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with a photo transistor, you simply pull it low and take the output 
from the emitter.  Connection wise, +5 connected to collector. 
emitter connected to pull-down resistor which is in turn connected to 
ground.  output is take from the emitter.  When the PT turns on (i.e. 
light), it will pull the output high, when its off (dark), the output 
is pulled low by the resistor.  You will need to play with the pull-
down resistor value to get the right triggering level.  I'd use a 50K 
or so pot but wouldn't run it down below 1K (or use 1K in series to 
protect it).  Once you get a relialable value for the pull-down you 
can substitute a fixed R.  Make sure your I/O pin doesn't have 
pullups enabled.  



--- In AVR-Chat@yahoogroups.com, jay marante <jaythesis@y...> wrote:
> hi!
> i wanna trigger a low-level interrupt on interrupt 1 using a 
phototransistor. what i wanted is that when there is an insufficinet 
light on the phototransistor or when the light is blocked, interrupt 
1 will trigger on low-level. how will i design this? thanks for the 
help...

> -jay
> 
> 		
> ---------------------------------
> Do you Yahoo!?
> Win a $20,000 Career Makeover at Yahoo! HotJobs

--- In AVR-Chat@yahoogroups.com, "Phil" <phil1960us@y...> wrote:

> with a photo transistor, you simply
> pull it low and take the output 
> from the emitter.

I would think you'd need a comparator so that the input to the MCU 
was a proper logic level and not some analog signal that could spend 
a lot of time floating around between the thresholds.  Especially if 
you're expecting it to generate an interrupt.

Graham.

One of the really nice things about the AVR processor line is that EVERY I/O
line is a schmitt trigger input.

By adjusting the load resistor of the photo-transistor, you should be able
to drive the AVR input pin directly.  Read the data sheet to see what the
thresholds are.  They are pretty well defined so your design will work from
chip to chip.

Cheers!

-----Original Message-----
From: Graham Davies

--- In AVR-Chat@yahoogroups.com, "Phil" <phil1960us@y...> wrote:

> with a photo transistor, you simply
> pull it low and take the output
> from the emitter.

I would think you'd need a comparator so that the input to the MCU
was a proper logic level and not some analog signal that could spend
a lot of time floating around between the thresholds.  Especially if
you're expecting it to generate an interrupt.

On 7 May 2004 at 0:32, Graham Davies wrote:

> --- In AVR-Chat@yahoogroups.com, "Phil" <phil1960us@y...> wrote:
> 
> > with a photo transistor, you simply
> > pull it low and take the output 
> > from the emitter.
> 
> I would think you'd need a comparator so that the input to the MCU 
> was a proper logic level and not some analog signal that could spend 
> a lot of time floating around between the thresholds.  Especially if 
> you're expecting it to generate an interrupt.
> 
> Graham.
> 

	So use the AVR internal analog comparator. It can even 
generate a direct interrupt or a timer capture interrupt.

	Mark Jordan

Or use a schmitt trigger..

--- In AVR-Chat@yahoogroups.com, "Mark Jordan" <mark@c...> wrote:
> On 7 May 2004 at 0:32, Graham Davies wrote:
> 
> > --- In AVR-Chat@yahoogroups.com, "Phil" <phil1960us@y...> wrote:
> > 
> > > with a photo transistor, you simply
> > > pull it low and take the output 
> > > from the emitter.
> > 
> > I would think you'd need a comparator so that the input to the 
MCU 
> > was a proper logic level and not some analog signal that could 
spend 
> > a lot of time floating around between the thresholds.  Especially 
if 

> > you're expecting it to generate an interrupt.
> > 
> > Graham.
> > 
> 
> 	So use the AVR internal analog comparator. It can even 
> generate a direct interrupt or a timer capture interrupt.
> 
> 	Mark Jordan