LDR Properties

[Martin Czech]

Old databooks are so usefull:

An old Valvo Optoelectronic book came into my hands and
voila there are the answers to my questions:

-most LDR are based on CdS, this means Tmax=70C

-maximum light sensitivity of CdS at 530 nm (blue-green)
 and about 640 nm (orange-red). High performance red leds
 thus are suitable.

-the first order formula for resistance: R2/R1=(E1/E2)^gamma
 and gamma=(log R1- log R2)/(log E2-log E1)
 where E is the light intensity.
 LDRs come with different flavours of gamma.

-temporal behaviour:
 an example for "light-on" shows 
 1ms delay for 1e7 to 1e2 Ohm (5000 lux), it then takes 1s down
 to 20 Ohm
 10ms for 1e7 to 1e3 Ohm (500 lux) , 90ms later it settles at 100 Ohm
 100ms for 10e8 to 1e3 Ohm (50 lux), 900ms later settling to 400 Ohm

 "light-off":
 from 20 Ohm to 1e4 (5000 lux) 100ms, to 1e6 1.5s !
 from 100 Ohm to 1e5 (500 lux) 400ms, to 1e6 1.5s
 from 400 Ohm to 1e5 (50 lux) ~400ms, to 1e6 1.5s

 Time delay for on is much shorter then for off, but 
 the time depends on light intensity, it can be quite fast.
 It takes almost the same time from any "on-state" to 
 "really-off" (1e6 Ohm). 
 
-the light intensity of an led is linear dependend on the 
 forward current, with very good approximation.


The nonlinear time behaviour makes it questionable, if
for example the ldr makes a triangle-sine shaper unneccesary,
or even a quare -tri shaper.

m.c.

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m.c. (alias mad scientist) has made it finally: 3 CDs à 72 minimum ready.
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