[TriEmbed] TriEmbed Digest, Vol 39, Issue 20

[Michael Monaghan]

Shane,

That's elegant and conserves power.  Only think I can think of that _might_
improve on it is to profile the beam break time to optimize the polling
period.  Chip might get by with 3 tests a second instead of hundreds and
that will feed his battery need.  ;)

I can confirm your suspicion that fog will reflect an IR beam.  We had a
gates in Fernbank Forest that used a reflected IR carrier.  In heavy fog or
rain, the sensor would false positive.  Another had remote sense instead of
reflected.  In the same conditions it wouldn't detect carrier at all.  Both
of these false positives would leave gates open for vehicle traffic
crossing unsigned pedestrian pathways.  In the end we put a timer on them
to close the gates and hoped no one ever stalled under them.

Mike

On Thu, Aug 25, 2016 at 10:46 AM, Shane Trent via TriEmbed <
triembed at triembed.org> wrote:

> Chip,
>
> I think you could set timer for the carrier generation to produced 15
> cycles and have the interrupt from the IR detector stop the pulse
> generation and set your result flag. So if you reach 15 cycles, the path to
> the reflector was blocked indicating an object in the path. This would let
> you keep your IR LED drive time as short as possible by turning off the
> drive as soon as the carrier is detected. So if your detector is awesome
> and locks in 7 cycles, it only get 7 cycles. If the detector needs 12
> cycles, it gets only 12 cycles.
>
> I wonder if heavy fog or rain would reflect enough of the IR to trick the
> receiver into believing it could ALWAYS see the reflective target?
>
> Shane
>
> On Wed, Aug 24, 2016 at 11:57 PM Chip McClelland via TriEmbed <
> triembed at triembed.org> wrote:
>
>> Shane,
>>
>> That makes a lot of sense.  By looking at a specific time and for a
>> specific carrier frequency (38 kHz), I can better see the signal against
>> the background noise.
>>
>> So, instead of a phototransistor circuit, I would use an IR receiver
>> sensor such as this one - http://www.vishay.com/docs/82474/tssp4p38.pdf
>>
>> To reduce the duty cycle, I would only run the 38kHz signal in bursts or
>> 10-12 pulses.
>>
>> Will check this out.
>>
>> Chi
>>
>>
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