[Laser] LED negative resistance

n5gui at cox.net n5gui at cox.net
Sun Feb 2 22:36:59 EST 2014


Ron

Thank you for your comments.   I did plot the numbers that I recorded from the testing using Microsoft Excel, which is how I got an estimate of the series resistor for the 6V LED replacement bulb.  The plot for the 3V bulb had a discontinuity, but the slope of the curve on either side of the discontinuity suggests about 9 or 10 ohms.


I have previously worked with red LEDs in the under 100 ma range, and I think they would function for communication demonstration at the range that I am considering - less than two miles, mostly under 500 yards.  They also cost less than a commercially available LED flashlight, at least for the active part of the system.  There are two reasons that I decided to pursue the "Talking Flashlight" project instead of building on the red LED experience that I had from the "Talk on a Beam of Light" demonstration.  The first reason is that I perceive that youth, and their adult leaders, would better understand the idea behind modifying a flashlight into a communication device.  They already know what a flashlight is and what it can do.  They know how to turn on a flashlight and to point it at something in the distance.  If I suggest to them that I want to modify a flashlight, they know what I am starting with.

The other reason that I want to modify a working flashlight instead of building the light transmitter from opto-electronic and optical parts is that while it is being used, it is still a flashlight.  If I design a game ( or maybe a skill learning module ) to be played at night in a large field, then to send a team, or an individual Scout out with the equipment, they have a flashlight that they can use on the trail and it will serve them very well as a flashlight.  They do not have to carry their own light in order to get into position, or to relocate to another, or to seek a specific goal.  The communication system serves as their travel light source.  They do not have to bring their own separate light, and the type or cost of their personal equipment should not affect how well they can perform on the game ( or skill learning module ).

I have already commented that a modified flashlight has the optics of the transmitter built in.  It also has a certain amount of standardization.  If I list a commercially available flashlight to modify, it is easier for groups across the nation to build similar performing equipment.  If I supplied a kit of parts that included the LEDs, lenses, and circuit boards ( wired or with a bag of parts to be installed ) but left them to scrounge up PVC pipe cut to length and hardware, the results don't seem to me to be as likely to be consistent.  I still have the problems of kitting the electronic boards and probably the optics for the receiver, but modifying a flashlight for the transmitter seems like a way to simplify much of the total project.  As for the cost, I don't see the $5 to $10 cost of a flashlight, while more than the cost of PVC, wood, screws, other hardware, a lens, and an LED,  as being an issue for the overall project.

Perhaps you would be willing to share with me what you use for a light receiver.  I have been much more successful building transmitters than receivers.   We might even find a way to work on a joint project that could be published together.


James
  n5gui







---- Ron Simpson <n6gkj.cm98 at yahoo.com> wrote: 
> Hello James,
> 
> Since you cannot measure the voltage and current at the LED because it is sealed, you have to do some math to get the true result. 
> 
> Do you know what LED the manufacture has chosen and do you know what kind of current draw it was designed around?
> 
> You can make a guess I suppose.
> 
> Keep digging I think you have a good idea there. If this is night time event, design it for red LED's.
> 
> With a little work you can take the filament bulb apart and put in a known LED and bias it for the current you need.
> 
> I play with the 3 watt LED's I get two current readings one at the LED is always different than the one at the supply. There is current in the dropping resistor, since they can get warm there is additional losses 
> 
> Publish your work please!
> 
> Our Ham Club helps out on BSA Events. They might like to do this too!
> 
> Ron,
> N6GKJ
> 
> Sent from Yahoo Mail on Android
> 
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