RGB Colour sensor on breadboard using LDR

Thread Starter

Ethan Fraser

Joined Aug 7, 2018
4
Hi all,

Need a bit of help on a project I have. We're trying to create a colour detecting circuit that, using a 5V power supply and an LDR will turn on a red, yellow or green light, dependent on which colour is shining on the LDR. We have measured the resistance created by the LDR with the different lights but need to establish how to effectively pair that with a circuit similar to a dark sensor. We have the dark sensor circuit working effectively but I personally am having trouble on how to piggy back that in a way to filter the current based on the LDR resistance. Any and all help will be greatly appreciated.

These are the components we have at our disposal:
1. Light Emitting Diodes (LED)
2. Light Detecting Resistors (LDR)
3. Zener diodes
4. Resistors
5. Mechanical switches
6. Double Pole Single Throw (DPST) relay (switch)
7. Single Pole Single Throw (SPST) relay
8. NPN/PNP – Transistors
9. Breadboard

Thank you!
 

MrChips

Joined Oct 2, 2009
20,868
You cannot sense colour with a single LDR unless you use coloured filters.
One technique is to use three LDRs with red, green, and blue filters in order to measure the colour of incident light.

You can measure colour patches with one LDR or photosensor if you illuminate the patch with red, green, and blue LEDs in sequence. This measures reflected light off the patch instead of incident light directly from the LED source.
 

Thread Starter

Ethan Fraser

Joined Aug 7, 2018
4
You cannot sense colour with a single LDR unless you use coloured filters.
One technique is to use three LDRs with red, green, and blue filters in order to measure the colour of incident light.

You can measure colour patches with one LDR or photosensor if you illuminate the patch with red, green, and blue LEDs in sequence. This measures reflected light off the patch instead of incident light directly from the LED source.
The method we are using is a perspex tube, wrapped in black tape to block ambient light, then shining the different coloured lights down the perspex tube directly onto the LDR
 

ericgibbs

Joined Jan 29, 2010
9,794
hi E,
Are you trying detect the RGB by measuring the current thru your ammeter.?
If yes, what level of change do you measure.?
E
 

Thread Starter

Ethan Fraser

Joined Aug 7, 2018
4
How are you going to detect different colours?
hi E,
Are you trying detect the RGB by measuring the current thru your ammeter.?
If yes, what level of change do you measure.?
E
To detect colours we are using an ammeter to measure the current from the LDR. I'll try to find where we wrote the separate colours but the lowest reading was 14K (yellow) and 125K (full dark)
 

MrChips

Joined Oct 2, 2009
20,868
To detect colours we are using an ammeter to measure the current from the LDR. I'll try to find where we wrote the separate colours but the lowest reading was 14K (yellow) and 125K (full dark)
Your meter is showing inverse of luminous intensity, not colour.
 

DickCappels

Joined Aug 21, 2008
6,301
The only way this would work is if you are only detecting the effects of LEDs you had already measured at the same intensity that you have already measured.

In the case in which the output of the LED is unknown you must have some method of discriminating by color, and that usually means colored filters or some diffraction element such as gratings.
 
Top