowest power use it will always contain samples from on and off. Then I walk that array and take the min/max of the values. Finally I create an ON threshold at a quarter of that difference above the min, and the OFF at a quarter below the max.  These numbers adjust during the day as the light levels change and make sure that it can always tell the difference between an on and an off.

I still get the rare oddball pulse. It’s so reliable in it’s timing that it’s either real coming from the meter or that is the shortest time frame that can be calculated with the arduino that I’m using. I’ve added code to ignore a pulse if the calculated power usage is more than 150 amps. It is possible that this might actually ignore a real peak power usage starting up a large AC unit with a hard start kit on it or something like that, but thats probably an unlikely occurrence that wont change the overall calculations that much.

Once doing that I am now reporting to XTension a value for the last reporting period, at least 5 seconds between reports, the number of pulses counted for a more accurate overall usage than adding the calculated values together and lastly the average power usage for the period and the peak usage for the period. All together making a very nice graph inside XTension.



thats just the AC cycling on and off the other morning. Interesting the compressor profile there where it uses a lot of power initially (though no big startup spike) which drops off and then as it comes up to full operating pressure returns almost to the starting load. Does it every time too.


This one is me cooking dinner the other day and you can see the electric stove elements cycling on and off and the spikes they generated when they first turned on. The graphing software needs some help there calculating the x axis lines ;) Dont know why that is so bad looking, but I never tested it with such a wide range of values.

This reports up to XTension through the serial port of an xBee, the next project will be to get it working with the wifi version of the xBee which I just received a couple of from Digi and will start messing around with shortly. Adding support for those and the traditional xbee radios to XTension really opens up a lot of possibilities. Here’s the Wifi one: