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Thread: Noise in phototransistors/photodiodes/light
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face BY : Thomas McGahee email (remove spam text)



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 removed}
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