[Laser] MORE XE2AT PART 2

[Art]

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.

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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