[TriEmbed] TriEmbed Digest, Vol 39, Issue 20

[Chip McClelland]

Mike,  interesting. Had not thought about reflected light. Is there a way
to use polarization to differentiate a signal reflected from the other side
and not just from the fog?

Thanks,

Chip

On Thursday, August 25, 2016, Michael Monaghan <mike at chipworks.net> wrote:

> 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
> <javascript:_e(%7B%7D,'cvml','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
>> <javascript:_e(%7B%7D,'cvml','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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-- 
Sent from my iPhone - please excuse the typos
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