Need a little design help on my latest project
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From: Charlie E. on 2 Mar 2010 15:46 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
From: Joerg on 2 Mar 2010 16:12 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 :-) -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Joerg on 2 Mar 2010 17:25 Charlie E. wrote: > 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... > Next step: Train it to recognize black&white cars with "illumination" on top :-)) -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Jim Thompson on 2 Mar 2010 17:35 On Tue, 02 Mar 2010 14:25:01 -0800, Joerg <invalid(a)invalid.invalid> wrote: >Charlie E. wrote: >> On Tue, 02 Mar 2010 13:38:02 -0700, Jim Thompson >> <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >> [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. I'm right here at the window. Mostly I see it out of my peripheral vision but, if I'm deep in thought I don't. > 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! Can I have two cameras on the same PC? If so I'll just add it on here, stick the camera on the window, then have an audible alarm. >> >> I knew some guys back in my toll road days that did this kind of >> stuff. By now, it should be pretty generic... >> > >Next step: Train it to recognize black&white cars with "illumination" on >top :-)) I don't have any black and white car issues... Aaron regularly rides shot-gun just for amusement ;-) But black helicopters may be coming from Obummer :-( ...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.
From: Jim Thompson on 2 Mar 2010 18:10
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 -- | 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. |