Ed KF6DXX

Thread Starter

kf6dxx

Joined Jul 23, 2008
8
Hello,
I'm working on a visible light project using a small mirror (3x5-inches) to direct a beam of sunlight to a small telescope with a photo-diode mounted on the eyepiece. Current from the diode is then fed into a small amplified speaker.
Currently I'm using a small DC fan at the mirror/transmitter end of a 650+ foot path to chop/modulate the mirror's light beam and generate an audible tone at roughly 600Hz.
This produces nice loud pulses in the speaker but I would like to get rid of the fan as it blocks too much light. Without the fan I can use a larger mirror and get far greater range.
I need suggestions on a better way (very simple, please) to modulate my photo-diode's ouput current of about 2-3volts. I've been working on this project for a long time and any help is greatly appreciated.
Thanks, Ed KF6DXX!
 

GopherT

Joined Nov 23, 2012
8,009
Use a simple 555 timer circuit that can easily be set up to pulse an LED at 600 hz (or what ever frequency you want). Could you explain more about the project. It sounds pretty rudimentary right now, we might be able to help you fairly quickly with some circuit design ideas or off-the-shelf module ideas.

Tell us what you are doing and why. Maybe a drawing or photo will help too
 

vk6zgo

Joined Jul 21, 2012
677
Ed is using sunlight as the source he is modulating,so the easy forms of modulation are out.

For simple Amplitude Modulation,this leaves a choice between a electromechanical shutter or mirror of some kind & some esoteric device which can change its light transmission capabilities with a modulating signal.

An LCD could do this,but would it be transparent enough to let a reasonable amount of light through?

Another possiblility is to change the light polarisation with modulation.
Maybe a quartz crystal,or a mechanically variable polarisation filter?

What happens if you drive a very thin mirror with the guts of a speaker?
 

alfacliff

Joined Dec 13, 2013
2,458
another way would be to vibrate the mirror. possibly glue a small mirror to a speaker cone and feed with 600 hz.route the reflection through a slot or hole to the telescope.
also, did you know that led's detect light near their own color? actually output voltage like a photocell.
 

THE_RB

Joined Feb 11, 2008
5,438
Have we lost the O.P.?

I've been waiting to ask him why on earth he needs to modulate a beam of sunlight into a telescope into a photodiode. :eek:
 

Thread Starter

kf6dxx

Joined Jul 23, 2008
8
Ed is using sunlight as the source he is modulating,so the easy forms of modulation are out.
What happens if you drive a very thin mirror with the guts of a speaker

Thanks for your reponse VK6XGO. I tried making a "Photo Phone" per Edison but decided it was not practical for my needs and capabilities.

What I'm going to try next is placing a tiny CPU cooler type fan directly behind the telescope eyepiece to chop the light just before the beam hits the photo-diode. Hopefully this design will pass a greater percentage of the beam to the photo-diode.

The photo-diode I'm using is something I picked up at a surplus store for US$0.50 which just about fits my budget. No part number or specs. Yes, it's a very low-tech project.

"Why on earth do you want to send sunlight signals to a telescope?" Simply to satisfy my intellectual curiosity. And the challenge of doing something a bit different. Ham Radio operators have a tradition of trying new ideas and building projects with stuff from the junk bin. Ed KF6DXX
 

Thread Starter

kf6dxx

Joined Jul 23, 2008
8

Thanks for your response. Yes, I have put together quite a notebook on free space optical communications, which includes many of Clint's designs and experiments and I have contacted him a couple of times a few years ago.

Clint KA7OEI, along with a few other guys, is in a class by himself for light comm's experimentation. Obviously a sharp guy working with a good crew!
Ed KF6DXX
 

Thread Starter

kf6dxx

Joined Jul 23, 2008
8
Could you explain more about the project.

Thanks for your response. Here's a little background. The sun occupies about 0.5 degree of arc as seen from the earth. The sun's image when reflected from a mirror creats a light beam with a width of about 0.5 degree as well.

So a reflected beam shining on a wall has a diameter of ~ tan(0.5) x distance. At 50-feet this is ~ 5.2-inches and the illumination is quite bright. With any increase in distance the light intensity decreases as the square of distance. So beam's strength quikly becomes very weak at any significant distance.

Interestingly (or maybe not) a large mirror produces the same diameter light spot as a small mirror but the light intensity at the receiving end is much greater. Obviously the aperture of your receiver is limited so big mirrors producing a stronger signal are what's needed. Ed KF6DXX
 

Thread Starter

kf6dxx

Joined Jul 23, 2008
8
I've been looking at this site http://www.levitationfun.com/sunsound.html as a possible solution. But from everything I've read about transformers they will not work with DC current.
This schematic shows what appears to be a transformer being fed from a solar cell or "battery". How can this circuit actually work?
Thanks, Ed KF6DXX
 

Thread Starter

kf6dxx

Joined Jul 23, 2008
8
Hello, I think I've found a circuit that will work for my project. It's at this site http://www.technologystudent.com/elec1/opamp3.htm
"Example Circuit-Light Activated Alerter"

Can someone explain what pin#2 on the 741 op amp is connected to, is it connected to the 100K resistor R2? Or is pin #2 N/C (no connection)?

I have draw literally hundreds of scematics but have never seen one drawn like this. Thanks for any help, Ed KF6DXX
 
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