Hi Tom.
You mean it is possible to detect some millivolts at the output of the
receiver caused by the modulation of the IR LED? Wouldn't you detect noise
fluctuations too thinking it would be a pulse?

AndrŽ Malafaya Baptista


-----Original Message-----
From: Tom Handley [mailto:thandley@TELEPORT.COM]
Sent: Quinta-feira, 12 de Novembro de 1998 15:32
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: IR barrier and sunlight


   AndrŽ, the reason to modulate the IR beam is to prevent the effect of
saturation. You can normally detect the modulation even under direct sun
light. While you can do some fancy analog techniques to overcome the
problem, a simple missing pulse detector will do the job. Given a saturated
receiver, you can still amplify the signal and detect the missing pulse. I
use to design IR limit switches for use in very harsh environments back in
the 70's. We also added a simple light shield (ie: tube), a low-cost IR
filter, and lenses. We used a 555 timer for the transmitter and a 741
opamp used as a preamp, followed by a 555 timer to detect missing pulses.
We tested the system out to several hundred feet with direct sunlight
focused into the receiver.

   - Tom

At 01:52 PM 11/12/98 -0000, AndrŽ  Malafaya Baptista wrote:
>Hi Tom.
>I think that is not the problem.
>What Leonardo says is that the light may SATURATE the IR receiver. That
>means the receiver will always be in ON state. Modulating the signal won't
>do any better because the receiver is already receiving IR.
>
>Regards,
>AndrŽ Malafaya Baptista
>
>
>-----Original Message-----
>From: Tom Handley [mailto:thandley@TELEPORT.COM]
>Sent: Quinta-feira, 12 de Novembro de 1998 09:29
>To: PICLIST@MITVMA.MIT.EDU
>Subject: Re: IR barrier and sunlight
>
>
>   Leonardo, in addtion to a light shield such as a tube, you want to
>modulate
>the IR beam. A simple way to do this is to pulse the beam at a given rate
>and
>have the receiver look for missing pulses. We use to use 555 timers for
this
>before microcomputers, in saw mills and other environments. You can also
get
>IR filters that are nearly opaque to ambient light. We used all three.
>
>At 03:33 PM 11/11/98 +0100, Leonardo De Palo wrote:
>>Hi PICer,
>>
>>I have the necessity to count object, using a 16F84.
>>
>>My idea is to use a couple of infrared diode and receiver (like the IR
used
>>on TV).
>>
>>But, thinking at the environment of the installation site, it possible
that
>>some sunlight can arrive to the sensor, and I suppose that sunlight can
>>saturate the receiver photodiode.
>>
>>I have the idea to try to minimize the effect of the undesidered light
with
>>some black plastic tube, but before to begin to build the prototipe, I
>would
>>like know if exist a method to drive the LED to ignore o reduce the
>sunlight
>>saturation effect.
>>
>>Anyone have some idea??
>>
>>Ciao
>>
>>Leonardo
>
>