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A radio-control (RC) servo can be added to the colour sensor unit for a bit of fun. Here, the servo arm is used as a pointer to indicate which sweet is under the sensor. The dial is made from a CD-ROM and the servo itself is simply connected to the servo driver PC board. PICAXE COLOUR Recognition System Use a PICAXE micro and a state-of-the-art optoelectronic IC to create this low-cost colour recognition system. By CLIVE SEAGER A LTHOUGH COLOUR recognition systems are not new, until recently they would have been far too complex and expensive to feature in a PICAXE project. Texas Advanced Optoelectronic Solutions (TAOS) from Plano, Texas, have changed all that with their new line of low-cost colour light-to-frequency (LTF) converter ICs. The TAOS TCS230 LTF converter was selected for this project because it integrates all of the functions neces70  Silicon Chip sary for colour sensing into a single miniature 8-pin package, including a digital output for easy interfacing to our PICAXE microcontroller. The project consists of two individual PC boards that plug together to form a complete colour recognition system. The first of these is the Colour Sensor Module, which includes the TCS230 sensor and a handful of support components. Revolution Education is supplying this board preassembled, as the TCS230 is only available in a tiny surface-mounted (SOIC) package that would be difficult to solder by hand. The second PC board is a PICAXE08M Servo Driver. Although primarily designed to control servos in robotic projects, this board is also suitable for use with the Colour Sensor Module. In this article, we’ll describe how to assemble the Servo Driver board, as well as how to connect it to the Colour Sensor Module to build a complete colour recognition system. Naturally, we’ll also show you how to program it to recognise colours! TAOS TCS230 colour sensor What makes the TCS230 sensor unique in the optoelectronic world is the integration of the light sensing, signal conditioning and analog-todigital conversion (ADC) functions in siliconchip.com.au Fig.1: the complete circuit diagram for the Colour Sensor Module. 33kW resistors set the default conditions (high or low) for the sensor’s input pins. Information on the function of the S0 & S1 inputs (here set for 100% relative frequency scaling using two 33kW pullup resistors) can be obtained from the TCS230 datasheet. A Mosfet (Q1) is used to switch the LEDs via the “L” signal on the connector. Fig.2: there’s even less to the Servo Driver circuit. As the sensor’s interface is digital, it is connected directly to the PICAXE port pins (via the 10-way header). A series diode (D1) reduces the 4-cell battery pack voltage to a safe level (5.4V nominal) to power both the Servo Driver and Colour Sensor boards. a single IC. The output from the sensor is a square wave of a frequency that is directly proportional to light intensity (irradiance). This can be connected directly to a microcontroller, thereby enabling extremely simple, cost-effective siliconchip.com.au light sensing solutions. So how can the TCS230 be used to differentiate one colour from another? Well, the sensor includes an array of 64 light sensors (photodiodes), organised in an array of 8 x 8. In all, 16 photo- diodes have blue filters, 16 have red filters, 16 have green filters and 16 are clear (no filters). All 16 photodiodes with the same filter colour are connected in parallel, with only one of the colours (red, green, blue or clear) June 2005  71 Fig.3: the TCS230 sensor is contained in a tiny 8-pin surfacemount package. The package is manufactured from a transparent material, allowing light to reach the photodiode array. can be gained simply by counting the number of pulses over a given sample period (eg, 50ms). This can be achieved with the PICAXE-08M’s count command. The process for measuring the RGB light intensity from a sample can be simplified as follows: (1). Select red filters (S2=0, S3=0) (2). Count pulses for sample period (result = red value) (3). Select blue filters (S2=0, S3=1) (4). Count pulses for sample period (result = blue value) (5). Select green filters (S2=1, S3=1) (6). Count pulses for sample period (result = green value) How accurate is it? enabled at any one time. Two digital control lines are provided so that external devices (such as our PICAXE) can select between the four arrays. By enabling each of the arrays (colours) in turn and measuring the proportional light intensity falling on the sensors, a good