Need a little design help on my latest project
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From: MooseFET on 1 Mar 2010 22:03 On Mar 1, 8:54 am, Charlie E. <edmond...(a)ieee.org> wrote: > Hi Guys, > Ok, going to put myself in harms way, and ask for a little design > advice. I have been working on this project for a while now, and it > has gone through several iterations, and I keep having the same > problem! > > First, the project: I am designing a small color reader for the > visually impaired. Really simple operation - put it against the > object you want to tell the color of, and it will say "RED" or > whatever the color is. Sensor is simply an RGB LED and a > phototransistor, buffered by some amps, and then digitized by a PIC24. > So, what's the problem? I can't get a stable reading. In normal > operation, this thing will run for about two seconds, and then be > turned off. To test, however, I run it in debug mode for hours. When > I first turn it on, and calibrate it to a white sample, I will get one > set of calibrations. Let it sit for about two minutes, and it starts > to drift. In about half an hour, I will have readings totally off the > scale. > > So, why am I baring my soul to ya'll? I need your help identifying > where the gain drift is coming from, and some ideas on how to control > them. I have the schematic here: > > http://edmondsonengineering.com/Documents/Rainbow%20color%20Reader%20... > > Basic description - MCU turns on an LED. The phototransistor is first > buffered by a non-inverting opamp with a gain of 2, and the signal is > split. Part goes directly to a PGA where it is first attenuated, and > then the PGA boosts it up. This gives me a calibration control to > deal with difference in output of the LEDs. The original and PGA > signal are added, and this is then applied to another non-inverting > opamp with a gain of 2. I also have one feed before this opamp to an > ADC input on the PIC. > > Problems I have already solved: > First, each LED has a different output level. Red needs a gain of > around 2, BLUE a gain of around 5, and GREEN a gain of about 7. The > PGA was added to give me an adjustable gain from around 2 to 14, with > the two different taps into the separate input channels of the PGA. > This gives me 16 different gain levels to play with. Using the tap to > the second ADC channel, it actually gives me 32 different levels. > > Right now, RED uses this lower gain channel, and is steady as a rock. > Part of this may be that my VCC is 3.3 volts, and only RED has a > forward voltage below this. Both GREEN and BLUE have forward voltages > of 3.4 volts. > > So, potential problems? could it be that repeated use warms up the > GREEN and BLUE LEDs so that they become more efficient? Could the > power supply drift higher as it warms up? Could the opamps drift with > slight changes in temperature? Any advice ya'll can give will be most > appreciated. > > Thanks, > Charlie How about: Schottky LED Vcc ---->!--------+-----/\/\---->!----GND ! ===C ! PIC port When you pull the port pin low, the capacitor gets charged. When you take it high again you get a pulse of light from the LED. From the shape of the pulse, you can get a reasonable guess at the forward voltage of the LED. Thus you can normalize for that drop.
From: news on 2 Mar 2010 03:56 On Mon, 01 Mar 2010 10:55:36 -0800, Tim Wescott <tim(a)seemywebsite.now> said: > Charlie E. wrote: > > On Mon, 01 Mar 2010 09:57:43 -0800, Joerg <invalid(a)invalid.invalid> > > wrote: > > > >> Joerg wrote: > >>> Charlie E. wrote: > >>>> On Mon, 01 Mar 2010 09:32:56 -0800, Joerg <invalid(a)invalid.invalid> > >>>> wrote: > >>>> > >>>>> Charlie E. wrote: > >>>>>> Hi Guys, > >>>>>> Ok, going to put myself in harms way, and ask for a little design > >>>>>> advice. I have been working on this project for a while now, and it > >>>>>> has gone through several iterations, and I keep having the same > >>>>>> problem! > >>>>>> > >>>>>> First, the project: I am designing a small color reader for the > >>>>>> visually impaired. Really simple operation - put it against the > >>>>>> object you want to tell the color of, and it will say "RED" or > >>>>>> whatever the color is. Sensor is simply an RGB LED and a > >>>>>> phototransistor, buffered by some amps, and then digitized by a PIC24. > >>>>>> So, what's the problem? I can't get a stable reading. In normal > >>>>>> operation, this thing will run for about two seconds, and then be > >>>>>> turned off. To test, however, I run it in debug mode for hours. When > >>>>>> I first turn it on, and calibrate it to a white sample, I will get one > >>>>>> set of calibrations. Let it sit for about two minutes, and