weak signal source for optical and more

[@coltronex]

This takes me back many years when I used a cluster of neon lamps modulated by an ECC83 and a LDR receiver to transmit music over 100ft,it worked and was a little noisey but fun,then I pointed the LDR receiver at the moon and listened to the moons noise pattern,all interesting stuff,keep up the experiments please,regards Colin.

[@W1VLF]

Thank you Colon.. Working a better bare bones beacon video now W1VLF

[@w8bya]

How cool was that ! No way I could stop watching Paul......tnx for the awesome science project. 73

[@W2TTT]

Paul, You got me hooked with this series of videos!

[@W1VLF]

We have to talk sometime.. more to come

[@paulwhite6995]

Lovely, most entertaining thanks.

[@W1VLF]

Thank you Paul.. W1VLF

[@kensejkora4424]

Great video, Paul. Another illustration of using non-conventional "antennnas" to feed the AirspyHF+. Ultrasonic transducers, photodiodes... what next? Does one have to be concerned with the polarity of the photodiode detector when connecting it to the input of the Airspy SDR? Does it make a difference if the anode is connected to the center conductor vs. the cathode, or does it make any difference at all? Obviously, a photodiode is a DC device with polarity, whereas the Airspy SDR is detecting an AC waveform. Also, might it be possible to increase the sensitivity of the photodiode by providing a few microamps of DC bias across the photodiode? Not sure if it would need to be forward biased or reverse biased. A 3-volt button or coin cell connected in series with a 10 megaOhm resistor across the photodiode would provide a bias current of 300 nA, or 0.3 uA. I'm not sure if that would be enough, but the resistor value could be changed accordingly. A potentiometer could possibly provide variable gain. Granted, adding DC bias might increase noise, but if the gain in signal is greater than the increase in noise, the SNR still benefits. Maybe fodder for another experiment. Again, great video. Keep pushing the envelope for innovative SDR "antennas". 73 Ken -- WBØOCV

[@W1VLF]

Hi Ken.. It does appear that the Photodiode polarity make any difference. Think about this in a no light condition.. The voltage output of the photodiode is zero. Any light impinging on the PD will cause an output voltage. In one direction the PD will have a + V compared to ground, reversed it will have a - V compared to ground. Either way the voltage is turning on and off at the rate the LED is turning on and off. In this case at 20 KHZ rate. Yes I love experimenting with SDR "Antennas" as you say !!! I hope to finish up a better optical video showing a barebones single LED beacon set up soon.. I am trying to get some of the guys in the local Microwave group interested in optical comms W1VLF Stay tuned old friend and thank you

[@kensejkora4424]

@@W1VLF Hi Paul. Your 'argument' makes sense, but after considering the signal waveform, would it make a difference? I agree you'd be looking at pulses of the opposite polarity coming from the photodiode, but considering that the SDR is sensing AC, it shouldn't be sensitive to the polarity of the incoming signal. Yes, you have a DC signal from the PD that is being pulsed at 20 kHz, but the SDR should be able to detect the pulsed DC regardless of polarity. Didn't you do another KZbin video a couple of years ago by shining your IR LED array up at the clouds and detect the reflected signal? Regardless, keep up the great experimentation. 73. -- Ken - WBØOCV

[@W1VLF]

Yes... I did ... but that one used the audio input of the (mic input ) or a computer. Try this .. Look at a square wave on an oscilloscope with the scope in the DC mode. If the square wave from the generator is referenced against ground.. The oscilloscope will show only 0 volts to the + voltage of the square wave... How ever put into the AC mode and only show the AC component of the and it block the DC component. The signal with go about and below DC 0 volts..

[@stevencianciolo5980]

Very informative! 🎉

[@W1VLF]

Thank you constant viewer.. always appreciate your views

[@loueckert4970]

WOW cool. Is that light polarized? 73 de KT1R Lou in WV

[@W1VLF]

Nope.. Just a run of the mill 10 cent LED... random polarization

[@WECB640]

Let's make a Christmas Tree with one of these IR LED's on top and PWM it at 17.2kHz sending CW "SAQ Christmas". 73 OM

[@W1VLF]

Hi Long time... We could set up a neighbor hood optical audio network via Christmas tree lights!!! W1VLF

[@WECB640]

@@W1VLF Great idea! Some use LPFM to play music as the cars drive through the cul-de-sac to see the pretty lights, BUT a ham might also wonder if there was a secret audio message (an Easter Egg) in the modulated light. 👍 Keep up the great work Paul. 73 OM

[@cthoadmin7458]

Hi Paul, can I ask a couple of questions? What can you tell me about that preamplifier you are using? Where would I get one, are they expensive? You are using PWM which is quite straightforward and cheap to generate, receive and demodulate. Is it possible to use weak signal modes to improve performance?

[@W1VLF]

There is no preamp in use on any of the videos in this series including this one. The only thing receiving the signal is a single photodiode, driving the receiver directly.. nto the 50 Ohm input impedance which is not optimal by any means. The photo diode I used is a 5 X Sharp PD49Pi Infrared Photodiode - General Photometry 1000 nm Wavelength listed on eBay as www.ebay.com/itm/274341608439 Yes we can use digital modes to take advantage of the huge gains in SNR ratio but that well beyond the scope of these initial videos.. That's to be covered in a future video... Its room experimentation

[@cthoadmin7458]

@@W1VLF The thing you are using at 16:25 Paul, that you call a preamp.

[@W1VLF]

@@cthoadmin7458 Yes... You are correct.. That is my mistake, I did not see which video you were asking about. The preamp something I had that I bought at auction many years ago. Its called a transimpedance amplifier that convert the output of the photodiode which wants to look into a high impedance to low impedance so it can drive another device. Look for video I just posted last night about a 200' foot path using no preamplifier. Sorry for the mistake. This video here was just to show can be done beyond the most primitive of systems. W1VLF

[@cthoadmin7458]

@@W1VLF You've really sparked my interest here Paul. I'd like to see if I can transmit and receive psk31 or one of the other protocols used for weak signal work.

[@W1VLF]

@@cthoadmin7458 Yes... It can be done easily. ACTUALLY we are already doing PWM .. the square wave being used to drive the LED ... is PWM right only thing is the duty cycle is fixed at 50%... we need to only modulated/ vary that 50 % duty cycle square wave W1VLF

[@paulwhite6995]

Paul, could such an optical link (or similar) through air ever compete with Ethernet (WiFi or wired)? Presumably never could compete with fibre.

[@KallePihlajasaari]

Free-Space Optical links have been around since before Ethernet and these days are able to do line of sight at a Gigabit or more in clear weather. The professional units have radio fallback for snow or fog. Commercial units seem to use multiple lasers and the hobby crowd seems to use Bright LEDs and a lens to spread the beam wider so rain is less of a problem and the beam does not diverge as fast and they are eye-safe so not regulated.. There was a community internet backbone project about a decade ago (EDIT: over two decades). Found it, RONJA and it has some information on wikipedia. These links compete favourably with fibre because you do not need to have access to a municipal right or way to dig trenches for fibre conduits. They ca also be installed as quickly as you can get a PoE cable to the roof of the building. Obviously they will not work if you cannot see the next site. The idea of the RONJA network was to have so many units that a mesh is created that allows everyone to connect and backup links to be available.

[@paulwhite6995]

@@KallePihlajasaari Many thanks, Kalle, I'll follow up your most interesting link.