Need a little design help on my latest project
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From: Joerg on 2 Mar 2010 22:29 dagmargoodboat(a)yahoo.com wrote: > 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. > I don't know enough about Charlie's LED but modulating might result in it sweeping through a narrow swath of spectrum all the time. And if Murphy strike this will modulate or beat with some other stuff from fluorescents in the room and the poor uC doesn't know what to do with all that. > 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 :) > Probably more like "The Intimidator", after I saw a video of him holding a glowing pipe upwards (!) into an induction heater coil, sans gloves. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: George Herold on 2 Mar 2010 22:31 On Mar 2, 9:48 pm, "neilrued" <neilr...(a)mail2Engineer.com> wrote: > >On Tue, 02 Mar 2010 13:38:02 -0700, Jim Thompson > ><To-Email-Use-The-Envelope-I...(a)My-Web-Site.com> wrote: > > >>On Mon, 01 Mar 2010 08:54:53 -0800, 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. > >>[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- Hide quoted text - > > - Show quoted text - Yeah lights get into everything! fluorescent lights are the worst. I dream of a DC powered white LED. (OK I've got one made from a resistor and DC power supply, but someone should sell me a commercial one.) George H.
From: Robert Baer on 2 Mar 2010 23:08 Charlie E. wrote: > On Mon, 01 Mar 2010 17:35:29 -0800, Joerg <invalid(a)invalid.invalid> > wrote: > >> Charlie E. wrote: >>> On Mon, 01 Mar 2010 14:30:26 -0800, Joerg <invalid(a)invalid.invalid> >>> wrote: >>> >>>> 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. >>> 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. > 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... ;-) > > Of course, no parts around here to play with. time to get out the > spice! > > Charlie Get a DC-DC converter that puts out (say) 8V or better. Use FETs or bipolars for level conversion and switching; circuit slightly more complicated but solves a lot of the problems you have been alluding to.
From: Charlie E. on 2 Mar 2010 23:07 On Tue, 02 Mar 2010 13:12:46 -0800, Joerg <invalid(a)invalid.invalid> wrote: >Charlie E. wrote: >> On Tue, 02 Mar 2010 10:48:15 -0800, Joerg <invalid(a)invalid.invalid> >> wrote: >> >>> Charlie E. wrote: >>>> On Mon, 01 Mar 2010 17:35:29 -0800, Joerg <invalid(a)invalid.invalid> >>>> wrote: >>>> >>>>> Charlie E. wrote: >>>>>> On Mon, 01 Mar 2010 14:30:26 -0800, Joerg <invalid(a)invalid.invalid> >>>>>> wrote: >>>>>> >>>>>>> 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. >>>>>> 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... ;-) >>>> >>> Consider it done :-) >>> >>> >>>> Of course, no parts around here to play with. time to get out the >>>> spice! >>>> >>> Nah, this does not need SPICE ... >> >> Ok, looks good. >> >> I still spice it, cause I don't have the parts on hand to breadboard >> (not that you can breadboard SM parts...) so gives me a chance to test >> things without releasing magic smoke! >> > >But it can cost some time to obtain models for the logic-level FETs. >Sometimes that takes as much time as just whipping it up. > > >> I really need to play with this analog stuff more. I know the basics, >> but don't have enough practice to know what works, and why. Ya'll >> have really pulled my bacon from the fire this time. Maybe if Jim has >> his BBQ next month, I will buy a round (or two... ) >> > >I think Jim needs to order some more global warming before people will >come, or everyone has to bring jackets :-) Actually, the FETs you suggested have spice models on their sites that worked like a charm. Order some complementary pairs this afternoon, some in DIP8 and some in SOT23 form factors. I will crank them up on friday and see how things perform! Naw, by April the weather will be perfect here and in Phoenix. I actually went swimming a week ago, and should be able to again next week! Just need a few days in a row of 76+ highs... Charlie
From: Charlie E. on 2 Mar 2010 23:08
On Tue, 02 Mar 2010 16:10:03 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >On Tue, 02 Mar 2010 11:32:20 -0800, Charlie E. <edmondson(a)ieee.org> >wrote: > >[snip] >> >>I really need to play with this analog stuff more. I know the basics, >>but don't have enough practice to know what works, and why. Ya'll >>have really pulled my bacon from the fire this time. Maybe if Jim has >>his BBQ next month, I will buy a round (or two... ) >> >>Thanks, >>Charlie > >With the economy like it's been I may need to "host" a BYOB party ;-) > >With all the help you've given me with understanding obscure PSpice >functions, I owe you! If there's something I can help you with just >ask... off-group if you prefer. > > ...Jim Thompson Thanks Jim, alway glad to help. But as they say, what goes around, comes around, and sometimes that is a good thing! If you host it, they will come... ;-) Charlie |