The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
From: Winfield Hill
Subject: Re: the strange phototransistors circuit
Date: 17 Oct 2002 06:58:06 -0700
Organization: Rowland Institute
X-Newsreader: Direct Read News 2.98
Keithe firstname.lastname@example.org (duqizhang) wrote...
> | |------ SIGNAL IN
> | /
> LED | NPN
> | \>
> | |
> | />
> LED | NPN
> | \
> | |
> --- ------- SIGNAL OUT
> look at the circuit above. There two phototransistor in series.
> And the emitters are connected together. The signal goes from
> one phototransistor's collector to the other phototransistor's
> collector. It seems the circuit won't work,but it works well in
> certain equipments.
> Why does the circuit use two phototransistors in series instead
> of just one? How can the circuit work?
It's a signal switch, as you show. When a single photo-
transistor is on at low current, it's a saturated transistor
which conducts signals well. In fact it can conduct current
in both directions. When the photo-transistor is off, if the
reverse voltage becomes high enough (greater than ~ 5V), the
reverse-biased emitter-base junction can break down, thereby
turning the switch on for those voltages. Using two photo-
transistors back-to-back prevents this problem, since one will
always be able to hold off high voltage in the right direction.
The modern replacement is two photoFETs wired back to back.
A photoFET or optoFET is a small MOSFET with its gate driven
by a series stack of photodiodes, enough to create 5V or more
when illuminated by an infra-red LED. Aromat explains their
NEC call them OCMOS, etc.
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup