07 January 2008

MCW optical TX - first test

Finally, more than a year after the idea of an optical RX-TX system was born, I have been able to send (pseudo)information through modulated light.

Instead of a laser pointer I used a 10mm clear-case hi-bri bulk red LED driven at 10mA average. The 488Hz modulation is obtained from a 4.00 MHz XTAL divided by 8192 with a 4060 CMOS chip, keying is achieved with the reset function of the chip as of this diagram:

The RX was the prototype of the future optical RX on experimenter's board, with an OPT202 sensor, without any post-amplification. Circuit per datasheet, single supply operation. Output fed directly into laptop MIC input. At first attempts I did not modify OPT202 feedback resistor, but then a 3M3 was added in series and the signal output increased while total bandwidth was reduced. No amplification lens was used.

The test was conducted in a lighted/dark room with the receiver next to the laptop screen with retroillumination. The TX was aimed at the ceiling and Spectran software in QRSS3 mode produced the following output:

I could not broadcast the received audio in the room.
I will soon try building bounce using two adjacent windows.
I did not reduce LED drive to test RX sensitivity.
I will try to overdrive the LED to get the max power out (destructive test).
I might then add an external LED drive control.

The overall distance was about 6 metres: TX to ceiling + ceiling to RX. Not a DX record, but a good starting point!