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1861  Using Arduino / General Electronics / Re: Wheatstone bridge? on: August 24, 2012, 03:46:42 pm
Well get out your books and reference the devices... a constant current diode is a jfet operated at it's "pinchoff voltage"... they are available in 2 lead diode like packages... a constant voltage power supply feeding the diode will produce a constant set of conditions that the thermistor can react to.
The Wheatstone Bridge is a fine tool but because of it's bi-polar nature is much more difficult to directly interface to the A/D inputs on the Arduino.
Reference the "Star" method of power supply distribution in your texts and see what I was talking about as far as noise both distributed from the power supply and created by individual elements...
Getting back to the thermistor issue, a constant current through the thermistor will produce a variable voltage, temperature dependent and that will give you the information you need from a stable source.
I hope this helps a little. Unfortunately I need to go and do some "Honey Doo's" now and I will be busy for the next 3 or 4 hours... I think.

Doc
1862  Using Arduino / General Electronics / Resistors, many and cheap on: August 24, 2012, 03:25:06 pm
Amazon, Amazing. I bought that 3400 1% Metal Film resistor assortment. I am looking at 7 bundles of 13 values each X 50 resistors all for $29.00 (7bundles X 13values  X 50 pieces = 4550 total resistors from 0 ohms to 10 Mohms... it came in a 10 X 4 X 3" box and weighs almost a pound... It's from Veroboard Too... quite a buy, I think... (free 2nd day shipping too with Prime)...

Doc
1863  Using Arduino / General Electronics / Re: digital oscope suggestions? <500$? on: August 24, 2012, 03:15:58 pm
Most "better analog scopes have both delayed and "other channel" trigger functions that could well have captured your event.
If you set channel B to trigger on the beginning of a sequence then channel A could be triggered from any event from the channel B source.
I do understand your choice and it was a good one based on your equipment available and the required task.
I frequently use delayed triggering from channel B to either enhance (brighten) a trace on channel A or to simply display the event triggered from the derived channel B event... like the system clock for example.
The point I guess is that because I never had the opportunity to use a GOOD Digital scope, I had to learn to use my "poor" analog scope...
The real issue is that "Digital"  is "sexy", most people see digital and a lot of numbers that are SLANTED to make the instrument look good while not displaying the shortcomings of the digital "guts".
The issue really comes back to "What" are you really looking for... faulty code, misused code, faulty design... (typically power supplies and by-passing) or a real issue with a part that isn't performing according to your understanding of the data sheet...
That was my MAJOR area of failure in design... NOT Very Carefully... READING the data sheet... or trying to "Bend" a part to do something not well covered or recommended by the part designer... Mostly I figured out a method to make them work... Because that is "Where the Rubber Meets the Road".
And No one really remembers your successes But NO ONE ever forgets your failures...

Doc
1864  Using Arduino / General Electronics / Re: 2 linear regulators on: August 24, 2012, 02:48:56 pm
Use the switchers if you possible can at all. use two and separate the loads according to function. A linear regulator can be thought of as a "Voltage Variable Resistor"... Heat in a regulator is wasted power... Much better to deliver that Power to the load than to warm the environment... The Environment won't turn your wheels... If you are really paranoid about regulator failure... something that never happened to me in production and use of Both buck and boost mode devices, a simple "crowbar" (SCR and a Zener diode)  device can be used to "Blow" a fuse and I recommend "Polyfuses" in the 12V supply to the switchers...
I used Tranzorbs... 1.5KE18's for the 12V stuff and 1.5KE 6.8's for the 5V stuff as they also provide spike clipping, should it happen... it never did for me But any protection at all is good.
Easy to find power issues... the higher current's drawn by "defects" cause the fuses to get hot...
Use Your 10 "digital wattmeters" or fingers.... to find a "hot" polyfuse
Linear regulators are easy to use but mostly really inappropriate for power control as they convert unwanted voltage... to heat, Wasted heat unless your "robot" is also your "coffee warmer"...

