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Colour
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 it is.
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.
2. Common ends of the LDRs should be connected to positive supply.
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 light conditions
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