[PLUG] powering relays with usb-serial adaptor

Eric Wilhelm scratchcomputing at gmail.com
Sat Jun 30 09:17:32 UTC 2007


The meaning of "usb-serial" in this case is the regular meaning :-)

I've been playing with a diode+relay setup which needs 5V for the relay, 
works great on the old dell laptop serial port (the little reed relays, 
plus an led and resistor (light-emitting optional.))

I'm going to hookup my external usb backup drive to only be on when 
backups are running (duh! turn off the lights when you leave.  Why 
don't more devices have power control?.)

I got some $9 usb-serial adaptors with the Prolific PL2303 chipset.  To 
my dismay they don't show any voltage between the ground pin and the 
signal pins.  WTF?

I jumper the ground pin to the usb ground (outer shield only) and viola! 
5V  (Seems like those would have been wired together to start with 
(btw, the ground pin5 is also not wired to the outer serial shield.  I 
thought that was supposed to be the case.  Further, the serial shield 
and usb shield are not connected, though the cable has the shielded 
jacket which I would assume is the usb shield.  Will learn more soon 
through dissection.))

What's the deal?  If I hook it to a modem it dials (though only at 
9600baud for some reason that I haven't thoroughly investigated.)  Why 
does the modem see a signal when the voltage meter doesn't?  Why are 
the usb shield and the ground pin at different levels?

If I'm only using it as a relay controller, is there anything wrong with 
soldering pin5 to the usb ground?

--Eric
-- 
"Everything should be made as simple as possible, but no simpler."
--Albert Einstein
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