Doc
1865  Using Arduino / General Electronics / Re: digital oscope suggestions? <500$? on: August 24, 2012, 02:21:12 pm
Most of my work was mixed signal from DC to daylight... really low frequency analog FSK type encoding and decoding... Sensors of all types from hand carried soil measurement devices to large Center Pivot control systems single and multiple radio controlled remote valve control from 1" to 36" valves from 27 to 800 MHz RF control...
An analog scope is great for analog signals and a digital scope is BEST for digital stuff. trying to extrapolate an analog signal from the digital "Artifacting" that occurs when a signal is rapidly chopped can be a real issue... is that "noise" an artifact or a harmonic from a frequency multiplier (RF Transmitter Vcc).
I use and used an analog scope for many years for RF development, Audio or low frequency data voice and basic control stuff.
When looking at a square wave signal on a digital scope is that spike on the leading edge of the waveform an artifact or a Power Supply issue???
The chioce is really simple, good digital scopes (Rigol) can be had for less than $500.00 fully loaded. A Good Used analog Tek scope (2235) can be had "Certified, Calibrated and guaranteed for about $150-$200 MAX used and the 2235 is a 100 MHz dual channel scope... I saw 4 or 5 on a quick Ebay scan @ approx $125.00 (AVG) price. The 100 Mhz bandwidth is the 3DB point...
They're as a class good to 150 MHz or more... easily meeting the Rigol analog measurement spec's. Both types of scopes are valuable for the things they do best...
However there is no scope (affordable by most of us) that will do both well... I see Digital "USB" "Scopes"??? with 10 - 20KHz effective vertical bandwidth's for $150 + dollars... Junk for anything but an Arduino and not very good even for that purpose.
A scope is a serious investment. If all you want to do is digital work then BUY Digital... But buy the very BEST you can afford. If your scope and your EXPERIENCE in properly using it is good you have a fine instrument...
But IF you have little experience in digital scopes you can spend a lot of time fixing both Digital AND Analog things that ain't broke....
Perhaps the easiest comparison would be a fine target competition Rifle... is NO BETTER than the man pointing it...
... IMNSHO

Doc
1866  Using Arduino / General Electronics / Re: simple OpAmp question on: August 24, 2012, 03:03:02 am
An LMC622 is a dual pin plugin rail to rail part that will include both the supply and ground rails in it's input circuit, that is you can operate the inputs right to ground or to within millivolts of the positive rail, it can even be used to the positive supply rail without drawing unusual input currents or latchup... I used them for years... Great old part.

Doc
1867  Using Arduino / General Electronics / Re: Time for a new iron on: August 24, 2012, 02:55:36 am
The Tips are GREAT. indistinguishable from the "Hakko" brand new in their little bags. Well worth a 2 week wait for 10 good tip sizes and to save nearly $50.00 (figuring fry's last price @$5.95 Ea)
I have several tips I bought new several years back from Fry's Electronics and these look great, Identical too in every respect. IF you have 220 Vac Mains service I know where You can buy a Hakko 936 for $50.00 New in the Box...
It might be a discontinued item but it is a valuable workhorse... I bought one from Electrodragon http://www.electrodragon.com/?f=2
and here is the Url for the ironhttp://www.electrodragon.com/?product=hakko-936907-soldering-station-esd-safe. I bought one and a $15.00 110 to 220 V transformer... Not Nearly as Ugly as the 888 iron is and still $20.00 cheaper...
My 926 has lasted for nearly 20 years... A GREAT soldering iron, a real good tool investment.
I cried when I paid $279.95+ tax for my Fluke 179 and besides horrendously expensive fuses (22 24 $ per set of 2, 440 mA and 11A fuses) in the long run though it was one of my best tool investments... buy them Right and Buy them once...

Doc
1868  Using Arduino / General Electronics / Re: digital oscope suggestions? <500$? on: August 24, 2012, 02:35:02 am
In all the years I worked for a living I never saw a digital scope that I liked... the digitizing artifacts annoy me somehow. I own a Tektronics 2213 dual trace 60 Mhz BW scope that I used (not the same one) for 10 years at work and except for the sample storage of a good digital scope I don't miss or want one. It was my experience that failures i that required Digital O'scopes could have been better engineered from the start. On the 3 or 4 times I thought I needed and in fact rented and used... A more carful read of the data sheets and some common sense was the fix and the scope was of little use except to verify what I already knew... I'd made a design error or worse forgot a bypass capacitor or placed it too far from the noise source.

