[mythtv-users] Simple transmitter-is it safe?

steve at nexusuk.org steve at nexusuk.org
Mon Jan 5 17:27:35 EST 2004

On Mon, 5 Jan 2004, Stuart Felenstein wrote:

> Also how can I know how much voltage my serial port can take ?

Your serial port won't be "taking" any voltage.  It will be _supplying_ a 
voltage between the DTR pin and ground.  ISTR the RS232 specs are quite 
broad (something like anywhere between 5v and 24v).  Modern serial ports 
won't go anywhere near 24v though (remember that the original RS232 specs 
were for driving teletype printers at very low speed over quite long 
distances).  A modern serial port will give you somewhere between 5v and 

The question you really need to ask is how much current can you safely 
draw from your serial port - serial ports are well buffered and current 
limited so in theory you can safely short the port out completely, in 
which case you should be more concerned with the LED's current rating - at 
12v if you put a 1k resistor in then you'll be drawing about 12mA...  So 
if for example, your LED is rated at 36mA you could get away with putting 
a 330 ohm resistor in instead.  The smaller the resistor, the brighter 
the LED (and the greater the distance it will work from).  But if it's 
going to be taped directly to the front of the receiver then bear in mind 
that the receiver might have problems with getting a very bright signal so 
there is a bit of trial and error if you have any problems with the 
initial circuit.

Of course, as with plugging any home-built device into your computer there 
is always a very tiny chance you'll blow stuff up.


 - Steve                                             http://www.nexusuk.org/

     Servatis a periculum, servatis a maleficum - Whisper, Evanescence

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