[TriEmbed] TriEmbed Digest, Vol 39, Issue 18

Wed Aug 24 21:53:58 CDT 2016

Chip,

You may have a hard time differentiating your pulses from ambient solar IR.
And getting the level just right to trigger an interrupt reliability under
changing solar brightness conditions may be a challenge. Good shielding of
your receiver will help.

You may need a modulated carrier to help pull your signal from the
background IR. Some demodulators (IR detector) can lock on to the carrier
in 10 cycles or so. In theory this means a demodulator could detect a
packet of 38 kHz pulses that contains 10 pulses and have a total length of
around 0.26 milliseconds.

So you could gate your 555 timer (or just use a timer on the uController to
generate the 38 kHz carrier) to generate packets of IR at high current but
at a low repeat rate and use the output of your demodulator to trigger your
interrupt. Using a 50% on/off ratio on a 38 kHz carrier you could sample
your IR optical path for completion at a rate of 1.9 kHz  (= 0.5 * 38 kHz /
10 pulses).

So just figure out how quickly you need to sample to not miss an any
objects and you are set. So if you decide you can sample at 38 Hz and catch
everything you want, I believe you would be running at about a 1%
duty-cycle on your IR emitter.

You would set a timer to fire every 26 milliseconds and generate 10 cycles
(may want to make it 11 or 12?) of 38 kHz (taking 0.26 milliseconds) and
after the end of the last pulse check to see for a interrupt from the IR
detector set the "detected" flag and go back to sleep until the 26
millisecond timer fires again. Or would that make the LED duty-cycle 0.5%
since it is only on for half of the 0.26 milliseconds? Anyway you can
really hammer on the LED since your duty-cycle is so low. And you will sill
have a low average current on the transmitter.

Shane

On Wed, Aug 24, 2016 at 9:49 PM Chip McClelland via TriEmbed <
triembed at triembed.org> wrote:

> Robert and all,
>
> So, here is what I am thinking, please let me know if there is a better
> way.
>
> 1) Transmitter - use a LMC555 timer to deliver 10 , 1uSec pulses a second
> in Astable mode.  The data sheet
> <http://www.ti.com/lit/ds/symlink/lmc555.pdf> for the LMC555 timer says
> it can handle  .  This circuit will be turned on and off by the Arduino but
> that is all no PWM or triggering required. It will drive a 100mA 850nm
> emitter
> <http://www.marktechopto.com/pdf/products/datasheet/MTPS8085P_2015-05-07.pdf> with
> a very focused beam.
>
> 2) Receiver  - I will use a matched 850nm phototransistor
> <http://www.vishay.com/docs/81527/bpw77n.pdf> wired as a common emitter
> amplifier.  That way, the received pulse will cause the interrupt pin to go
> LOW and wake the Arduino.
>
> 3) Software, I will create a simple interrupt service routine that will
> register a pulse and determine how many if any pulses have been missed.  I
> can use a running average smoothing function to get rid of noise so I don’t
> count a falling leaf.
>
>
> I hope I am not missing something.  Is there a better way to do this?
> Will I run into issues with changing ambient light or rain / snow causing
> problems?  Will see but any heads up are appreciated.
>
> Thanks,
>
> Chip
>
>
>
> On Aug 24, 2016, at 10:00 AM, triembed-request at triembed.org wrote:
>
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>   1. Re: TriEmbed Digest, Vol 39, Issue 16 (Robert Gasiorowski)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Tue, 23 Aug 2016 16:16:57 -0400
> From: Robert Gasiorowski <rgresume at gmail.com>
> To: Chip McClelland <chip at mcclellands.org>
> Cc: "triembed at triembed.org" <triembed at triembed.org>
> Subject: Re: [TriEmbed] TriEmbed Digest, Vol 39, Issue 16
> Message-ID:
> <CAEU50hUQ+S3f4_KTHTAXRCEmMx_tGiuGeb1k61Wq1atiCf12dw at mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
>
>
>
> If you decide to use 38kHz carrier, you will either use a lot of energy or
> limit your distance.
> Here's my suggestion:
> 1. set IR LED's current to 100mA or higher (up to 1A,)
> 2. send 1us-5us  pulses, 10 to 50 per second,
> 3. use pin interrupt and timer to detect and count missing pulses
> 4. use lens (LED) and IR filter (detector)
>
> Alternatively, you could use short bursts of 38kHz carrier (also at higher
> current, but probably not as high as above.) This way, you could use one of
> Vishay's detectors, which usually come with IR filters.
>
> On Tue, Aug 23, 2016 at 1:23 PM, Chip McClelland via TriEmbed <
> triembed at triembed.org> wrote:
>
> Alex, yes, bought and tested that one. But humans are not reflective sir
> only about 12'. Cars to 30' easy so will eventually use it to replace the
> pneumatic tubes across the road.
>
> Bought a matched pair of IR emitter and phototransistor yesterday. Will
> see if I can build my own circuit as this seems to be a new and novel
> application (!?).
>
> Thanks, chip
>
> On Tuesday, August 23, 2016, <triembed-request at triembed.org> wrote:
>
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> Today's Topics:
>
>   1. Re: TriEmbed Digest, Vol 39, Issue 15 (Alex Davis)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Mon, 22 Aug 2016 13:28:21 -0400
> From: Alex Davis <alexd at matrixwide.com>
> To: triembed at triembed.org
> Subject: Re: [TriEmbed] TriEmbed Digest, Vol 39, Issue 15
> Message-ID:
>        <1471886901.1337427.702662641.4268E605 at webmail.
> messagingengine.com>
> Content-Type: text/plain; charset="ISO-8859-1"
>
>
> All,  any ideas on a low power (5mA and 3.3-5V) IR beam sensor
> (transmitter
> and reflector) I could use in an outdoor setting?  Need to count
> pedestrians walking on a gravel road and the width - 30' - is too great
> for
> ultrasonic. Ideally, could buy the sensor components directly and
> integrate
> them with my data logger board.
>
>
> Chip,
>
> looks like this one (https://www.sparkfun.com/products/11724) can be had
> in a 10-meter version, and uses 2.9 mA average, so maybe ultrasonic
> would work.
>
> I think decent-range beam-break is going to take some decent current.
>
>
>
> Alex
>
> --
> "The theater of noise is proof of our potential."
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