You might be better served using a transconductance amplifier on the output of the LED. That of course also depends on how fast, how noisy, how sensitive etc you wish the device to work.  You may be able to use the device as a linear isolator without much difficulty as well (using a transimpedance amplifier on the input LED). I am curious what type of bandwidth can be obtained from a simple coupler like that.  Are you using high power LED's or normal 'high efficiency'?

Stephen

> ----- Original Message ----- 
> From: "Michael Algernon" <agscal@gmail.com>
> To: "Microcontroller discussion list - Public."
> <piclist@mit.edu>
> Sent: Saturday, November 22, 2008 8:08 PM
> Subject: [EE]: DIY opto-isolator furrgetaboutit
> 
> I have built a do-it-yourself opto-isolator using twoT13/4
> red LEDs
> pointing at each other.  Thanks to suggestions from the
> PIClist , it
> occurred to me to add a 2N4401 transistor to the circuit to
> increase
> the current gain.  I used a 74C14 ( CMOS hex inverter with
> schmitt
> triggers ) to make an oscillator and a detector.  This is
> only useful
> to automate turning the driving LED on and off.
> 
> see http://oh-god.com:5080/dir/diy-optoiso/
> 
> 001.jpg is the schematic.  I used RsubF and CsubF  to set
> the period
> for an on-off oscillator at about 1Hz.  RsubL is 1K ohms. 
> V+ is 7
> volts.  So about 5ma is going thru the LED on the left. 
> RsubB is 100K
> ohms.  And it works !  yeaaaa.
> 002.jpg shows an out of focus photo of the breadboard. 
> Used a MacBook
> and the PhotoBooth application.  The silver thingie
> consists of the
> two LEDs glued ( superglue gel ) together facing each other
> and coated
> with silver paint.  I thought the paint might block
> external light and
> maybe reflect internal light.  The 2N4401 has a red blob on
> it and the
> 74C14 is to the right.  I used 1K ohm resistors for zero
> ohm jumpers
> where 1K ohm was essentially zero in the circuit.
> 003.jpg shows the silver thingie thru a magnifying glass.
> 
> I plan to use spare inverters on the 74C14 to
> differentially drive
> small transformer at 100KHz.  The rectified DC voltage on
> the other
> side of the transformer will be regulated by the
> opto-isolator turning
> off the 100KHz oscillator when the voltage is high enough. 
> Thus I
> achieve a low current isolated DC supply for my truck to
> power $2 DMMs
> I bought at Harbor Freight.  The DMMs don't work unless
> they are
> isolated from the truck ground.
> 
> MA
> 
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