Need a little design help on my latest project
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From: Charlie E. on 1 Mar 2010 17:11 On Mon, 1 Mar 2010 13:44:55 -0800 (PST), "dcaster(a)krl.org" <dcaster(a)krl.org> wrote: >On Mar 1, 4:54�pm, 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! >> > >> Thanks, >> Charlie > >Have you considered using an incandescent lamp and two >phototransistors with red filter in front of one and a blue filter in >front of the other. If you run the lamp with a slightly low voltage, >it should have a long life. A 49 lamp runs on 2 volts and 60 ma. > > >Dan Definitely see too many variables in this scheme, with lamp voltage changing color, time on lamp changing color, only two data points. What I have isn't too great, but probably a little more reliable... Thanks, though! Charlie
From: Charlie E. on 1 Mar 2010 17:25 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... Charlie
From: linnix on 1 Mar 2010 17:29 On Mar 1, 2:25 pm, Charlie E. <edmond...(a)ieee.org> wrote: > On Mon, 01 Mar 2010 14:02:09 -0800, Joerg <inva...(a)invalid.invalid> > wrote: > > > > >Charlie E. wrote: > >> On Mon, 01 Mar 2010 12:54:27 -0800, Joerg <inva...(a)invalid.invalid> > >> wrote: > > >>> Charlie E. wrote: > >>>> On Mon, 01 Mar 2010 11:12:58 -0800, Joerg <inva...(a)invalid.invalid> > >>>> wrote: > > >>>>> Charlie E. 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. > > >>>>> 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... > > Charlie Now you don't care about cost? $1.04 ea @2k. Probably $2 in small qty.
From: Joerg on 1 Mar 2010 17:30 langwadt(a)fonz.dk wrote: > On 1 Mar., 20:12, Joerg <inva...(a)invalid.invalid> wrote: >> Charlie E. 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. >> 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. >> > > something like: http://www.national.com/ds/LM/LM2705.pdf > could be used either as constant current or as high voltage supply > AFAIK those become iffy unde 2.5V. Something like this could work, and it's cheap: http://www.diodes.com/datasheets/AP6714.pdf If Charlie would prefer a nicely regulated 3.3V as well which would be nice to keep the detector side in check he could use the same chip for that rail. > I think you could have three npns floating on top of a shared sense > resistor > to do the switching between leds. > That's a good option. Just make sure any load change reactions have petered out when the measurement window cometh. -- 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 17:33
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 |