11

FN1050.6

March 4, 2005

Differential Light Detector

In the circuit shown in Figure 11, the CA3240E converts the

current from two photo diodes to voltage, and applies 1V of

reverse bias to the diodes. The voltages from the CA3240E

outputs are subtracted in the second stage (CA3140) so that

only the difference is amplified. In this manner, the circuit

can be used over a wide range of ambient light conditions

without circuit component adjustment. Also, when used with

a light source, the circuit will not be sensitive to changes in

light level as the source ages.

FIGURE 11. DIFFERENTIAL LIGHT DETECTOR

3

2

+15V

6

2

3

7

+15V

CA3140

+

6

5

4

8

4

1

7

-15V

-15V

1/2

CA3240E

+

-

1/2

CA3240E

+

0.015

µF

100K

0.015

µF

100K

+15V

5.1K

1.3

13K

K

C30809

PHOTO

DIODE

C30809

PHOTO

DIODE

-

2K

200K

2K

OUTPUT

-

200k

CA3240, CA3240A