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sensor is an interesting project for hobbyists. The cir- cuit can
sense eight colours, i.e. blue, green and red (primary colours);
magenta, yellow and cyan (secondary colours); and black and white.
The circuit is based on the fundamentals of optics and digital
electronics. The object whose colour is required to be detected
should be placed in front of the system. The light rays reflected
from the object will fall on the three convex lenses which are fixed
in front of the three LDRs. The convex lenses are used to converge
light rays. This helps to increase the sensitivity of LDRs. Blue,
green and red glass plates (filters) are fixed in front of LDR1,
LDR2 and LDR3 respectively. When reflected light rays from the
object fall on the gadget, the coloured filter glass plates
determine which of the LDRs would get triggered. The circuit makes
use of only ‘AND’ gates and ‘NOT’ gates.
When a primary coloured
light ray falls on the system, the glass plate corresponding to that
primary colour will allow that specific light to pass through. But
the other two glass plates will not allow any light to pass through.
Thus only one LDR will get triggered and the gate output
corresponding to that LDR will become logic 1 to indicate which
colour it is. Similarly, when a secondary coloured light ray falls
on the system, the two primary glass plates corres- ponding to the
mixed colour will allow that light to pass through while the
remaining one will not allow any light ray to pass through it. As a
result two of the LDRs get triggered and the gate output
corresponding to these will become logic 1 and indicate which colour
When all the LDRs get triggered or remain untriggered, you
will observe white and black light indications respectively.
Following points may be carefully noted :
1. Potmeters VR1, VR2
and VR3 may be used to adjust the sensitivity of the LDRs.
Common ends of the LDRs should be connected to positive
3. Use good quality light filters.
The LDR is mounded
in a tube, behind a lens, and aimed at the object. The coloured
glass filter should be fixed in front of the LDR as shown in the
figure. Make three of that kind and fix them in a suitable case.
Adjustments are critical and the gadget performance would depend
upon its proper fabrication and use of correct filters as well as
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