Sean,

To maximize the distance, consider the following:

1) Maximize radiated power.

   A) Power IR LEDs with narrow pulses and LARGE instantaneous
      currents. This will make best use of the super-luminant
      effect. To aid in getting these large currents delivered,
      use fets or transistors that can handle the current pulses.
      Drive them HARD, with clean, fast rise and fall times.
      If you do not drive the FET/TRANSISTOR hard and fast enough,
      then it will spend too much time in the linear region
      and overheat.

      Keep all wiring between LED and local power source SHORT
      and eliminate inductance as much as you can. Use a LOCAL
      current source such as a low ESR capacitor close by the
      fet/LED combination. Use at least one local storage cap
      for each series string of LEDs. Cap must be fully charged
      during the LED "off" time. By "fully charged" I mean to about
      90% of the nominal power supply voltage.

      For best results use IR LEDs that come in metal casings,
      and mount the IR LEDs in snug holes drilled through
      a bar of aluminum such that the lens of the IR LED
      just barely sticks out. Using a heat sink lets you
      push the LEDs just a bit more.

   B) Use multiple IR LEDs. One very useful technique is to
      run several IR LEDs in series. This greatly reduces
      average current drain and minimizes wasted current,
      since you can now use a smaller resistor value for the
      current limiting resistor. More LEDs not only increases
      the average amount of light generated, it also widens
      the field of view of the light source.

   C) Use a condensing lens arrangement to collimate the light
      from the LED(s) so that the light is aimed in the direction
      of interest.

2) Detect signal and minimize interference

   A) Use an IR filter at the receiving end. Do NOT use any
      filter at the transmitting end. The narrower the bandwidth
      of the filter, and the closer it is matched to the IR
      bandwidth of the LED(s), the better.

   B) If operating in low light environments, it is sometimes
      useful to use a local IR LED to flood the IR sensors with
      a very weak DC IR beam that just barely gets the sensor
      to operate in its active region.

   C) Use a modulated IR beam. You get this for free if you are
      pulsing the LEDs. Keep the pulse width ratio LOW. Something
      around 5% works well in most cases.

   D) An optical lens can help gather light from a larger
      area and concentrate it. Large lenses can also help
      increase the useable viewing angle.

   E) Once the signal has been detected, AC couple the modulated
      frequency to a High Q Narrow BandPass Filter. This
      helps you differentiate between noise and signal,
      and rejects anything outside the band of interest.
      I have found LC networks to have the best characteristics
      for the bandpass elements. A parallel LC arrangement
      in the feedback section of an inverting opamp circuit
      works very well.

Fr. Tom McGahee

----- Original Message -----
From: Sean Breheny <shb7@CORNELL.EDU>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Sunday, June 04, 2000 4:18 PM
Subject: [EE]: Noise in phototransistors/photodiodes/light


> Hi all,
>
> For a project that I am working on, I need to detect the output of a few IR
> leds at about 300 feet (abt. 100 meters) away. The output is pulsed, to
> help distinguish it from background light.
>
> I have succeeded in doing this out to about 100 feet, but am having
> difficulty beyond that because of noise. I have found appnotes which
> explain photodiode noise to some extent, but nothing on phototransistor
> noise. Could someone point me to a good reference for both?
>
> Also, does light emitted by an incandescant bulb have a significant noise
> due to the discrete nature of the photons (like shot noise in electric
> current)? How would one go about calculating the approximate amount of
> noise from such a source? I have been using flashlights to simulate intense
> background light (like the receiver is trying to pick out the source with
> the sun as a backdrop) and the amount of noise I have been seeing is much
> larger than I would expect due to electrical effects. I was wondering if I
> am seeing real fluctuations in the light intensity.
>
> Thanks,
>
> Sean
>
> |
> | Sean Breheny
> | Amateur Radio Callsign: KA3YXM
> | Electrical Engineering Student
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