it starts > >>>>>> to drift. In about half an hour, I will have readings totally off the > >>>>>> scale. > >>>>>> > >>>>>> So, why am I baring my soul to ya'll? I need your help identifying > >>>>>> where the gain drift is coming from, and some ideas on how to control > >>>>>> them. I have the schematic here: > >>>>>> > >>>>>> http://edmondsonengineering.com/Documents/Rainbow%20color%20Reader%20Schematic.pdf > >>>>>> > >>>>>> > >>>>>> Basic description - MCU turns on an LED. The phototransistor is first > >>>>>> buffered by a non-inverting opamp with a gain of 2, and the signal is > >>>>>> split. Part goes directly to a PGA where it is first attenuated, and > >>>>>> then the PGA boosts it up. This gives me a calibration control to > >>>>>> deal with difference in output of the LEDs. The original and PGA > >>>>>> signal are added, and this is then applied to another non-inverting > >>>>>> opamp with a gain of 2. I also have one feed before this opamp to an > >>>>>> ADC input on the PIC. > >>>>>> > >>>>>> Problems I have already solved: > >>>>>> First, each LED has a different output level. Red needs a gain of > >>>>>> around 2, BLUE a gain of around 5, and GREEN a gain of about 7. The > >>>>>> PGA was added to give me an adjustable gain from around 2 to 14, with > >>>>>> the two different taps into the separate input channels of the PGA. > >>>>>> This gives me 16 different gain levels to play with. Using the tap to > >>>>>> the second ADC channel, it actually gives me 32 different levels. > >>>>>> > >>>>>> Right now, RED uses this lower gain channel, and is steady as a rock. > >>>>>> Part of this may be that my VCC is 3.3 volts, and only RED has a > >>>>>> forward voltage below this. Both GREEN and BLUE have forward voltages > >>>>>> of 3.4 volts. > >>>>>> > >>>>>> So, potential problems? could it be that repeated use warms up the > >>>>>> GREEN and BLUE LEDs so that they become more efficient? Could the > >>>>>> power supply drift higher as it warms up? Could the opamps drift with > >>>>>> slight changes in temperature? Any advice ya'll can give will be most > >>>>>> appreciated. > >>>>>> > >>>>> As Jim hinted, the first order of business would be to regulate the > >>>>> current that goes through each LED. A resistor that drops only very > >>>>> little voltage isn't going to cut it. > >>>> So, my first stupid mistake was using a VCC of 3.3 volts with LEDs > >>>> that required 3.4! Not an easy problem to fix. > >>>> > >>>> Makes me really wish that the PSOC1s had built in debug capability. > >>>> They could run at 5 VDC... > >>>> > >>> It can be fixed. You'd need three step-up (boost) converters and a small > >>> resistor in front to each and then regulate. If it has to be cheap you > >>> could try to regulate with the uC. Alternatively one step-up and three > >>> linear ones but LDOs are often rather buggy and if you boost to 6V or > >>> more for a proper linear regulation you'll waste a lot of power. > >>> > >>> Yet another option: Supply the PIC with 3.3V or whetever it needs and > >>> the LED with more. But even with several volts of headroom a resistor is > >>> not a constant current source. However, you can make a uC-controllable > >>> current source with two transistors and two resistors per LED, no big > >>> deal. AoE figure 2.25, just flipped around using NPNs and R1 goes to a > >>> PIC port pin. > >>> > >>> This will improve things massively. If that's still not low-drift enough > >>> you can use an opamp current source. > >>> > >> P.S.: If the cathodes are tied together the last idea only works if you > >> leave the current source up against positive, like in the AoE example. > >> Then drive either with a 5V uC or a logic level FET and add a resistor > >>from base to positive supply. > > > > Joerg, > > Thanks for the advice. Yes, I had been concentrating so hard on the > > amplifiers I never really considered the LEDs. In my mind, they would > > just 'work' and I could then adjust accordingly. Didn't realize that > > they would vary that much. Will have to look at maybe adding a higher > > voltage, and go with the constant current drives for them. This does > > need to be pretty accurate! > > Consider using a photodiode instead of a phototransistor -- it _should_ > be more linear and predictable. Yep. Phototransistors are horrible for this sort of application. Linearity, unit-to-unit repeatability, temperature drift will all ruin your measurement. A photodiode with transimpedance amplifier should do what you want. If you have a large dynamic range to cover, consider an integrating amp - this allows you to control the overall gain by varying the integration time, which should save a few parts. HTH R.
