[Laser] MORE XE2AT PART 2

[Andrew T. Flowers, K0SM]

Hi folks,
I found that I could plug a K3PGP front-end into the  LP filter on the 
ramsey board--R2 if I remember correctly.  The PGP receiver is much more 
sensitive than the phototrasistor, but as others point out, it will 
saturate more easily than the phototransistor.  For line-of-sight 
communication, PGP's "daytime" version would probably be better.  I've 
been using the PDB-V117 (~$5) from digikey--the detector area is just 
big enough for a 10" fresnel.   The PGP front-end replaces the 
transimpedence amp (first IC) and runs directly into the LP filter and 
headphone amp.  You'll need to sheild it from outside lightby putting it 
in a box--no "open frame" lesnses.

BTW, I took a ramsey TX/RX pair and mounted them in a separate box with 
RCA connectors to go to the optics.  This allows me to control the 
lasers without disturbing the laser or the optics.  Essentially, this 
gives me a nice "laser transceiver" all in one box.  I used computer 
headers wired to SPDT swtiches to replace the jumpers.  This is nice for 
sending the "test tone" for aligning the lasers and then switching back 
to the microphone for the contact.  It has a connector for a 8-pin mic, 
external audio input (WSJT, tone for CW, etc), headphone and line out. 
I'll take some pictures if anyone is interested in doing this and 
improving upon it.  The transmitter puts out a PWM square wave (5V) that 
can be used to drive any power laser assuming you use a proper current 
limiting resistor.  It works with my 1mw laser pointer and my 100mw IR 
laser heads.

I await summer when I'll have more time to play with this stuff!

Andy K0SM/2
  
Art wrote:

> 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=2592863461 
>
>
>
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