Need a little design help on my latest project
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From: Joerg on 1 Mar 2010 17:37 Charlie E. wrote: > On Mon, 01 Mar 2010 14:02:09 -0800, Joerg <invalid(a)invalid.invalid> > wrote: > >> Charlie E. wrote: >>> On Mon, 01 Mar 2010 12:54:27 -0800, Joerg <invalid(a)invalid.invalid> >>> wrote: >>> >>>> Charlie E. wrote: >>>>> On Mon, 01 Mar 2010 11:12:58 -0800, Joerg <invalid(a)invalid.invalid> >>>>> wrote: >>>>> >>>>>> Charlie E. wrote: >>>>>>> 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. >>>>>>> >>>>>> That will require switch mode conversion, no other choice. >>>>>> >>>>>> >>>>>>> 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... >>>>>>> >>>>>> Nope, it ain't quite that easy. It doesn't have an external sense >>>>>> resistor and, consequently, the "accuracy" to which it holds the current >>>>>> is really horrid. Look at the Iout versus Vin, that's just not good >>>>>> enough. If you want to use a chip (or three) you need to find one with >>>>>> at least and external Rsense. >>>>>> >>>>>> It is usually easier and less expensive to boost that voltage from the >>>>>> two AA cells to 5V and add the analog current source circuits I >>>>>> mentioned in my other post (one per LED section). The PIC could be >>>>>> supplied directly from the AA cell if it's happy with 2.5V. >>>>> You are probably right. One problem is that the opamps for the >>>>> phototransistor (ambient light sensor) should also probably stay on >>>>> the battery, to prevent overdriving the ADC inputs for the PIC. There >>>>> are the other LED drivers with an external current sense resistor, so >>>>> will look into the tradeoffs involved... >>>>> >>>> The opamps can be handled with series resistors and, if necessary, BAV99 >>>> double-diodes. But if you have suitable opamps you might as well run >>>> them off the PIC rail. >>>> >>>> I'd really consider just one li'l boost converter that makes 5V. Tons of >>>> those available. Then current sources for a clean control. >>> Actually, just found the MCP1252 series - switchable 3.3 or 5.0 boost >>> converters (charge pump) that don't need inductors! Two of these will >>> be cheaper than the inductor based solution I have now, and give me >>> both rails regulated... >>> >> Careful, the 5.0V version doesn't have any oomph below 3V input and >> AFAIK it quits once Vbat drops below 2.7V. I'd use a real boost >> converter that can comfortably cover your battery voltage range all the >> way to the end of discharge. That one little inductor isn't going to >> kill ya :-) > > but, could I cascade them? With the 5V being fed by the 3.3V? > Assuming everything is less than 120mA... > Probably, but its also an exotoc part. Digikey stock is zero and for me that's usually a red flag. At least if you want to produce this in larger qties. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Charlie E. on 1 Mar 2010 17:42 On Mon, 01 Mar 2010 14:37:22 -0800, Joerg <invalid(a)invalid.invalid> wrote: >Charlie E. wrote: >> On Mon, 01 Mar 2010 14:02:09 -0800, Joerg <invalid(a)invalid.invalid> >> wrote: >> >>> Charlie E. wrote: >>>> On Mon, 01 Mar 2010 12:54:27 -0800, Joerg <invalid(a)invalid.invalid> >>>> wrote: >>>> >>>>> Charlie E. wrote: >>>>>> On Mon, 01 Mar 2010 11:12:58 -0800, Joerg <invalid(a)invalid.invalid> >>>>>> wrote: >>>>>> >>>>>>> Charlie E. wrote: >>>>>>>> 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. >>>>>>>> >>>>>>> That will require switch mode conversion, no other choice. >>>>>>> >>>>>>> >>>>>>>> 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... >>>>>>>> >>>>>>> Nope, it ain't quite that easy. It doesn't have an external sense >>>>>>> resistor and, consequently, the "accuracy" to which it holds the current >>>>>>> is really horrid. Look at the Iout versus Vin, that's just not good >>>>>>> enough. If you want to use a chip (or three) you need to find one with >>>>>>> at least and external Rsense. >>>>>>> >>>>>>> It is usually easier and less expensive to boost that voltage from the >>>>>>> two AA cells to 5V and add the analog current source circuits I >>>>>>> mentioned in my other post (one per LED section). The PIC could be >>>>>>> supplied directly from the AA cell if it's happy with 2.5V. >>>>>> You are probably right. One problem is that the opamps for the >>>>>> phototransistor (ambient light sensor) should also probably stay on >>>>>> the battery, to prevent