[Laser] Re: MORE XE2AT PART 2 (Art)

Wed, 11 Feb 2004 10:43:35 -0500

I was able to attenuate the ambient light by placing an optical filter in
front of the photodiode. For "red" LASER's, I found that the red plastic
covers that were used over LED numeric displays work reasonably well, and
can be salvaged from old equipment.
walt

-----Original Message-----
From: [email protected] [mailto:[email protected]]On
Behalf Of [email protected]
Sent: Wednesday, February 11, 2004 10:24 AM
To: [email protected]
Subject: [Laser] Re: MORE XE2AT PART 2 (Art)

Would there be any advantage to transformer coupling the photodetector
to the rest of the system?  While doing some experimentation on my own
design, I frequently had problems with ambient illumination causing
saturation effects while using a fixed load resistor for the photodetector.

Also, a phototransistor is usually not the best photodetector.  They tend
to be somewhat noisy.  For my design, I've been using a PIN photodiode
with a moderately large detector area (Osram SFH-214).

Additionally, my design uses an op-amp based active filter, which can
be tuned to select the frequency.  The system I demonstrated for our
club meeting a few months ago was a base-band modulated CW (MCW, A2)
system, using a slightly modified laser pointer being driven by an 800 Hz
square wave.  For more information, here's a pointer to the club's web-page,
which has a link to the laser transmitter/receiver demonstration in the
lower
right corner:

http://www.scaresclub.50megs.com

I can't claim any distance records with the equipment yet, but it does
work very well over about a 40 foot distance.  :*)

Dave
WA4QAL

-----Original Message-----

   1. Re: MORE XE2AT PART 2 (Art)

--__--__--

Message: 1
Date: Wed, 11 Feb 2004 01:37:06 -0500
To: [email protected]
From: Art <[email protected]>
Subject: Re: [Laser] MORE XE2AT PART 2
Reply-To: [email protected]

Hello Al,

I checked the Ramsey website and found the full schematic for the
transmitter there, along with the manual! The description of the system in
the manual is very good!

The transmitter is simple pwm at an 18 Khz rate. So, the front end needs to
have 18 Khz bandwidth.

I think you have severe limitations placed on your range because of the
phototransistor (Q1) in the receiver.

First, it has a small active area and focusing light from a large lens onto
that small active area is not easy, much of the gain from your 4 inch lens
is lost due to this.

Second, the phototransistor has a 1 meg ohm load resistor! While this is
good if you want to hear 50 Hz and 60 Hz, there will be a very poor
response from the 18 Khz data that the transmitter is sending. What makes
this even worse is that the active filters in the receiver will not respond
to 50/60 Hz, but the phototransistor saturates easily because of the large
value 1 Meg ohm resistor-meaning that the front end overloads because it is
much more responsive to 50/60 Hz.

---------------------------------------

I think the Ramsey kit can easily go 25 miles with a modest modification to
the receiver though.

Remove Q1 and substitute a photodiode and op amp with bandwidth set to
approximately 18 Khz. If you want an easy to build  photodiode/op amp
assembly, you can use a Burr-Brown OPT-201 IC with the feedback resistor
chosen to provide approximately 18 Khz bandwidth. Because of the higher
gain, you might need to shield the OPT-201 to prevent excessive hum and to
minimize the possibility of self oscillation. OPT-101's (an improved
OPT-201) are currently being offered on ebay for $2.50 each, but you must
buy a minimum of 4 units. The OPT-101 is easier to use because it's a
single supply chip and because it only draws 120 microamps of supply
current!

Click here to go to the ebay listing:

http://cgi.ebay.com/ws/eBayISAPI.dll?MfcISAPICommand=ViewItem&item=259286346
1

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