approximation of the red-green-blue (RGB) content of the light source can be established. As white light is composed of these three primary colours, it’s a relatively simple task for the microcontroller to differentiate any colour across the spectrum. Selection of each of the photodiode arrays is achieved via the S2 and S3 digital input pins, as defined in Table 1. The output of the sensor is a square wave with the frequency directly proportional to the light intensity. Therefore, a reliable indication of intensity Most colour sensors vary in accuracy across the RGB spectrum and the TAOS TCS230 is no exception. In theory, the three RGB sensors should record an equal value for pure white light but in practice, they don’t. This problem can be addressed by performing a white balance test. Its purpose is to calculate a scaling factor to apply to each colour to correct the error. In the case of a video camera, for example, this makes the playback of the colours more accurate on a television screen. However, with a simple microcontroller colour detection system, it isn’t necessary to scale the readings, as we won’t be reproducing the colours elsewhere. We are simply interested in a threshold point for each colour. Therefore, if we account for the imbalance within the thresholds set for each colour by experimentation, no white Fig.4: the Colour Sensor Module is supplied preassembled but we’ve provided the overlay diagram here for reference. All you have to do is fit the two LEDs (and their “posts”) as shown in the photo above right. Links J1-J4 are left open for this project. 72  Silicon Chip Table 1: Filter Selection S2 S3 Filter 0 0 Red 0 1 Blue 1 0 None 1 1 Green balance test is required. However, for more advanced applications, a white balance test can easily be added to the program if required. Note that background lighting conditions and distance from the sample will also make a small difference to the readings, so you may need to recalibrate the sensor when moving its position. Colour sensor module As well as the TCS230 sensor IC, the Colour Sensor Module also includes its own light source in the form of two white LEDs. These are angled at 45° to provide a point of light to illuminate the sample. The reflected light is then focused by a small lens (as typically used in CCD cameras) onto the TCS230 chip. The lens also filters out unwanted background infrared light. The full circuit diagram for this module is shown in Fig.1 and the board overlay in Fig.4. These are included mainly for reference, as apart from the two LEDs, the board is supplied preassembled. Instructions supplied with the kit show how to install the LEDs. The most important point to remember is that the cathode (K) side of a LED is identified by its shorter lead and a “flat” side on the housing, as indicated on the circuit and overlay diagrams. In addition, the LED must be threaded into the right-angle post so that it will be angled towards the centre of the board when installed in the PC board holes. When testing the assembly later, note that you should check to make sure that the two LEDs are correctly aligned. Ideally, their light output should merge to produce a single light dot at a focal length of about 30mm from the PC board. If you have a “figure-8” light pattern instead, try tweaking the angle and position of the LEDs slightly. As the LEDs consume considerable power in comparison to the other parts of the circuit, battery life can siliconchip.com.au Fig.5: follow this diagram when assembling the Servo Driver board. In particular, check that you have the 33mF capacitors, diode (D1) and microcontroller (IC1) around the right way. The 10-way socket mounts on the opposite side of the board to the other components. be maximised by switching the LEDs on only when a “scan” is to take place. PICAXE control of the sensor As presented here, control of the Colour Sensor Module requires only three outputs and one input of a PICAXE microcontroller. A suitable circuit could be constructed on a prototyping board but a better way is to use a PICAXE-08M Servo Driver board for the job. This board includes a 10-way dual-row header socket to mate with the header on the Colour Sensor Module. Fig.2 shows the circuit diagram for the Servo Driver board. Output 0 of the PICAXE-08M controls the LEDs on the Colour Sensor Module, whereas outputs 1 & 4 connect to the TCS230’s S2 & S3 inputs to select the desired photodiode array (colour). Input 3 is connected to the TCS230’s frequency output. On the '08M chip, this leaves only one output (output 2) available for other uses. Naturally, if you require more input or output pins for a project, then the circuit and program is easily ported to the PICAXE-18X or 28X. If desired you could also leave the white LEDs permanently on, freeing up a PICAXE pin for use elsewhere (eg, connected to a pushbutton switch to activate sensing). Note that this board requires 6V (4 x AA cells) instead of the more usual 4.5-5V supply. This higher voltage is needed because typical RC servos require at least 6V in order to generate useable amounts of torque. A series diode (D1) drops the rail down to about 5.4V to power the PICAXE micro and the Colour Sensor Module, Table 2: Sample RGB Values Sweet Red Value Green Value Blue Value blue green 0<w4<50 50<w6<150 200<w5<350 0<w4<50 200<w6<300 100<w5<200 red 50<w4<100 20<w6<80 20<w5<100 yellow 150<w4<250 230<w6<350s 80<w5<120 siliconchip.com.au June 2005  73 Program Listings Listing 1 '****************************************************** ' PICAXE-08M input/output pins symbol LED = 0 symbol S2 = 1 symbol ser = 2 symbol CSI = 3 symbol S3 = 4 'colour sensor white LEDs (output 0) 'colour sensor select S2 (output 1) 'servo or serial LCD (output 2) 'colour sensor pulse (input 3) 'colour sensor select S3 (output 4) '****************************************************** ' Variables (w4-w6 uses b8-b13!) symbol red_value = w4 symbol blue_value = w5 symbol green_value = w6 'colour sensor red content 'colour sensor blue content 'colour sensor green content '****************************************************** ' Scan and display every second main: gosub colour 'scan the colour sertxd ("Red =", 9, #red_value, 9) sertxd ("Blue =", 9, #blue_value, 9) sertxd ("Green =", 9, #green_value, CR, LF) pause 1000 goto main '****************************************************** ' Sub to scan colours colour: high LED low S2 low S3 count 3, 50, red_value high S3 count 3, 50, blue_value high S2 count 3, 50, green_value low LED return 'LED on 'read red into w4 'read blue into w5 'read green into w6 'LED off Listing 2 servo 2, new_pos pause 1000 goto main 'move the servo '*********************************************** ' Sub to scan colours colour: high LED low S2 low S3 count 3, 50, red_value high S3 count 3, 50, blue_value high S2 count 3, 50, green_value low LED return 'LED on 'read red into w4 'read blue into w5 'read green into w6 'LED off '*********************************************** ' Sub to evaluate colour and then set the servo position evaluate: new_pos = 190 'preload reject position ' Now identify correct colour using the threshold values if red_value > 150 and red_value < 250 then test_yellow if red_value > 50 and red_value < 100 then test_red if red_value < 50 then test_blue_or_green return test_blue_or_green: if blue_value > 200 and blue_value < 350 then test_blue if blue_value > 100 and blue_value < 200 then test_green return test_blue: if green_value > 50 and green_value < 150 then is_blue return is_blue: new_pos = 170 return main: gosub colour 'scan the colour serout 2,N2400,(254,128,"R=",#red_value, " ") serout 2,N2400,(254,136,"B=",#blue_value, " ") serout 2,N2400,(254,192,"G=",#green_value, " ") pause 1000 goto main test_green: if green_value > 200 and green_value < 300 then is_green return Listing 3 test_red: if blue_value > 20 and blue_value < 100 then test_r2 return '****************************************************** ' PICAXE-08M input/output pins symbol LED = 0 symbol S2 = 1 symbol ser = 2 symbol CSI = 3 symbol S3 = 4 'colour sensor white LEDs (output 0) 'colour sensor select S2 (output 1) 'servo (output 2) 'colour sensor pulse (input 3) 'colour sensor select S3 (output 4) '****************************************************** ' Variables (w4-w6 uses b8-b13!) symbol new_pos = b1 symbol red_value = w4 symbol blue_value = w5 symbol green_value = w6 'new servo position 'colour sensor red content 'colour sensor blue content 'colour sensor green content '****************************************************** main: gosub colour 'scan the colour gosub evaluate 'set the servo position 74  Silicon Chip is_green: new_pos = 90 return test_r2: if green_value > 20 and green_value < 80 then is_red return is_red: new_pos = 145 return test_yellow: if blue_value > 80 and blue_value < 120 then test_y2 return test_y2: if green_value > 230 and green_value < 350 then is_yellow return is_yellow: new_pos = 120 return siliconchip.com.au Fig.6: the output from the test program, as it appears in the Programming Editor’s serial terminal window. which have a maximum