In a message dated 15/08/02, gjharris@............. writes:

> A very sensitive and inexpensive two axis sensor can be made as follows:
> 
> Above the top surface of the pendulum, place an LED with the front surface 
> 

> On the top surface cement a 6mm square mirror (stock H43866 from Edmund).
> On each of the four sides of the LED place small solar cells (stock 980-0150
> from allied) in such a position that each will be half illuminated by the 
> 

Hello George Harris,

      Nice to have some extra input! I am having a problem in visualising the 
optical layout you suggest and the orientation and position of the various 
components. Is the LED 'water clear' plastic or what, please? What size? Are 
they IR or visible?  
       When you cut the end off a plastic LED and polish the end, looking 
into it, you see a bright central square chip surrounded by a ring of light 
from the plastic case. Putting a mirror on the end just reflects most of the 
light back through the base.
       Is that any chance of a quick sketch / 'paint' drawing, please?

       Edmund optics seem to be at http://www.edmundoptics.com/  There is no 
response to H43866, but there is a 10 mm square surface mirror NT45-517 
@.......
  
> The photocells can be connected in parallel, plus to minus, on the opposite 
> sides across the inputs to a low noise operational amplifier with about a 
> megohm in the feedback.  The resulting output is very sensitive 
> 

      The photo output of LEDs varies exponentially with temperature and you 
get about a factor of five reduction in light output at a constant current if 
you increase the temperature from 0 C to 100 C. Since you can expect ambient 
changes of >10 C deg and the LEDs themselves can heat up considerably, the 
variations are significant. How do you stabilise the photo output please?

    Regards,

      Chris Chapman
In a message dated 15/08/02, gjharris@............. writes:



A very
 sensitive and inexpensive two axis sensor can be made as follows:



Above the top surface of the pendulum, place an LED with the front surface 

flattened so that it become a point source about 1/4 inch above the surface.



On the
 top surface cement a 6mm square mirror (stock H43866 from Edmund).

On each of the four sides of the LED place small solar cells (stock 980-0150

from allied) in such a position that each will be half illuminated by the LED.



Hello George Harris,



      Nice to have some extra input! I am having a problem in visualising the optical layout you s
uggest and the orientation and position of the various components. Is the LED 'water clear' plastic or what, please? What size?
 Are they IR or visible?  

       When you cut the end off a plastic LED and polish the end, looking into it, you see a 
bright central square chip surrounded by a ring of light from the plastic case. Putting a mirror on the end just reflects most 
of the light back through the base.

       Is that any chance of a quick sketch / 'paint' drawing, please?



       Edmund optics seem to be at http://www.edmundoptics.com/  There is no response to
 H43866, but there is a 10 mm square surface mirror NT45-517 @.......

  

The ph
otocells can be connected in parallel, plus to minus, on the opposite sides across the inputs to a low noise operational amplif
ier with about a megohm in the feedback.  The resulting output is very sensitive (nanometers) and linear.



      The photo output of LEDs varies exponentially with temperature and you get about a factor of
 five reduction in light output at a constant current if you increase the temperature from 0 C to 100 C. Since you can expect a
mbient changes of >10 C deg and the LEDs themselves can heat up considerably, the variations are significant. How do you sta
bilise the photo output please?



    Regards,



      Chris Chapman