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    Color sensor

    I built this color sensor to equip my third implementaton of brick sorter. Michael Gasperi presents such a sensor in an article of Nuts and Volts magazine. His circuit is based on an input multiplexor. Three CdS photo-resistors, connected to this multiplexor, receive light through RGB colored filters (cleverly done with Lego transparent bricks).

    This one takes a somewhat opposite route: a multiplexor (similar to Michael's) drives three RGB leds that cast light onto the object. Reflected light is measured by a single photo-resistor. Another (non-Lego) home built color sensor, based on the same idea, can be found in David Cook's robotics pages.

     

    Circuit analysis
    The heart of this circuit is a CD4017 (IC1), CMOS counter with decoded output. On each pulse received on its clock input, this IC drives high the next output (all the other are driven low).

    Output 0, 1 and 2 drives red, green and blue LEDs through transistors Q2, Q3, Q4 (emitter follower is used to avoid the need of a base resistor). During the first three phases, output 4 is driven low and provide ground reference for photo-resistor R8: the RCX can read light level reflected in each color. In phase 4 photo-resistor is no longer grounded, value returned to RCX is maximum (note the 1 Mohm resistor paralleled with R8 to prevent maximum value to be reached in phases 1/2/3). RCX can detect this special condition and synchronize its readings to CD4017 state.

    The oscilloscope hard-copy shows clock pulse on top trace, output 1/2/3 below.

    To generate a clock pulse, you have to switch from active sensor mode (sensor voltage is higher than Zener D4 threshold, Q1 is conducting, clock input is low) to passive sensor mode (sensor voltage is lower than D4 threshold, Q1 is blocked, clock input is high) and back to active sensor. C1 filters out short power supply pulses occuring in active mode evey 3ms while RCX reads value.

    The oscilloscope hard-copy shows clock pulse on top trace, sensor voltage on bottom trace showing 2 short reading pulses and wide command pulse generated by sensor mode switching.

    When reaching step 5, CD4017 activates D04 and reset itself to step1 through R4/C3. This RC network delays reset pulse well after clock transition: without it, the current rush of the LED, coupled through power supply, modifies clock threshold and CD4017 goes directly to step 2...

    Power supply is rectified by D1 and filtered by C2. As in my other sensors, I keep on with single diode instead of full bridge to minimize components count (3 diodes instead of 8!) but as you can see I built the breadboard with a full bridge module I constructed to ease my prototypes.
     


    The power-supply module I use for breadboards.


    The breadboard 



    Component selection

    Nothing is really critical: D1 to D3 are small signal diodes (i.e.  1N4148), Q1 to Q4 are general purpose NPN transistors (BC548, 2N2222...). Photo-resistor I used is a MPY54C569 (any similar size LDR will probably work fine). Leds are high efficiency 3mm ones. I used Agilent: red HLMP-K105 for D5, blue HLMP-KB45 for D8, green HLMP-1540 for D6 and D7. I was somewhat disappointed by green Leds efficiency. Though the photoresistor has its peak sensitivity around green wavelength, read value was fairly low with green lighting. After testing different Led types and brands, I finally chose to use two green leds with maximum current. Even then color sensor sometimes has troubles to distinguish between green and black (!) Lego bricks... Green bricks reflects surprisingly very little light, I guess the problem is that my green leds have a too yellowish wavelength. Most of the components were bought from Farnell.

    Led current is limited by R5, R6 and R7. To set their values, I placed the breadboard in front of a white piece of paper and adjusted them to get about the same reading for all colors.

    Building color sensor: photo gallery


    I used two right-angle mounted perf. boards


    Everything fits in a block composed of
    four 4 x 2 hollowed out bricks


    The main board receives power supply, CD4017 and clock
    generation. It uses standard through-holes components.


    The other one receives Leds and photoresistor on one side,
    transistors and resistors are SMD mounted
    on the other side to keep size down. 
    Photoresistor is shielded from Leds direct light by heat-shrink tubing.
    (I used white tubing for leds hoping to get a few more photons...)
    Leds are slanted as much as possible towards the center .

     

    Test programs

    test programs (requires RCX 2.0 firmware that you can get here)

    Real applications

    • Brick Sorter 3: see it here !
    • Color follower: when I was a Mindstorms newbie, I tried the roverbot-and-light-sensor line follower on my tiled floor. Unfortunately there is not enough contrast between tiles to be able to get a reliable working. With the color sensor, the difference between blue and red channels enables to tell blue from grey tiles and is relatively independent of ambient light level. The robot must be slow though because of the time it takes to cycle through color sensor states.

      Get NQC program for the color follower.



    Color follower on tiled floor



    QuickTime movie of the color follower (200K)

     

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