Need a little design help on my latest project
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From: Joerg on 2 Mar 2010 21:45 dagmargoodboat(a)yahoo.com wrote: > On Mar 2, 9:15 pm, Joerg <inva...(a)invalid.invalid> wrote: >> dagmargoodb...(a)yahoo.com wrote: >>> On Mar 2, 1:48 pm, Joerg <inva...(a)invalid.invalid> wrote: >>>> Charlie E. wrote: >>>>> On Mon, 01 Mar 2010 17:35:29 -0800, Joerg <inva...(a)invalid.invalid> >>>>> wrote: >>>>>> Charlie E. wrote: >>>>>>> On Mon, 01 Mar 2010 14:30:26 -0800, Joerg <inva...(a)invalid.invalid> >>>>>>> wrote: >>>>>>>> langw...(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. >>>>>>> Joerg, et.al. >>>>>>> Ok, I think I like the idea of using the 1253adj in the current >>>>>>> feedback mode, with three transistors to switch the anodes of the >>>>>>> LEDS. Now, for a really controversial subject - transistor selection! >>>>>>> I could just throw 2N2222s in there, but are there any better options >>>>>>> available, like logic-level FETs that I should use? Looking through >>>>>>> the Digikey selections, I found AO9926B, dual FETS that look pretty >>>>>>> good, while still being big enough to solder by hand! >>>>>>> Any good, cheap through hole logic level FETs for prototyping? >>>>>> Don't you need P-channel? These look good but there won't be much in >>>>>> through-hole, that era is over: >>>>>> http://www.diodes.com/datasheets/ds30933.pdf >>>>>> But make sure the source doesn't get much above 3.3V or it won't turn >>>>>> off and your current regulator could hang. IIRC on of your LEDs may >>>>>> require more than 3.3V. So you could, for example, hang a BAV99 up front >>>>>> to drop 1.2V and a resistor from gate to source. >>>>>>> Thanks again for all the great advice! >>>>>> As one SW guy put it, we are here to serve :-) >>>>>>> Charlie >>>>>>> (at least, this has been on on-topic discussion... ;-) ) >>>>>> Yeah, amazing, not even the slightest rant. >>>>> Ok, this has been bothering me all night. The circuit is now looking >>>>> like >>>>> Vout from regulator >>>>> | point A >>>>> FET switch >>>>> | point B >>>>> LED >>>>> | point C >>>>> FB resistor >>>>> | >>>>> GND >>>>> (Ok, it isn't ASCII art, but hopefully gets the point across...) >>>>> So, working from the bottom, point C is at 1.21 volts. The green LED >>>>> has a Vforward of 3.4V, so B is at 4.6V. I have 3.1VDC (typically) >>>>> from the PIC pin to switch the FET. Not an easy problem. >>>> It is easy: Spring for two logic level FETs that are guaranteed to have >>>> low Rdson at 3V drive. One P and one N. The P-channel goes where your >>>> FET switch ist between points A and B, source to point A. It's gate has >>>> a resistor of 10k or whatever to "Vout from regulator". Now place a >>>> N-channel, source to GND, drain to gate of the P-channel and it's gate >>>> is directly driven by the PIC. >>>>> Unfortunately, the RGB LED is common cathode, so needs to be switched >>>>> above, not below, so need to somehow raise the level of the turn on >>>>> signal to be able to control the FET, and still be able to turn it >>>>> off. Of course, this is just one of three circuits, so I need to be >>>>> sure the others don't turn on at the same time... ;-) >>> The RGB LED is common-cathode? Oh. Then this would work: >>> +3.3v >>> -+- >>> | >>> .--------+--------+--------. >>> | | | | >>> |<' |<' |<' |<' >>> -| Q1 -| Q2 -| Q3 -| Q4 >>> |\ |\ |\ |\ >>> | | | | >>> | | | | >>> | | LED-R |LED-G | LED-B >>> | V ~> V ~> V ~> >>> | --- --- --- >>> | | | | >>> | D1 | | | >>> +---|<---+--------+--------' >>> | | >>> |_ L1 --- C1 >>> )|| --- >>> )|| | >>> _)|| === >>> | >>> +--->Vsense (to switching current regulator, >>> | e.g. ZXSC310) >>> .-. >>> | | >>> | | Rsense >>> '-' >>> | >>> === >>> GND >>> This approach is efficient, stable, inexpensive, small, and provides a >>> wide-compliance range. It needs no level-translation, which saves six >>> parts or so. A resistor-DAC to the Vsense node could modify the >>> individual LED currents, if desired. >> A negative voltage can work and if you use FETs instead of the BJTs you >> save three more parts, the base resistors. > > I personally like FETs; I put in the PNPs for you. They're cheaper. > <grin> > But only in mass production where SMT placement is cheap :-) >> However, make sure the 3.3V hangs on, is strong enough. Also the >> switcher frequency must be high so load changes are handled fast enough. >> C1 can't be too large. > > Or omit it altogether. C1's optional, really. > Careful, then you modulate the LED current with tons of switcher ripple. That could lead to weird color shifts. