"Clinton" Bypass Not Working

[tommy.genes]

Having salvaged an LED/BJT optoisolator from a thrown-out UPS at work, I decided to try RG's "Clinton" bypass circuit (and I don't think he was talking about George).

The isolator is an Isocom IS201 (datasheet).

My circuit is as follows:


Behavior I observed:
1. Blue LED did not turn on, weak signal passed through transistor of IS201
2. Voltage drop across blue LED and 4.7k of about 4 - 5 volts
3. Bypassing diode of IS201 - blue LED worked fine
4. Connecting +9V to base of IS201 transistor (pin 6) did turn it on
5. Connecting blue LED to pin 6 did not turn on transistor, fried LED

My questions:
1. Is there something stupidly wrong with the schem that I'm not seeing?
2. Is the diode of the IS201 blown? Did I do something to blow it?
3. Am I supplying too much or too little current/voltage to the IS201 diode?
4. Do I need to do something to "partly" bias the transistor via pin 6? The dual and quad versions of this chip do not have this base connection.

Any and all help appreciated. If I had the parts on hand, I probably would have tried a Millennium bypass instead.

Thanks,
-- T. G. --

[R.G.]

Congratulations! You're the first person to ask about that. And you understood the innuendo, too. 

What's wrong is that you used the wrong optoisolator. Photo transistors don't work very well if at all in this setup. Only the LED-photoFET versions work. The H11F1/F2/F3 series work fine.

[tommy.genes]

Thanks for the reply. It's good to know that I understood the concept, beyond "selecting" (it was what I had) the wrong part.

I would ask for a bit more theory as to why one type of part works and another doesn't, but I'm pretty sure I wouldn't have the time to properly understand the answer.

Thanks again,
-- T. G. --

[jrc4558]

Congratulations! You're the first person to ask about that. And you understood the innuendo, too. 

What's wrong is that you used the wrong optoisolator. Photo transistors don't work very well if at all in this setup. Only the LED-photoFET versions work. The H11F1/F2/F3 series work fine.

Yep! That's the way Voodoo Labs's Sparkledrive is bypassed too.

[R.G.]

I would ask for a bit more theory as to why one type of part works and another doesn't, but I'm pretty sure I wouldn't have the time to properly understand the answer.

It's not that complicated. Bipolar transistors like in the LED/phototransistor isolator make lousy signal switches. FETs as in the LED/photo-FET are inherently voltage variable resistors at small signal levels. You just happened to grab the wrong one.