How to Design Schematic Circuit of Light Comparator.

 This device can be used to compare two light sources by their intensities. Several optical experiments can be conducted using this. You can compare colors of surfaces by reflected light or compare colors using special filters.

The sensors used are Cds photoresistors or LDRs (Light Dependent Resistors) that have peak sensitivity in the red part of the spectrum (7350 angstroms or 735 nm).

As the name suggests, the LDR has a resistance which varies as light falls on its sensitive surface. As the light intensity increases, resistance falls. In total darkness resistance can be high as 1,000,000 ohms or more. illuminated by direct solar light, the resistance falls to 100 ohms or less.

Our circuit is a simple Wheatstone Bridge equilibrated by two LDRs and an adjustable potentiometer (Pl ).

When the two LDRs are receiving the same amount of light, P1 can be adjusted to get a null indication on the ammeter (M1).

lf we use one LDR as reference, we can compare another source of light with this by using the other LDR as sensor. A new adjustment of Pl can reveal the difference between the two light sources.

The schematic diagram of the light comparator is shown in circuit. No transistors are used in this project <nlv resistors.

Components layout, using a terminal strip as chassis, is shown in circuit. The terminal strip can be fixed in the plastic box using screws. Wires to the sensor can be as long as 6 feet without instability problems.

The LDRs should be mounted into two small opaque tubes to receive light from different sources without outside interference. Ml should be a zerocenter microammeter. A 50-0-50 uA microammeter would be suitable.

The circuit can be powered from two or four AA cells, and current drain is very low, extending battery life to many months.

Parts List - Light Comparator

LDR1, 2 - Common 1cm LDRs (Light Dependent Resistors)

             or Cds photosensors

M1 - 50-0-50 uA ammeter - see text

R1,2 - 1,000 ohm, 114W,5% resistor

R3 - 4,700 ohm, 114W,5% resistor

P1 - 10,000 ohm potentiometer

51 - SPST toggle or slide switch

81 - 3 to 6V - two or four M cells