From: Robert Baer on 2 Mar 2010 06:22 Charlie E. wrote: > Hi Guys, > Ok, going to put myself in harms way, and ask for a little design > advice. I have been working on this project for a while now, and it > has gone through several iterations, and I keep having the same > problem! > > First, the project: I am designing a small color reader for the > visually impaired. Really simple operation - put it against the > object you want to tell the color of, and it will say "RED" or > whatever the color is. Sensor is simply an RGB LED and a > phototransistor, buffered by some amps, and then digitized by a PIC24. > So, what's the problem? I can't get a stable reading. In normal > operation, this thing will run for about two seconds, and then be > turned off. To test, however, I run it in debug mode for hours. When > I first turn it on, and calibrate it to a white sample, I will get one > set of calibrations. Let it sit for about two minutes, and it starts > to drift. In about half an hour, I will have readings totally off the > scale. > > So, why am I baring my soul to ya'll? I need your help identifying > where the gain drift is coming from, and some ideas on how to control > them. I have the schematic here: > > http://edmondsonengineering.com/Documents/Rainbow%20color%20Reader%20Schematic.pdf > > Basic description - MCU turns on an LED. The phototransistor is first > buffered by a non-inverting opamp with a gain of 2, and the signal is > split. Part goes directly to a PGA where it is first attenuated, and > then the PGA boosts it up. This gives me a calibration control to > deal with difference in output of the LEDs. The original and PGA > signal are added, and this is then applied to another non-inverting > opamp with a gain of 2. I also have one feed before this opamp to an > ADC input on the PIC. > > Problems I have already solved: > First, each LED has a different output level. Red needs a gain of > around 2, BLUE a gain of around 5, and GREEN a gain of about 7. The > PGA was added to give me an adjustable gain from around 2 to 14, with > the two different taps into the separate input channels of the PGA. > This gives me 16 different gain levels to play with. Using the tap to > the second ADC channel, it actually gives me 32 different levels. > > Right now, RED uses this lower gain channel, and is steady as a rock. > Part of this may be that my VCC is 3.3 volts, and only RED has a > forward voltage below this. Both GREEN and BLUE have forward voltages > of 3.4 volts. > > So, potential problems? could it be that repeated use warms up the > GREEN and BLUE LEDs so that they become more efficient? Could the > power supply drift higher as it warms up? Could the opamps drift with > slight changes in temperature? Any advice ya'll can give will be most > appreciated. > > Thanks, > Charlie Wild guesses: 1) the photodetector is temperature sensitive, #2: ambient light is driving it (big variable). Not guess: LED output color and intensity vary with temperature, and at any reasonable drive for "bright" they warm up. Heatsink them as much as possible - 1 to 10W heatsinking on each lead...
From: Jan Panteltje on 2 Mar 2010 07:52 On a sunny day (Mon, 01 Mar 2010 16:28:59 -0800) it happened Charlie E. <edmondson(a)ieee.org> wrote in <8rmoo5h0f7i9s6c07el0t5iqumslahu7g1(a)4ax.com>: >Hi Jan, >As you read the rest of the thread, everyone is pretty convinced the >problem is improper LED drive. At present, the three different LEDs >all have different outputs, making it necessary to have a variable >input amp to get the voltages to the right levels. I did some experiments some time ago with R, G, and B LEDs to make white: ftp://panteltje.com/pub/rgb_leds_series_white_img_0837.jpg Using the same current (here in series) gives too much blue. If you look at the color TV system, and I dunno your background, but let's do some math, they 2 make color-difference signals, those signals chosen so they are zero in a BW picture. Then those 2 signals are used to modulate 2 90 degrees shifted carriers. The resulting vector has a amplitude corresponding to the color *saturation*, and an angle corresponding to the color. You have to use these 2 if you uniquely want to point to a color. It goes something like this (and assume your LEDs have similar efficiency for a moment, but of course they do not, but as a starting point: For the human eye sensitivity 'white' 'Y' can be composed of: y = (.3 * red) + (.59 * green) + (.11 * blue) So for less blue is needed, a bit of red, and mostly green. You can then make 2 signals, 'r - y' and 'b - y', Those can be both positive and negative. Now use them like this: + r - y | |/ v - -- -- b - y + | | - Now calculate a vector v, its phase will be the color, and it's amplitude the saturation. Now you can compare one number only, the angle, to a table of colors. And that is independent of any saturation variations. 