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: neilrued on 2 Mar 2010 21:48 >On Tue, 02 Mar 2010 13:38:02 -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. >>[snip] >> >>When you figure it out, let me know... >> >>I need to recognize the presence of large yellow objects (*) about 60' >>from the front of the house, and sound a buzzer. >> >>(*) aka School Bus ;-) >> >> ...Jim Thompson >Hi Jim, >Differrent problem. Suggestion: Get a cheap wi-fi camera aimed at >the spot. Take a netbook or other small PC, and get some of the PD >visual processing software that should be out there for security. Set >up a zone of interest, and trigger off yellow! > >I knew some guys back in my toll road days that did this kind of >stuff. By now, it should be pretty generic... > >Charlie > Hi, I would like to add something from my own experience I had several years ago. I was experimenting with a phototransistor and used a voice modulated LED to build a photonic intercom. It worked fine in daylight, then at night I was getting a hum. By using an oscilloscope I discovered that when I switched off the LED transmitter, there was a 50Hz waveform superimposed on the phototransistor amplifier. I connected the oscilloscope to my power supply, but there was no 50Hz leaking through. After a few hours of fruitless head scratching, I thought I'd turn in, get a good night's sleep and make a fresh start the next day. When I switched off the incandescent light, I noticed I forgot to switch off both the oscilloscope and the phototransistor amplifier circuit, as I went to switch the light back on, I happened to notice the oscilloscope trace was flat; no 50 Hz leakage. I then switched the light back on and the 50Hz waveform returned, I then realised that the phototransistor was picking up the 50Hz as interference from the incandescent light. I never got the chance to try out the circuit with a fluorescent lamp. Looking at your schematic, I cannot see any filter topology around the MCP6024A Op Amp? Perhaps a low pass filter designed for fc <= 40Hz, with a flat passband response may be a good option, such as a Butterworth? Neil
From: George Herold on 2 Mar 2010 21:48 On Mar 2, 12:06 pm, dagmargoodb...(a)yahoo.com wrote: > 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- Hide quoted text - > > - Show quoted text - Excellent! And I'll vote for feedback... (as if a vote matters) George H.
From: George Herold on 2 Mar 2010 22:11 On Mar 2, 9:10 pm, dagmargoodb...(a)yahoo.com wrote: > On Mar 2, 1:48 pm, Joerg <inva...(a)invalid.invalid> wrote: > > > > > > > Charlie E. wrote: > > > On Mon, 01 Mar 2010 17:35:29 -0800, Joerg <inva...(a)invalid.invalid> > > > wrote: > > > >> Charlie E. wrote: > > >>> On Mon, 01 Mar 2010 14:30:26 -0800, Joerg <inva...(a)invalid.invalid> > > >>> wrote: > > > >>>> langw...(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. > > >>> Joerg, et.al. > > > >>> Ok, I think I like the idea of using the 1253adj in the current > > >>> feedback mode, with three transistors to switch the anodes of the > > >>> LEDS. Now, for a really controversial subject - transistor selection! > > >>> I could just throw 2N2222s in there, but are there any better options > > >>> available, like logic-level FETs that I should use? Looking through > > >>> the Digikey selections, I found AO9926B, dual FETS that look pretty > > >>> good, while still being big enough to solder by hand! > > > >>> Any good, cheap through hole logic level FETs for prototyping? > > > >> Don't you need P-channel? These look good but there won't be much in > > >> through-hole, that era is over: > > > >>http://www.diodes.com/datasheets/ds30933.pdf > > > >> But make sure the source doesn't get much above 3.3V or it won't turn > > >> off and your current regulator could hang. IIRC on of your LEDs may > > >> require more than 3.3V. So you could, for example, hang a BAV99 up front > > >> to drop 1.2V and a resistor from gate to source. > > > >>> Thanks again for all the great advice! > > > >> As one SW guy put it, we are here to serve :-) > > > >>> Charlie > > > >>> (at least, this has been on on-topic discussion... ;-) ) > > > >> Yeah, amazing, not