21 January 2008

LED ERP vs. current

While searching for optical RX-TX systems that would carry voice, I stumbled across an article of the only Italian HAM (AFAIK) that has documented optical experiments, I4VIL. He mentions that LED efficiency at some point decreases even if the current flow increases (in Italian).

I managed to replicate his observations using my MCW TX and the simple OPT301 RX, just the detector without the active bandpass filter. I monitored the received signal strength by feeding the detector output to my laptop MIC input. Spectran software did the measurements. The TX was aimed at the ceiling, as well as the OPT301, without any lens.

I monitored the total circuit input current, not the one flowing into the LED. The MCW TX draws about 6 mA @12V in stand-by, so you can figure out how much current really flows into the LED. But at 100mA average when transmitting, 6mA are negligible.

The Y-axis shows dB reading on Spectran for the 488 Hz line, relative to the first dot (17 mA); the X-axis is the total current flowing into the circuit.

It is obvious that the emitted light reaches the maximum power somewhere between 80 and 100 mA. Going further just heats the diode.

Delta ERP is +17 dB. That would be almost 3 S-points in RF terms.

Interesting, isn't it?

18 January 2008

HP iPAQ h5450 dead/flat battery

So, you forgot to recharge your iPAQ h5400 PPC handheld for a week and it doesn't power up anymore? Not even if you leave it plugged to the external supply for days?

Might be too late, or not.

First of all, if you read this before your battery has flatten out, go to the HP website ("software and drivers"), download and install firmware upgrades if you've never done it. There is a fix for the flat battery that doesn't get recharged.

If your h5450 has already passed away without upgrades, you need to find another h5450 (h5400 series), do the patching and then recharge your battery on it. Most probably in less than 24h you'll have your h5450 working again.

If you have a h5500 series handheld (h5550 for example), HP has not released the battery charging patch, while it seems to be affected as well (real world experience). Since h5400 and h5500 batteries are the same, let a patched h5400 recharge your battery and you'll be back on track.

Important: always do these chargings with direct connection to the external power supply. Do not use the USB charging feature.

Good luck.

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!