overdriving the ADC inputs for the PIC. There >>>>>> are the other LED drivers with an external current sense resistor, so >>>>>> will look into the tradeoffs involved... >>>>>> >>>>> The opamps can be handled with series resistors and, if necessary, BAV99 >>>>> double-diodes. But if you have suitable opamps you might as well run >>>>> them off the PIC rail. >>>>> >>>>> I'd really consider just one li'l boost converter that makes 5V. Tons of >>>>> those available. Then current sources for a clean control. >>>> Actually, just found the MCP1252 series - switchable 3.3 or 5.0 boost >>>> converters (charge pump) that don't need inductors! Two of these will >>>> be cheaper than the inductor based solution I have now, and give me >>>> both rails regulated... >>>> >>> Careful, the 5.0V version doesn't have any oomph below 3V input and >>> AFAIK it quits once Vbat drops below 2.7V. I'd use a real boost >>> converter that can comfortably cover your battery voltage range all the >>> way to the end of discharge. That one little inductor isn't going to >>> kill ya :-) >> >> but, could I cascade them? With the 5V being fed by the 3.3V? >> Assuming everything is less than 120mA... >> > >Probably, but its also an exotoc part. Digikey stock is zero and for me >that's usually a red flag. At least if you want to produce this in >larger qties. I have noticed that on the Microchip power supply chips, either little stock, or requires big minimums from DK. Not a good sign... Charlie
From: Charlie E. on 1 Mar 2010 17:48 On Mon, 01 Mar 2010 14:37:22 -0800, Joerg <invalid(a)invalid.invalid> wrote: >Charlie E. wrote: >> On Mon, 01 Mar 2010 14:02:09 -0800, Joerg <invalid(a)invalid.invalid> >> wrote: >> >>> Charlie E. wrote: >>>> On Mon, 01 Mar 2010 12:54:27 -0800, Joerg <invalid(a)invalid.invalid> >>>> wrote: >>>> >>>>> Charlie E. wrote: >>>>>> On Mon, 01 Mar 2010 11:12:58 -0800, Joerg <invalid(a)invalid.invalid> >>>>>> wrote: >>>>>> >>>>>>> Charlie E. wrote: >>>>>>>> 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. >>>>>>>> >>>>>>> That will require switch mode conversion, no other choice. >>>>>>> >>>>>>> >>>>>>>> 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... >>>>>>>> >>>>>>> Nope, it ain't quite that easy. It doesn't have an external sense >>>>>>> resistor and, consequently, the "accuracy" to which it holds the current >>>>>>> is really horrid. Look at the Iout versus Vin, that's just not good >>>>>>> enough. If you want to use a chip (or three) you need to find one with >>>>>>> at least and external Rsense. >>>>>>> >>>>>>> It is usually easier and less expensive to boost that voltage from the >>>>>>> two AA cells to 5V and add the analog current source circuits I >>>>>>> mentioned in my other post (one per LED section). The PIC could be >>>>>>> supplied directly from the AA cell if it's happy with 2.5V. >>>>>> You are probably right. One problem is that the opamps for the >>>>>> phototransistor (ambient light sensor) should also probably stay on >>>>>> the battery, to prevent overdriving the ADC inputs for the PIC. There >>>>>> are the other LED drivers with an external current sense resistor, so >>>>>> will look into the tradeoffs involved... >>>>>> >>>>> The opamps can be handled with series resistors and, if necessary, BAV99 >>>>> double-diodes. But if you have suitable opamps you might as well run >>>>> them off the PIC rail. >>>>> >>>>> I'd really consider just one li'l boost converter that makes 5V. Tons of >>>>> those available. Then current sources for a clean control. >>>> Actually, just found the MCP1252 series - switchable 3.3 or 5.0 boost >>>> converters (charge pump) that don't need inductors! Two of these will >>>> be cheaper than the inductor based solution I have now, and give me >>>> both rails regulated... >>>> >>> Careful, the 5.0V version doesn't have any oomph below 3V input and >>> AFAIK it quits once Vbat drops below 2.7V. I'd use a real boost >>> converter that can comfortably cover your battery voltage range all the >>> way to the end of discharge. That one little inductor isn't going to >>> kill ya :-) >> >> but, could I cascade them? With the 5V being fed by the 3.3V? >> Assuming everything is less than 120mA... >> > >Probably, but its also an exotoc part. Digikey stock is zero and for me >that's usually a red flag. At least if you want to produce this in >larger qties. Ah, but the MCP1253 (higher frequency version) has lots in stock!
