Need a little design help on my latest project
in [Design]
|
Prev: Magnetism is really -- WHAT?
Next: low-end scope
From: Charlie E. on 1 Mar 2010 14:01 On Mon, 01 Mar 2010 10:38:45 -0800, Joerg <invalid(a)invalid.invalid> wrote: >Charlie E. wrote: <snip original problem...> >> 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! >> > >Where is your VCC coming from? Regulator? If so, what's the minimum >voltage going into that regulator? If it is a battery that won't drop >below about 4.5V and has low load ripple (low source resistance, added >capacitors) fixing this part of the circuit would become fairly simple. Hi Joerge, I only have two AA batteries, so voltage is only about 2.5-3.1 volts. That was why I added in the power supply, to try and stabilize that voltage. Most of the parts were pretty power tolerant, but I figured (somewhat correctly) that the LEDs would be pretty voltage sensitive. What do you think of this idea? Take an LED driver chip, like an LM3519 to do the voltage step up and current control, and then three fets to switch that current to each of the LEDs. Means a chip, a small inductor and schottkey, a couple of caps, and three fets. Shouldn't take up too much board space or budget... Charlie
From: George Herold on 1 Mar 2010 14:15 On Mar 1, 2:01 pm, Charlie E. <edmond...(a)ieee.org> wrote: > On Mon, 01 Mar 2010 10:38:45 -0800, Joerg <inva...(a)invalid.invalid> > wrote: > > >Charlie E. wrote: > > <snip original problem...> > > >> 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! > > >Where is your VCC coming from? Regulator? If so, what's the minimum > >voltage going into that regulator? If it is a battery that won't drop > >below about 4.5V and has low load ripple (low source resistance, added > >capacitors) fixing this part of the circuit would become fairly simple. > > Hi Joerge, > I only have two AA batteries, so voltage is only about 2.5-3.1 volts. > That was why I added in the power supply, to try and stabilize that > voltage. Most of the parts were pretty power tolerant, but I figured > (somewhat correctly) that the LEDs would be pretty voltage sensitive. > > What do you think of this idea? Take an LED driver chip, like an > LM3519 to do the voltage step up and current control, and then three > fets to switch that current to each of the LEDs. Means a chip, a > small inductor and schottkey, a couple of caps, and three fets. > Shouldn't take up too much board space or budget... > > Charlie Could you run the whole thing from a 5 volt supply? Regulate that down to 3.3 for the PIC and have the 5 volts for the LED's. (making some sort of constant current source.) George H.
From: Jim Thompson on 1 Mar 2010 14:16 On Mon, 01 Mar 2010 11:11:55 -0800, Charlie E. <edmondson(a)ieee.org> wrote: >On Mon, 01 Mar 2010 12:08:17 -0700, Jim Thompson ><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: > >>On Mon, 01 Mar 2010 09:30:34 -0800, Charlie E. <edmondson(a)ieee.org> >>wrote: >> >>>On Mon, 01 Mar 2010 10:25:21 -0700, Jim Thompson >>><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >>> >>>>On Mon, 01 Mar 2010 08:54:53 -0800, Charlie E. <edmondson(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%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 >>>> >>>>What keeps the LED's at their _constant_ current? >>>> >>>> ...Jim Thompson >>> >>>Hi Jim, >>>Basically, it is whatever the PIC outputs will put out. They are >>>rated at 18mA, but I suspect that they are putting out a whole lot >>>less... >>> >>>Charlie >> >>Don't you want them to stay at a constant current? >> >> ...Jim Thompson >Yeah, I realize this now... ;-) > >this is my first foray into photometry. Wanted something cheap and >easily reproducible. I just 'assumed' that an LED would give the same >output with the same drive... > >Oh well, I guess the boss will just fire me! > >Charlie Maybe 3xAA (or 4xAA), run uP off of two cells, then some little Mickey mouse OpAmp mirror games to hold the LED currents constant? Does uP have an on-board reference? ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
From: Joerg on 1 Mar 2010 13:38 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! > Where is your VCC coming from? Regulator? If so, what's the minimum voltage going into that regulator? If it is a battery that won't drop below about 4.5V and has low load ripple (low source resistance, added capacitors) fixing this part of the circuit would become fairly simple. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Tim Wescott on 1 Mar 2010 14:51
Charlie E. wrote: > On Mon, 01 Mar 2010 12:08:17 -0700, Jim Thompson > <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: > >> On Mon, 01 Mar 2010 09:30:34 -0800, Charlie E. <edmondson(a)ieee.org> >> wrote: >> >>> On Mon, 01 Mar 2010 10:25:21 -0700, Jim Thompson >>> <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >>> >>>> On Mon, 01 Mar 2010 08:54:53 -0800, Charlie E. <edmondson(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%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 >>>> What keeps the LED's at their _constant_ current? >>>> >>>> ...Jim Thompson >>> Hi Jim, >>> Basically, it is whatever the PIC outputs will put out. They are >>> rated at 18mA, but I suspect that they are putting out a whole lot >>> less... >>> >>> Charlie >> Don't you want them to stay at a constant current? >> >> ...Jim Thompson > Yeah, I realize this now... ;-) > > this is my first foray into photometry. Wanted something cheap and > easily reproducible. I just 'assumed' that an LED would give the same > output with the same drive... > > Oh well, I guess the boss will just fire me! > > Charlie Well, there's drive and there's drive. In the case of an LED, the drive is current. And even then, it's not generally reasonable to expect that a power conversion device (like an LED, that transforms electric power into light) is really at all precise. Use an LED, or a motor, or a solenoid, or whatever, 'in the raw', and you have to expect it's performance to vary. The more efficient it is, and the less expensive it is, the more you can expect it to vary. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com |