even the slightest rant. > > > > Ok, this has been bothering me all night. The circuit is now looking > > > like > > > > Vout from regulator > > > | point A > > > FET switch > > > | point B > > > LED > > > | point C > > > FB resistor > > > | > > > GND > > > > (Ok, it isn't ASCII art, but hopefully gets the point across...) > > > > So, working from the bottom, point C is at 1.21 volts. The green LED > > > has a Vforward of 3.4V, so B is at 4.6V. I have 3.1VDC (typically) > > > from the PIC pin to switch the FET. Not an easy problem. > > > It is easy: Spring for two logic level FETs that are guaranteed to have > > low Rdson at 3V drive. One P and one N. The P-channel goes where your > > FET switch ist between points A and B, source to point A. It's gate has > > a resistor of 10k or whatever to "Vout from regulator". Now place a > > N-channel, source to GND, drain to gate of the P-channel and it's gate > > is directly driven by the PIC. > > > > Unfortunately, the RGB LED is common cathode, so needs to be switched > > > above, not below, so need to somehow raise the level of the turn on > > > signal to be able to control the FET, and still be able to turn it > > > off. Of course, this is just one of three circuits, so I need to be > > > sure the others don't turn on at the same time... ;-) > > The RGB LED is common-cathode? Oh. Then this would work: > > +3.3v > -+- > | > .--------+--------+--------. > | | | | > |<' |<' |<' |<' > -| Q1 -| Q2 -| Q3 -| Q4 > |\ |\ |\ |\ > | | | | > | | | | > | | LED-R |LED-G | LED-B > | V ~> V ~> V ~> > | --- --- --- > | | | | > | D1 | | | > +---|<---+--------+--------' > | | > |_ L1 --- C1 > )|| --- > )|| | > _)|| === > | > +--->Vsense (to switching current regulator, > | e.g. ZXSC310) > .-. > | | > | | Rsense > '-' > | > === > GND > > This approach is efficient, stable, inexpensive, small, and provides a > wide-compliance range. It needs no level-translation, which saves six > parts or so. A resistor-DAC to the Vsense node could modify the > individual LED currents, if desired. > > -- > Cheers, > James Arthur- Hide quoted text - > > - Show quoted text - Dang, Sorry I don't get this. I turn on one of the transistors and nothing happens right away because of the inductor. But current starts to flow with an L/R time constant... now what? George H.
From: dagmargoodboat on 2 Mar 2010 22:21
On Mar 2, 9:45 pm, Joerg wrote: > dagmargoodb...(a)yahoo.com wrote: > > On Mar 2, 9:15 pm, Joerg wrote: > >> dagmargoodb...(a)yahoo.com wrote: > >>> The RGB LED is common-cathode? Oh. Then this would work: > >>> +3.3v > >>> -+- > >>> | > >>> .--------+--------+--------. > >>> | | | | > >>> |<' |<' |<' |<' > >>> -| Q1 -| Q2 -| Q3 -| Q4 > >>> |\ |\ |\ |\ > >>> | | | | > >>> | | | | > >>> | | LED-R |LED-G | LED-B > >>> | V ~> V ~> V ~> > >>> | --- --- --- > >>> | | | | > >>> | D1 | | | > >>> +---|<---+--------+--------' > >>> | | > >>> |_ L1 --- C1 > >>> )|| --- > >>> )|| | > >>> _)|| === > >>> | > >>> +--->Vsense (to switching current regulator, > >>> | e.g. ZXSC310) > >>> .-. > >>> | | > >>> | | Rsense > >>> '-' > >>> | > >>> === > >>> GND > >>> This approach is efficient, stable, inexpensive, small, and provides a > >>> wide-compliance range. It needs no level-translation, which saves six > >>> parts or so. A resistor-DAC to the Vsense node could modify the > >>> individual LED currents, if desired. > >> A negative voltage can work and if you use FETs instead of the BJTs you > >> save three more parts, the base resistors. > > > I personally like FETs; I put in the PNPs for you. They're cheaper. > > <grin> > > But only in mass production where SMT placement is cheap :-) > > >> However, make sure the 3.3V hangs on, is strong enough. Also the > >> switcher frequency must be high so load changes are handled fast enough. > >> C1 can't be too large. > > > Or omit it altogether. C1's optional, really. > > Careful, then you modulate the LED current with tons of switcher ripple. > That could lead to weird color shifts. I actually /was/ going to suggest modulating the LEDs deliberating, then using a.c. amplification and synchronous demodulation on the detector side. That rejects ambient light and eliminates drift. But, Charlie doesn't seem to need that level of precision (or nuisance) just now. (P.S. D1 = 1n914, as a gift for Tim 'The Timinator' Williams :) -- Cheers, James Arthur |