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computer controlled power switch

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Michael Daly

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Problem: DSL modem locks up periodically, usually when I'm away.
Cheap solution: Cold boot modem.

Since my computer polls the modem already, if it detects the modem is knackered, I'd like to have the computer cold boot the modem. I figured this would be easy, but existing products (server power controls, X10 etc) cost more than a new modem.

I have RS-232 and USB available. After considering several alternatives, I figured the cheapest and easiest for a noobie like me (very limited digital electronics fooling around ~20 years ago) would be to have an RS-232 controlled relay to switch off the power in the 5V output of the modem's switching power supply wall-wart.

So I dreamed up RS-232 on computer to a little box next to modem (in another room near phone line entering house) containing:

- MAX232 for RS-232 to TTL conversion
- something
- 74xx107 configured as a toggle
- toggle to drive a transistor
- transistor drives NC relay to open the circuit
- relay controls power in 5V line.

I send two signals - the first to shut off power, the second to power up.

My problem is - what is the easiest "something". I just need a single signal to the toggle to change the state of the switch. A UART like 8250 is a tad too complicated. I found a post on this forum showing a 74HC299 instead - I could use the high bit to toggle. Any other ideas? Better solutions?

Mike
 
That looks like it.

I was so focussed on the use of the data transmit line that I forgot I could mess with the flow control lines.

Thanks!
 
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