I wish I had an Oscilloscope ... Anyone selling one cheap?

I have the specs on the LED and they are rated at 100mA continuous. The 
circuit is simple, and mounted on a "sea-of-holes" just 7 x 20 with room 
to spare... 6 resistors + 1 F628A. There is no circuit board (and will 
never be). I have lots of space to solder/desolder. My biggest issue is 
that I am unfamiliar with the parts available... I have to learn what a 
Sziklai pair is, and if my (small) local electronics store has it. I 
have no training in real electronics... I am a computer programmer doing 
this "for fun". I have interest, but no experience....

I do have an opto-coupler on hand (excess from another "learning" 
project), and that will probably do the trick... worth a try (I was 
planning on using it to drive multiple LED's at 100mA each). I believe 
the switching time would be fine. Need to (re-)read the specs on it though.

I am hoping for a range greater than 20 meters ( 50' ).

Thanks for a new avenue of research...

Rolf

Spehro Pefhany wrote:

> At 02:46 PM 10/22/2005 -0400, you wrote:
>
>> Hi all.
>>
>> I have just completed a "prototype" project (hobby), that is an 
>> Infra-Red remote control for a Nikon D70s camera. The primary 
>> motivation is to create an "extended range" relative to the remote 
>> that Nikon sells (I set the camera on a tripod, focussed on a bird 
>> feeder. I want to be really far away and still trigger the camera... 
>> maybe even set up a motion detector ... ;-).
>>
>> Right now it is working, but it's range is "only on par" with the 
>> Nikon remote (and a lot more clunky ;-).
>>
>> I have a bunch of IR LED's available. They are 100ma with 1V volt 
>> drop. They have a 20deg "half-intensity" beam angle. I am using 2 AAA 
>> cells to power the system (nominal 3V).
>> Because the IR Signal is a 40KHz 50% duty cycle when on, and for each 
>> "trigger" it will only be on (at 40KHz) for 8ms for the complete 
>> trigger sequence of 85ms, I believe I could exceed the 100mA 
>> "continuous" rating. It's "pulse" rating is something like 10A for a 
>> few uSec.
>>
>> I am using a 16F628A with INTOSC@4MHz
>>
>> To Drain the current of 100mA, I am using 4 PORTB pins as a sink 
>> (25mA current per pin as per datasheet). My calculations indicate 
>> that to get 100ma through the LED with 1V forward, and 3V system, I 
>> need 20Ohm resistance. To sink that through 4 pins, I connected an 82 
>> Ohm resistor to each pin, and then the LED through the 4 parallel 
>> resistors, giving a net parallel resistance of about 20Ohm.
>
>
> Have you put an oscilloscope across any of the resistors to see what 
> current you are
> actually getting? I suspect a lot less than 100mA total, both because 
> of the PIC at
> such a low power supply voltage and because I suspect the voltage drop 
> of the IRLED
> is a fair bit greater than 1V at anything like 100mA.
>
> You should probably use a very low voltage MOSFET (eg. Zetex 
> ZXMN2A02N8, which
> has 40mohm Rds(on) at Vgs=2.5V) or a bipolar (eg. Sziklai pair-- cheaper)
> to drive the LEDs, with a single port pin.  After you get it working 
> at ~100mA
> (assuming your LED is actually rated to work at that current) you can
> re-evaluate the range and see if you need to do anything more.
>
> If you do decide to use more than one LED, it's best to use a separate
> resistor for each to ensure that the current splits more evenly.
>
> Best regards,
>
> Spehro Pefhany --"it's the network..."            "The Journey is the 
> reward"
> speff@interlog.com             Info for manufacturers: 
> http://www.trexon.com
> Embedded software/hardware/analog  Info for designers:  
> http://www.speff.com
> ->> Inexpensive test equipment & parts 
> http://search.ebay.com/_W0QQsassZspeff
>
>

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