Doc
1869  Using Arduino / General Electronics / Re: Tips on double sided PCBs... on: August 24, 2012, 02:17:47 am
Take your time and practice, You Will get better... It's all a process and each step is different requiring a different technology and skill set. I made my first boards by cleaning my own material and literally coating and curing the photo resist in the same environmental chamber the production girls used to heat their lunches. I did my artwork with pen and ink and those Bishop Graphics pads and tape... at 4X scale I had the 'Artwork lithographed at 1 - 1 scale (reduction hides many small sins). I used a photo contact exposure frame to hold the negative to the PCB and the sun to expose the sensitized copper PCB's. I made a bubble etcher from an old Aquarium filter aerator tank and some PVC tubing and silastic silicon caulk to "Glue" gown the bubble tubing and the air pump supplied the air for the bubbles...
I drilled holes in the corners to attach nylon strings to pull the etched boards from the etch tank... total cost in 1967 was about 15 dollars for a bubble etcher that could handle two 8 X 10 boards every 20 minutes. For plate through's I used 30 Ga Kynar wire wrapping wire stripped and placed through the holes prior to insertion of the socket. I would insert and flatten the wires to the board and place a piece of masking tape on the solder side of the board to hold the wires long enough to insert the socket... I had no real issues with my setup and it was the first time I ever had made a PCB in my life...
Days long gone... for the better I think... Very hard to do a DRC on a tape layout or a BOM for that matter...

Doc
1870  Using Arduino / General Electronics / Re: Precision Resistors (.01% or better) - What are they good for? on: August 24, 2012, 01:44:18 am
I specified a lot of .1 and .01% resistors for op-amp feedback networks, Very frequently and not because I had any use what so ever for the accuracy of the part which was really important but the real reason was temperature stability.
Thermocouple scaling amplifiers are one case that comes to mind. I once had a problem with a Max666 in an old design where there was a voltage divider for a battery reference comparator and both values were common E12 series resistors a 6.8 Mohm and a 3.3 Mohm resistor. Until I changed to 10 PPM/Deg C resistors none would pass a temperature test without showing a bad battery. The other part of my answer was done already... hard to make 1% accuracy cheaply with 5% parts and trimming... Believe it or not but thermistors were a frequent part of a design just to compensate for temperature changes.It's Really harder to manufacture .01% accuracy test equipment with 1% parts, It can be done and has for many years BUT not on a real production basis... like cell phones or MP3 players or television receivers, Even with digital calibration done automatically with EEprommed constants and Digipots... It's still a difficult process.

Doc
1871  Using Arduino / General Electronics / Re: Free (almost) Solder Suckers on: August 24, 2012, 01:18:32 am
Yes the partner shipping is usually as fast as prime. I was told that the devices would be here no later than Sept. 9th... and they came in the mail today from a Wednesday order. If I buy from Amazon directly as I did with the solder suckers.When I go to an 'affiliate'store then the store rules are in force...
I never usually do that... the difference in cost (savings?) is eaten up by the shipping charges from the 'Affiliate' store and or the shipping time. My feeling is that the usual small change saved isn't worth giving up the Amazon guarantee.

Doc
1872  Using Arduino / General Electronics / Re: Arduino Mega, Screw (Terminal) it on: August 24, 2012, 12:59:28 am
Does anyone have a clue as to a shield for the mega that would allow a 40 pin cable, short but long enough to access the other I/O on the mega... I have an LCD (Itead) and a shield for my mega but there is NO access to anything under that shield...
I would hate to make one... 40 dollars or so??? the time isn't an issue just a one of or two of circuit board is more than I want to pay. Any Idea's?