From: whit3rd on 1 Mar 2010 18:39 On Mar 1, 2:09 pm, Charlie E. <edmond...(a)ieee.org> wrote: > On Mon, 1 Mar 2010 12:57:49 -0800 (PST), whit3rd <whit...(a)gmail.com> > wrote: > > >On Mar 1, 8:54 am, Charlie E. <edmond...(a)ieee.org> wrote: > > >> Sensor is simply an RGB LED and a > >> phototransistor, buffered by some amps, and then digitized by a PIC24. > > >Your digitizing scheme is not what you really need; the small current > >can easily be integrated onto a capacitor, and a comparator ... > >Current-input D/A conversion is easy > So, I just feed the emitter of the phototransistor directly to a cap, > and use a comparator input to determine when it has reached a > reference voltage. I probably need to hit it before and after with a > ground to clear the cap for the next sample... Yes, that would work; consider using an op amp integrator (feedback capacitor and photodiode connect to (-) node), to keep the photodiode bias steady and so you can filter (at the (+) node) to get rid of power supply hash... either way, it's gonna give you an average of current over a longish time period (maybe turn the illumination on for a few milliseconds per color). I'd prefer to see a 'silicon blue' corrected photodiode for color pickup, phototransistors have LOTS of red and IR sensitivity, not so much in the green/blue range. Brown 'amorphous Si' solar cells are somewhat broader response.
From: Jan Panteltje on 1 Mar 2010 19:04
On a sunny day (Mon, 01 Mar 2010 14:33:41 -0800) it happened Charlie E. <edmondson(a)ieee.org> wrote in <s3goo59u9hh1khbfe0tsuds56j1b8b8bk8(a)4ax.com>: >On Mon, 01 Mar 2010 22:10:00 GMT, Jan Panteltje ><pNaonStpealmtje(a)yahoo.com> wrote: > >>On a sunny day (Mon, 01 Mar 2010 08:54:53 -0800) it happened Charlie E. >><edmondson(a)ieee.org> wrote in <54rno5h38v0fq45a1i4gbkqfl2qc6no2gh(a)4ax.com>: >> >>>Hi Guys, >> >>I would likely do this very differently and only use one PIC and no other stuff. >>How about: >> >>Red on >>Read level1 >>Red off >>Read level2 >>Subtract level1 from level2, this is the red component independent of the environment light strength and changes. >> >>Repeat for green and blue. >> > >Hi Jan, >In this case, level2 is almost zero, but level1 varies from measurment >to measurement. Trying to get that accurate and precise is the real >problem... > >Charlie I do not get it, if you have a PIC with a 10bit ADC? That is good for 5 mV resolution (1024 steps) at 5 V supply, less if you use a lower reference. Without drift. I have seen a lot more signal from a photo transistor. So what drifts? 1) the incoming light? 2) the photo transistor? 3) your circuit? 4) any or all of the above? LOL I would connect the photo transistor directly to the ADC. |