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1066  Using Arduino / General Electronics / Re: mosfet as current regulator on: February 04, 2014, 04:58:07 pm
Doh! I meant to say two MOSFETs in -parallel-, half the current in each.

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Purpose of mosfet should not be voltage drop, just current limiting, so why it should drop so much?

Because that is how you are controlling current. If you have an 18V power supply charging a 12V SLA battery, and the voltage limit to the battery is 13.8V, the excess 4.2V must be dropped across something.

No offense meant, but perhaps you should start with a simpler project.

BTW, there is a schematic using an LM338 as a 12V lead acid battery charger in that datasheet I gave you.
1067  Using Arduino / General Electronics / Re: Which Handheld Oscilloscope? on: February 04, 2014, 04:54:58 pm
At this point in time, I'd strongly suggest you stay away from pocket and handheld 'scopes. I recently bought a DSO 203 aka DSO Quad, and I'm sending it back. Buggy, false claims of the analog bandwidth, etc.

There are plenty of smaller bench scopes that are very portable with decent specs. Hantek, Rigol, etc. Better to save a bit more than waste your money on junk.
1068  Using Arduino / Project Guidance / Re: Ideas for detecting reflected laser light on: February 04, 2014, 02:48:41 pm
Yes, LDRs are not very sensitive, and they are slow to respond. The lower the light level, the slower they are to respond.
1069  Using Arduino / Project Guidance / Re: battery powered low power object detection on: February 04, 2014, 02:47:03 pm
You've not yet said what they are. Are they metal? Wood? Plastic? Flesh? Is this to detect an animal sticking its head into a feeder?
1070  Using Arduino / Project Guidance / Re: Stepper or servo? on: February 04, 2014, 02:43:14 pm
That stepper from MPJA looks like it came out of a printer. 0.8A, 3.84V. Not particularly powerful.

The OP doesn't need something really powerful, as long as care is given to not creating a lot of friction. A leadscrew converts speed to torque.
1071  Using Arduino / General Electronics / Re: mosfet as current regulator on: February 04, 2014, 02:38:31 pm
SLA means sealed lead acid battery. Constant voltage charging sets the maximum voltage at 2.45V. Constant current charging will mean a battery voltage less than that, but not below 2V.

Still, 4A at 3V is 12W. Why not use a computer power supply and use the 3.3V output as the charging output? Cuts way down on the power dissipated in the MOSFETs.

If you put two MOSFETs in series and adjust the gate voltage so the total current is still 4A, you have half the current through each, therefore half the power (both still dropping the same voltage).

Why not put a logic level MOSFET across R2 in the LM338 voltage regulator circuit? Use a small series resistor to measure current. Then the Arduino smoothed PWM output can drive the gate of the MOSFET to regulate current. With the transistor ON, voltage at the output is 1.25V. To drive more current, turn the PWM down.

http://www.ti.com/lit/ds/symlink/lm338.pdf

You need a minimum of 3V differential across the LM338. So to charge a single SLA cell, takes about 5.5V minimum. Call it 6V at 4A, the LM338 needs a heat sink large enough to dissipate 24W.
1072  Using Arduino / Project Guidance / Re: Stepper or servo? on: February 04, 2014, 08:03:51 am
How old are the printers? IE, how large are the motors, and are you sure they are steppers? Most inkjet printers now use a servomotor system, hence the encoder strip. I expect you knew that.

The stepper would certainly be easier, as long as you get steppers strong enough that they'll never miss a step.

I'm helping a friend of mine do something nearly the same, only to drill a series of holes. His previous setup had a handle with a magnet to lock it into the same position on each turn, and he'd just count turns.

He'd put a stepper motor from a 5-1/4 floppy drive on it and I was just running it back and forth in this video.
https://www.youtube.com/watch?v=NEkU4zXn4Ik
1073  Using Arduino / General Electronics / Re: Help with electronic components on breadboards on: February 04, 2014, 07:54:05 am
You can solder short chunks of 22 gauge solid wire onto the leads. It is easier to hold if you don't trim the wires off until after soldered on.
1074  Using Arduino / General Electronics / Re: Help with electronic components on breadboards on: February 04, 2014, 07:51:29 am
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I hear Digikey is having a sale on 1% tolerance sarcasm detectors.

