Byron A Jeff wrote:
>> [deleted for brevity ]
>
>> 10 kOhms between base and emitter should cut the NPN when contol is
>> floatting.
>> When driving, the voltage divided will still be able to drive the
>> NPN. If the IR Diode forward voltage is less than 2.5V, you could
>> double transmission power, by using two diodes in series, of course
>> reducing the Rx value for the appropriate current.
>
> Good idea! I'll do that.
>
>> If you need some support for the IR modulation and codes, even
>> examples for any uC coding, check my webpage, I made some text for
>> different remote brands. http://www.ustr.net
>
> Great pages! I already took a look and the Hitachi protocol, which
is
> my
> initial target is quite well explained.
>
> Thanks,
>
> BAJ


Byron, two other things to include in the "try and buy" bench tests;

1) You can try to change the control logic symmetry to the transistor.
Sometimes you get further control distance by finding the best
symmetry, 50/50, 45/55, 40/60. Sometimes the room reflects so much IR
and it keeps bouncing that it is better to reduce the ON time to 48,
45 or lower%, so the receiver can better "discriminate" IR from non
IR.  Sometimes the transistor or the LEDs are lazy enough to reduce by
themselves this On/Off symmetry, so you need to increase the ON time
at the uC, to compensate.

2) If you have more uC port pins available, you could tie several of
them together and sink the IR diodes directly, without any transistor.
I made several CNC remote control using to tie 5 AVR port pins and
direct drive the diodes, it works perfectly, and you save the
transistor and two resistors.


---.           o +5V
   |           |
 A |----.     _V_-> IR.D1
   |    |      |
 B |----o     _V_-> IR.D2
   |    |      |
 C |----o      |
   |    |      R
 D |----o      |
   |    |      |
 E |----o------'
   |
---'

A very small capacitor in parallel with the current limiting resistor,
can improve the diodes "turn on".  At 38kHz, a 1nF capacitor offers an
 impedance of 4200 Ohms, so, at least fast 1mA will be via the cap.
If you have a scope, you can find the best cap, not big enough to blow
up the uC port pins...  A way to see that is installing an extra
resistor of very low value (shunt) and scope the Vdrop. The High Vdrop
positive pulse will tell you the relative current.  When you find the
best cap, then you can remove the shunt resistor



---.           o +5V
   |           |
 A |----.     _V_-> IR.D1
   |    |      |
 B |----o     _V_-> IR.D2
   |    |      |
 C |----o      o----.
   |    |      |    |
   |    |      R   === 1nF
 D |----o      |    |
   |    |      |    |
 E |----o--R---o----o---> Scope
   |      1 Ohm
---'      Shunt



5V----.        .-----
      |        |
Cap > '-.      |
Peak    |      |
knee    |      |
        |      |
        '-----.|
0V             '

CapPeakV / 1 Ohm = Current


As an extra "tip", try to use metal film resistors from now on. They
cost a little more (will not hurt the pocket), they are not carbon
coiled around the body, so metal film does not create "inductance" for
higher frequencies.  I use to buy 1% metal film from Digikey at
$6/package of 200 units, not bad at all, and they also have lower ppm,
what suffer less changes in resistance based on temperature changes.


/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/
Wagner Lipnharski - UST Research Inc
Orlando FLorida - USA - www.ustr.net
/_/_/_/ Atmel AVR Consultant /_/_/_/

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