input voltage of 5.5V. Assembling the servo driver Assembly of the PC board is very straightforward. Install the low-profile components first, starting with the resistors. When installing the two 33mF tantalum capacitors, make sure that you have their positive (+) leads oriented as shown (see Fig.5). Also, take care that you have the PICAXE08M (IC1) around the right way; the notched (pin 1) end must be next to the programming socket. Leave the 10-way socket until last. It must be mounted on the opposite side of the board to all the other components (see photos) so that it can mate with the corresponding header on the colour sensor module. Once assembly is complete, fit 4 x 12mm threaded spacers in each corner mounting hole using the supplied M3 x 6mm screws. The colour sensor module can now be plugged into the servo driver’s socket and a further 4 x 30mm spacers fitted as legs to support the whole assembly. You should end up with a “tower”, as shown in the lead photograph. Testing To check that your completed unit is working properly, a simple program can be run to “learn” the reflected light properties of various samples. We’ve used some small sweets as samples but you can use what ever you have on hand. Note that you may need to adjust the height of the tower to compensate for the height of your samples. The test program is shown in Listing 1. You can type this into the Programsiliconchip.com.au ming Editor directly or download it from the SILICON CHIP website at www. siliconchip.com.au. Download and run the test program in the PICAXE-08M and then go to the terminal menu (via PICAXE -> Terminal from the toolbar) and choose a baud rate of 4800. The terminal window will then display the RGB data being output via the sertxd command. If you wish to perform the testing away from your computer, you can use a serial LCD module (Part No. AXE033) connected to output 2. Listing 2 shows the “main” section of the program altered to support a serial LCD module. Par t s Lis t For AXE024 Servo Driver Colour identification Capacitors 2 33mF 16V tantalum 1 100nF MKT polyester Table 2 shows a list of values for each of the sweet colours that were determined by experimentation. Note that we’ve used very broad thresholds to allow for the variations seen with even slight movements of the sweets from measurement to measurement. Regardless of the broadness of our figures, the important point is that each colour of sweet can be uniquely identified from these values. The third program adds a radiocontrol type servo for a bit of fun. The servo arm is used as a pointer to indicate which sweet is under the sensor. A simple dial is made from a blank CD-ROM, which is then placed under the servo arm. The servo itself is simply connected to output 2 of the PICAXE chip. This time, the program is a little more involved as it has to determine which sweet is which colour using mathematical comparisons. This is achieved by testing the threshold values for each of the RGB values in Table 2. Summary The TCS230 light-to-frequency 1 Servo Driver PC board 1 3.5mm stereo socket 1 battery clip 1 4 x AA battery holder 1 8-pin IC socket 1 10-way 2.54mm (0.1-inch) SIL header (separate into 3 x 3-way headers) Semiconductors 1 PICAXE-08M microcontroller (IC1) 1 1N4001 diode (D1) Resistors (0.25W 5%) 1 22kW 1 10kW 3 330W Also required (not in kit) PICAXE Programming Editor software (v4.1.0 or later) PICAXE download cable (Part No. AXE026) 4 x AA alkaline cells 1 Colour Sensor Module (Part No. AXE045) 1 10-way 2.54mm (0.1”) pitch DIL header socket Mounting hardware (standoffs, screws) converter is an economical solution for many colour-sensing projects. It is easily interfaced to a PICAXE microcontroller and is a versatile sensor that can be incorporated into many mechatronic and robotic applications. More information on TAOS sensors can be obtained from their website at SC www.taosinc.com. Obtaining Kits & Software The design copyright for this project is owned by Revolution Education Ltd. The Colour Sensor Module can be purchased individually (Part No. AXE045) or as part of a kit (Part No. AXE112S). The kit includes a PICAXE Servo Driver kit (Part No. AXE024), a 10-way header socket and the mounting hardware necessary to allow the boards to be stacked together. All items are available from authorised PICAXE distributors – see www.microzed.com.au or phone Microzed on (02) 6772 2777. The PICAXE Programming editor software can be downloaded free of charge from www.picaxe.co.uk or ordered on CD (Part No. BAS805). June 2005  75