IR Transmitter circuit.

[Sam V. <mr...@co...>]

Anyone care to comment on the following rough circuit for a transmitter I 
intend to build, perhaps saving me the trouble of dealing with a fried port? 

It's signalled slightly differently than typical circuits, so I probably 
won't be able to use it with lirc out of the box, but I'll cross that bridge 
when I come to it.  I haven't checked, but if lirc keeps RTS pulled high 
while frobbing DTR, it might work if I reverse DTR/RTS designations on the 
following diagram. 

The basic problem I'm trying to solve is to be able to use a fairly long 
cable from the serial port, but still generate a strong signal.  The sample 
circuit that's linked from lirc.org's page is relatively complicated, 
because it's trying to both charge the circuit and signal it with a single 
pin.  I suspect that if a second pin is used to charge the capacitor, the 
circuit becomes much simpler: 

GND---------------+--------+------------------------+
                 |        |                        |
                 |       \_/                       |
                 |       ---                       |
                 |        |                        |
         5-11V+----+ 5V   |     //                 |
DTR ---->|-----| VR |------+---->|----/\/\/\/--|    |
              +----+             I            |    |
                                              /    |
                                            |/     |
RTS ---->|--------------/\/\/\---------------+\     |
                                            | \    |
                                              |    |
                                              +----+ 

VR is a 5 volt regulator, "I" is the transmitter.  The way I figure it, no 
matter how much of a voltage drop I get from a long cable, I'll end up with 
a precise 5V.  This should allow me to use the right resistor to get 
precisely the maximum amperage that the transmitter is rated for, then use 
RTS to switch the transmitter on/off through an NPN transistor.  The 
transistor should be switchable with very little current, so all I need the 
resistor for is to cap the maximum current.  I'm not sure if sticking 
another voltage regular before the resistor/transistor would mess up the 
signal timings. 


-- 
Sam