The circuit shown in Figure 1 is a two-scale light comparator using two LEDs as indicators.
By adjusting the potentiometer to erase the two LEDs, we have in its scale the point that gives us the intensity of the light that is focusing on the LDR at that moment.
The scale can be graded based on a common photometer.
The scale that places R2 in the circuit is the most sensitive, to work with very small intensities of light.
The assembly on a printed circuit board is shown in Figure 2.
The resistors are 1/8 W with any tolerance and the common LEDs, red or other color.
The LDR is round type and for greater sensitivity and directivity it must be mounted on an opaque tube with a converging lens.
Material list
CI-1 - 741 - operational amplifier
LED1, LED2 - Common LEDs
LDR - round common LDR
P1 - 1 M ohms - potentiometer
S1 - Single switch
S2 – Push button
S3 - 1-pole x 2 positions switch
B1 - 9 V - battery
R1 - 10 k ohms - resistor - brown, black, orange
R2 - 100 k ohms - resistor - brown, black, yellow
R3 - 1 M ohms - resistor - brown, black, green
R4, R5 - 1k ohms - resistors - brown, black, red
C1 - 100 uF x 12 V - electrolytic capacitor
Several:
Printed circuit board, mounting box, battery connector, wires, solder, etc.