I hear not everyone is an old hand at this, and may not get sarcasm in a text without facial expression, body language, or vocal intonation.

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Without the benefit of paralinguistic cues such as gesture, emphasis, and intonation, it can be difficult to convey emotion and tone over electronic mail (e-mail). Five experiments suggest that this limitation is often underappreciated, such that people tend to believe that they can communicate over e-mail more effectively than they actually can. Studies 4 and 5 further suggest that this overconfidence is born of egocentrism, the inherent difficulty of detaching oneself from one's own perspective when evaluating the perspective of someone else. Because e-mail communicators "hear" a statement differently depending on whether they intend to be, say, sarcastic or funny, it can be difficult to appreciate that their electronic audience may not. (PsycINFO Database Record (c) 2012 APA, all rights reserved)

http://psycnet.apa.org/journals/psp/89/6/925/

http://www.wired.com/science/discoveries/news/2006/02/70179

http://www.csmonitor.com/2006/0515/p13s01-stct.html
1075  Using Arduino / General Electronics / Re: Help with electronic components on breadboards on: February 03, 2014, 10:43:32 pm
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Let's see. The video was made at MIT, a rather respected institution. They specifically said to print on glossy magazine paper.  So no, I guess the type of paper doesn't matter.

It absolutely does matter. Don't think because magazine paper is cheap, that it doesn't matter. Glossy magazine paper is clay coated paper. Glossy ads may work, too. It depends on the exact toner in your laser printer.

I run Homebrew_PCBs on Yahoogroups. Some people use clay coated glossy inkjet paper, or foil transfer sheets, or some variety of glossy magazine or the Sunday ads. But don't think that means that just any paper will work.
1076  Using Arduino / General Electronics / Re: ultrasonic transmitter circuit on: February 03, 2014, 07:22:20 pm
OK, great, so we know the circuit is working, because 40kHz will trigger the receiver.

Can you post your code? Don't forget to use the code tags.
1077  Using Arduino / General Electronics / Re: USB Oscilloscope Advice Desired on: February 03, 2014, 04:43:47 pm
My review of the DSO 203, based on the seller claiming 72MHz analog bandwidth:

http://www.amazon.com/SainSmart-DSO203-Portable-Oscilloscope-Bandwidth/product-reviews/B0057M7YLE/ref=cm_cr_pr_hist_1?ie=UTF8&filterBy=addOneStar&showViewpoints=0&sortBy=bySubmissionDateDescending
1078  Using Arduino / General Electronics / Re: Arduino digital input, 80Vdc on: February 03, 2014, 11:10:37 am
Perhaps you could distill your requirements into one message. We've asked a lot of questions, you've given a lot of answers, but it is now spread out over 4 pages.
1079  Using Arduino / General Electronics / Re: USB Oscilloscope Advice Desired on: February 03, 2014, 11:08:49 am
If I could only have one scope, I would make it a bench scope. The current benchtop DSOs like Rigol, Hantek, and others are small compared to the CRT scopes I used to have to carry around, and with larger screens.

Look for a sample rate 10x the bandwidth. Nyquist/Shannon is NOT a recommendation, it is a limit that says you'll see in-band aliasing if you do not limit bandwidth to 1/2 the sampling frequency. It does NOT say anything about the fidelity of the capture.

When I can buy a USB scope with 1Gsps dual channel 100MHz bandwidth, I'll buy it.

FYI, those DSO203 pocket scopes being sold as "72MHz analog bandwidth"? They are 36Msps per channel, 4MHz analog bandwidth. Yes, all the ads lie. There is a lowpass filter on the last Op Amp, single pole with -3dB point at 4.4MHz. Take into account stray circuit capacitance and other stages in the circuit, I'd call it 4MHz. That's just about right for 36Msps.

I consider that just about right for a scope for audio work and digital signals <100kHz.
1080  Using Arduino / General Electronics / Re: Arduino digital input, 80Vdc on: February 02, 2014, 06:15:15 pm
I think I'd probably use a comparator with high value resistors.
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