'Saturated yellow' versus 'yellow'. It is called the color circle', what a TV vectorscope would show. ftp://panteltje.com/pub/vectorscope.gif In the circle you see some colors marked: red, green, blue, manganese, cyan, yellow, each color has its own angle. That is how I would probably go about it. I would not use this idea with the LEDs emitting, as it cannot measure the color of *emitted* light. One of of the things measuring the color of emitted light would be useful for is to adjust monitors. This subject was discussed here a while ago, and somebody recommended a chip, IIRC the TCS230D, and pointed to a project, lost the link, but googling for TCS230 finds many: http://www.flickr.com/photos/arms22/3241371664/ Hope any of this helps :-)
From: dagmargoodboat on 2 Mar 2010 12:06
On Mar 1, 10:03 pm, MooseFET <kensm...(a)rahul.net> wrote: > On Mar 1, 8:54 am, Charlie E. <edmond...(a)ieee.org> wrote: > > > > > Hi Guys, > > Ok, going to put myself in harms way, and ask for a little design > > advice. I have been working on this project for a while now, and it > > has gone through several iterations, and I keep having the same > > problem! > > > First, the project: I am designing a small color reader for the > > visually impaired. Really simple operation - put it against the > > object you want to tell the color of, and it will say "RED" or > > whatever the color is. Sensor is simply an RGB LED and a > > phototransistor, buffered by some amps, and then digitized by a PIC24. > > So, what's the problem? I can't get a stable reading. In normal > > operation, this thing will run for about two seconds, and then be > > turned off. To test, however, I run it in debug mode for hours. When > > I first turn it on, and calibrate it to a white sample, I will get one > > set of calibrations. Let it sit for about two minutes, and it starts > > to drift. In about half an hour, I will have readings totally off the > > scale. > > > So, why am I baring my soul to ya'll? I need your help identifying > > where the gain drift is coming from, and some ideas on how to control > > them. I have the schematic here: > > >http://edmondsonengineering.com/Documents/Rainbow%20color%20Reader%20... > > > Basic description - MCU turns on an LED. The phototransistor is first > > buffered by a non-inverting opamp with a gain of 2, and the signal is > > split. Part goes directly to a PGA where it is first attenuated, and > > then the PGA boosts it up. This gives me a calibration control to > > deal with difference in output of the LEDs. The original and PGA > > signal are added, and this is then applied to another non-inverting > > opamp with a gain of 2. I also have one feed before this opamp to an > > ADC input on the PIC. > > > Problems I have already solved: > > First, each LED has a different output level. Red needs a gain of > > around 2, BLUE a gain of around 5, and GREEN a gain of about 7. The > > PGA was added to give me an adjustable gain from around 2 to 14, with > > the two different taps into the separate input channels of the PGA. > > This gives me 16 different gain levels to play with. Using the tap to > > the second ADC channel, it actually gives me 32 different levels. > > > Right now, RED uses this lower gain channel, and is steady as a rock. > > Part of this may be that my VCC is 3.3 volts, and only RED has a > > forward voltage below this. Both GREEN and BLUE have forward voltages > > of 3.4 volts. > > > So, potential problems? could it be that repeated use warms up the > > GREEN and BLUE LEDs so that they become more efficient? Could the > > power supply drift higher as it warms up? Could the opamps drift with > > slight changes in temperature? Any advice ya'll can give will be most > > appreciated. > > > Thanks, > > Charlie > > How about: > > Schottky LED > Vcc ---->!--------+-----/\/\---->!----GND > ! > ===C > ! > PIC port > > When you pull the port pin low, the capacitor gets > charged. When you take it high again you get a pulse > of light from the LED. Absolutely. Instead of using a PIC port, he could tie the cap to his DC-DC converter's switch node. That makes a common Vled supply, ... Vcc -+- | V D1 ._. --- _| |_ C1 | D2 Vled >------||---o-->|--o-----. | | --- C2 | --- | | | === | | .--------+--------+ | | | R1 R2 R3 | | | | LED-R |LED-G | LED-B V ~> V ~> V ~> --- --- --- ----. | | | | Q1 | | | |-|>-' Q2 | | PIC |------|>-----' Q3 | |---------------|>-----' | ----' The phototransistor Hfe drifts a lot with temperature, but that's common to all channels; software can remove it. Seconded: Tim's suggestion for optical feedback on the LEDs' output. -- Cheers, James Arthur |