Infra Red Remote Transponder

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From: Shishin Yamada

[...] I built a remote-transponder, which lets me send signals from a small receiver into closed (opaque door) cabinets, and around corners, etc. Anyways, the cheif problem is just stray environmental noise with any slowly changing amplitude modulated IR signal (lots of 60 Hz noise, and sunlight noise). Most IR remotes work around a 40KHz carrier, so that they can just pulse this digitally, and just bandpass filter it at the receiving end. This boosts the range of unfocused IR remotes to tens of feet (around 20-30 feet). Adding two IR Leds helps alot, by sending out more IR signals.

Anyways, the following is a cutout from the group sci.electronics. Most of which had been posted in the past month (I guess many news-servers might not go back one month, so I re-posted. Hope nobody minds). There is included transmitter and receiver modules. Personally, I find the Sharp IR Receiver module (avaiable at Radio Shack) to be the easiest darn thing to use. You just pop in +5V and ground, and you get the demodulated signal out (a high/low version of whatever was riding on the 40KHz IR signal). It comes with simple application notes. Hope this info helps you out.

Oh, lastly, if you're looking for ANALOG transmitters, I would look into Radio-Electronics recently for posts about their "Air-Hop" voice transmitter, or into Forrest Mims III "Circuit Cookbook." They both show how to do FM modulation onto IR (much, much better than AM).

IR DETECTOR CIRCUIT

                           30Hz BANDPASS FILTER          gain = 1
                                                            Q = 4
 
        +5V                  .1uF
         |                    ||            330K
         \            +-------||----+-------/\/\/--------+
         /            |       ||    |                    |
         \ 100K       |             |                    |
         /            |             |                    |
         \            |             |                    |
         |            |      .1uF   |     ___            |
         |     39K    |       ||    |    |   \___        |
         +----\/\/\---+-------||----+----|-      \___    |
IR       |            |       ||         |           \   |
Detector |            |                  |   LM3900   >--+---+
         |            |                  |        ___/       |
    ||---+            \             +----|+   ___/           |
    ||                /             |    |___/               |
    ||---+            \ 5.6K        \                        |
         |            /             /                        |
         |            \             \ 1M                     |
         |            |             /                        |
         |            |             \                        |
        GND          GND            |                        |
                                   +5V                       |
                                                             |
                                                             |
                                                             |
              +----------------------------------------------+
              |
              |
              |
              |
              |
              |                  ___
              |        120K     |   \___
              +-------\/\/\-----|-      \___
     SCHMITT TRIGGER            |           \
                                |   LM3900   >--+------ Vout
                                |        ___/   |
                           +----|+   ___/       |
                           |    |___/           |
                           |                    |
                           |                    |
                 1.1M      |         1.1M       |
           +5V --\/\/\-----+---------\/\/\------+
                                                |
                                              ----- 1uF
                                              -----
                                                |
                                                |
                                               GND

Notes

The LM3900 is a quad unipolar op-amp IC. Vcc of this chip must be on +5V DC.
I found that the 100K pull-up resistor on the IR detector provided the optimum sensor performance. Any increase or decrease in the resistance caused a decrease in the signal quality from the IR detector.
The 0.1uF capacitors that are part of the filter eliminate the effects of DC light, such as direct sunlight, something that causes an unfiltered IR passive detector to become swamped.
The H->L and L->H threshold voltages in the Schmitt trigger can be changed by adjusting the 120K and 1.1M resistors.
The 1uF capacitor was added to eliminate voltage spikes at the H->L and L->H trasitions.
Vout is a TTL/CMOS level signal that outputs a logical high 30 times/second when an object has gotten within 6-12 inches of the IR detector.

IR EMITTER CIRCUIT

           +5V                       555         +5V
            |                       TIMER         |
            /                   +-----------+     |
            \                  1|           |8    |
            / 10ohms   GND -----|GND     Vcc|-----+
            \                   |           |     \
            /                   |           |     /
            |                  3|           |     \ 2.2K
            +-------------------|OUT        |     /
            |                   |           |7    \
            |                   |      DISCH|-----+
            |                  4|-          |     \
IR LED  ||--+          +5V -----|R          |     /
       <||                      |           |     \ 200K
        ||--+                   |           |     /
            |                  2|           |6    \
            |               +---|TRIG  THRES|-----+------+
            |               |   |           |     |      |
            |               |   +-----------+     |      |
           GND              |                     |    ----- .1uF
                            |                     |    -----
                            +---------------------+      |
                                                         |
                                                        GND

Notes:

Adding more IR LED's will increase the range of the IR detector.
The signal on pin 3 of the 555 is 30Hz and has a duty cycle of 50%
Due to the very small pull-up resistor the 555 sinks about 109mA. The specs say the 555 can sink up to 225mA so it's well below the danger level.

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