Most of the cost of mining is usually the mining card, so there is usually no reason to turn off the GPU. However, if your old card happens to have an extremely low electrical efficiency, then it may be worthwhile to turn off the GPU. You should find out first before going through the trouble of setting up a cron job to turn the card on and off.
Well with the old card I don't have a problem, it is lost in the noise and the machine runs 24/7 for other reasons anyway.
If I decide to get into this seriously however I plan to buy a high end card (ATI 6xxxx series) and dedicate (one or more) old/cheap machines to it.
THEN the power costs get significant - particularly with a 20/7 (plus HVAC cost!) ratio.
My present thought is to simply put the whole machine into hibernation or standby mode via a cron job.
Right now I have 3 old machines sitting around that could be put to use under a counter (or something).
Cron is probably your best bet. It's simple and as long as you've got your clock properly synced to your timezone it should manage itself.
I've got a script running on cron that runs every 5 minutes just to check that the miners are indeed working and that my graphics cards aren't exploding. It's quick, simple and it works.
I like your idea of checking to see if things are still alive - hadn't though of that one! Thank you!
Of course all of this depends on the ratio of cost of mining to the value of Bitcoins - right now at US$15/BTC it works nicely, but in 3 weeks - who knows?
