Relay switching circuit

On a sunny day (Fri, 06 Jun 2008 10:11:30 -0400) it happened Phil Hobbs wrote in :

That same idea, of getting that info, I did recently read is now done somehow by tracking cell phones to a few centimetres? What they probably do is send frequent request to the cell phones to identify themselves... Now if I could only remember where I did read this (in the last few weeks).

Nice, clever idea :-)

themselves...

Yes, I heard about that too. Makes me the invisible man. ;)

Thanks. There turned out to be a big volume of the parameter space that made it possible to build good sensors for really cheap, which was what was so much fun. The real kick is doing something hard with nearly zero apparatus.

Cheers,

Phil Hobbs

So I guess using a latching relay is not an option then.

I don't know. I use a Mac, and the way an external hard drive works is that when attached (or powered up, in case of an external hard drive I have connected via Firewire) is that the drive icon pops up on the desktop indicating it's "mounted" as they say in MacOS. In order to safely "unmount" the drive I would have to drag the hard drive icon to the trash icon (which in this case turns into an "eject" icon) whereby the hard drive icon vanishes, the drive powers down and I can safely turn off its physical power switch.

But I don't know of a way to power up the drive and NOT have it spin up, so this is why I was thinking of physically breaking its power lines. Would it be better to switch something inside the SATA to Firewire/IDE bridge board (which is the interface between the Mac and the hard drives)? I'm pretty sure the manufacturer isn't interested in handing me the schematics for it though.

example:

Ah yes, I believe that pretty much describes what I was after in the first place. I assume the latching is released when power to the circuit is turned off. This isn't a biggie as I can have the relays for each drive connected to the TFF circuit so that each hard drive doesn't received power, but its TFF switch has to be pressed in order for it to start.

The biggest problem now it seems is finding out where to switch the power or control signals...

There are ATA to SATA style power adapter plugs to be found (look for "SATA 'Y' power adapter cable here:

which means that SATA drives work fine without 3.3V. From what I've read +3.3V is only needed for "hot-swapping", but then again perhaps this is exactly what I'll be doing when enabling and disabling each drive. Hmmm..

Yes, I need something more or less readily available. By that I mean I can handle making a PCB out of a simple schematic, but I'm no good at designing a circuit out of theories or from scratch.

Back in the stone age, we had to use an external EPROM, and you could build a programmer one of those for about $10.00 with all new parts.

Of coure, learning the micro itself is part of it - newbies don't necessarily know programming - I'm already a programmer, so I just basically use the instruction chart from the data sheet.

But doing it with "discrete" logic, you used to be able to get chips virtually free at a hamfest or flea market (by stripping surplus boards}; and a solderless breadboard is about $10-20.00 and reusable, and doing it that way teaches more about the actual operation of the thing.

Hope This Helps! Rich