Need a little design help on my latest project
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From: Charlie E. on 2 Mar 2010 12:18 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
From: dagmargoodboat on 2 Mar 2010 13:08 On Mar 1, 11:54 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. <snip> > http://edmondsonengineering.com/Documents/Rainbow%20color%20Reader%20... Slightly OT, but--won't that 10uF regulator input cap--C14--be a bit hard on power switch SW1? An ohm or three in series would make that a lot nicer. -- Cheers, James Arthur
From: Joerg on 2 Mar 2010 13:48 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 ... -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Charlie E. on 2 Mar 2010 14:32 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! 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
From: Jim Thompson on 2 Mar 2010 15:38
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 -- | 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. |