Doc
1873  Using Arduino / General Electronics / Re: Free (almost) Solder Suckers on: August 24, 2012, 12:39:17 am
I like solder wick my self and sometimes you need 3 hands to use it as well, the only drawback is the bad stuff is really hard on PCB's and the good stuff is expensive. Personally I like the shield braid from RG316 (teflon RG174) and that wax type rosin flux you can buy at radio shack (Part # 64-022). I strip the Kevlar jacket off and just push toe center conductor out... works great and I usually always have some scrap co-ax.
That having been said a solder sucker is a great tool Especially a Vacuum commercial type.... and Now hot air rework stations are a requirement for really serious techie work anyway.
At $0.51 Ea (Yes I have Prime so shipping is free) because at that price when they become full I can leave all that toxic lead inside of it and discard it properly... and just grab a new one. Amazon Prime is great, second day shipping is free... IMO

Doc
1874  Using Arduino / General Electronics / Re: High voltage bipolar sawtooth wave generation on: August 24, 2012, 12:27:28 am
Find an old diagram for a solid state scope... having said that...
you might have "ripped" too much.
The device has a deflection circuit that might gave been used the horizontal frequency is 15,734 Hz and the vertical rate is 60 hz the color burst frequency 3.579545 MHz is the key as it is the source for the local color injection... and it is the master clock for both the horiz remember the odd 734 instead of the old 15750... turns out that 15734 is a multiple of 3.59 color burst osc.
 I digress too far really but some technical background is necessary.
The Horizontal frequency won't shift by much before high voltage regulation issues, brightness and more important Focus issues will be an issue...
Now comes the vertical and that one is much easier as it can be changed in frequency by factor of 2 both ways and in most tv sets the vertical output circuit is an oscillator and a power amp. Frequently in the same IC. It is very likely that there is some kind of Synch generator driving both the horiz and vert sweep sections of the TV and locked to the color burst frequency, So you get an inkling of what you are facing.
Last is the easiest and that is the video amplifier which is most likely an IC... back in the day it was 2 or 3 class A amplifiers with some video EQ... peaking coils to improve the high frequency response which leads to a clearer for lack of a better word... like copies of copies of old VCR tapes with out the EQ. I mention it because a video driver for a scope should be as flat as possible...
In Short, Yes it can be done, the above list is really a list of the shortcomings of attempting to convert an old tube or CRT type TV set to an O'scope. There are many Many shortcomings there.
Making a Good scope from a processor is very possible....  Great TFT LCD devices are coming available and there are some Very impressive chips available.
But the available data is limited by the processor controlling the device functions and the A/D conversions both speed, depth and quality (Noise and distortion due to Nyquist like effects)...
If you 'sweep' the sampling clock on an A/D converter around a multiple of an input being measured you will find "Aliased" products in the A/D data stream that are interactions between the sample and the sample frequency...
And Finally we get to the meat in this mess... An Arduino isn't fast or the A/D deep enough to keep track of itself let alone be a valid measuring tool for much beyond measuring voltage or time and neither is any more accurate than the Vref for the A/D converter and or the clock used by the Micro.
You could spend a lot less money and time by finding an "Oldie but Goodie" like I did, I looked for a month or so and found a Tektronics 2213, a Basic dual trace 60 Mhz scope....
Old but highly reliable, I paid $55.00 for it and $50.00 for shipping and thought I got a good deal. It's worked very well for 3 years now. In my view a good analog scope is much better for certain types of work and a digital scope is better for other things like deep event storage for those really hard to find problems. Many years ago a man I worked for wanted to rent a "storage" scope (it has two types of phosphors... and I said why... we don't need to see it, we know when the FF resets prematurely... Fix that and the issue is dead... the issue turned out to  be a snubber network and a .1u
f cap across the FF power leads. After those changes were made all was well, Permanently.

Doc
1875  Using Arduino / General Electronics / Re: LM2917 Frequency to Voltage Converter on: August 23, 2012, 11:34:20 pm
Although not the high current issue, The reason for the discrepancy in the speed is due to that series capacitor and diode... at some point probably around 2000 - 2200 rpm that capacitor is going to be charged high enough to start to 'block' the input pulses... connect a meg or so from the diode capacitor junction to ground... Not enough to affect accuracy, just enough to make sure the diode operates as a diode. What happens is that the diode is cut off by the dc generated from the input pulses. At some input condition the diode becomes reverse biased and cut off except for the diode and capacitor leakage and the load resistance which is the reason it 'sorta' works above 2 - 2.5K rpm's.

Doc
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