The MagPi Magazine - Aimed at learners - Printed edition Kickstarter

I hate to be so negative, but the writing in this magazine seems to be getting worse. I have read every one so far, but in the most recent one there seems to be a number of things that just leave the user hanging or have obviously been written by someone who is an expert in what they are doing, but can't convey that information properly to a beginner.

I did look at the Kickstarter on this, but couldn't justify it on the grounds that better information is either available freely on the web, or via one of the many books popping up recently that are cheaper than the bound set of these magazines.

I was however happy to see the Ciseco Eve Kickstarter get funded and I have my board from them already :-)

Steve

I hate to be so negative

Yea it's not like you have a history of that in Rapberri Pi forums or anything. Oh, wait a minute.

John Beetem wrote: You can have upper NPN drivers provided that you can get the base voltage higher than the emitter voltage.  The L298 diagram doesn't show the details of the AND gates that drive the upper NPNs.  I suspect they convert TTL input levels to "Vs".

You're right. I found a data sheet for the L293 and the first stage transistors on the motor drivers are tied to the motor supply.

I've left a comment on the blog and also just tried the raspberrypi.org, it seems I only needed to write a name and e-mail address. I don't see it published, but I guess it is moderated first.

Blake Girardot wrote:

 

As to the wiki, unfortunately, with the unofficial, but still definitive RPi wiki, it is very out of date in many areas. It seems about the last thing people do is decide to go back and edit the wiki when things change and there is certainly a dedicated community around the RPi so I do not think that is the part that is lacking. My main reason for not editing the wiki much is that I feel I am not qualified to edit what is essentially a definitive source of information.

I also hesitate to update the RasPi wiki if I'm unsure about my information, but if I think it's important I'll edit and include caveats ("One user has found that...") and links to the original discussions.   It's really nice to have a single starting point for RasPi questions, and the wiki is the obvious place for this.  Most of my contributions to the wiki have been troubleshooting hints.  It's so much better to be able to tell a newbie to "first check the Troubleshooting Wiki (link)" and get 90% of the problems taken care of instead of having to answer FAQs.  Sometimes FAQs indicate that the Wiki's answer is poorly worded (or formatted) or hard to find, in which case fixing a couple of lines can be quite beneficial.

I haven't read MagPi Magazine myself, but I've read good reviews of it.  Its audience is new RasPi users who don't have strong technical backgrounds and don't know where to start.  Old timers like yours truly are excited by RasPi because of its ridiculously low cost for a GNU/Linux "box" and we know what can be done with such a board: basically the same things we did with $295 boards five years ago, but now the penalty for smoking a board is the cost of a good dinner.  But we're not the official RPF target audience.  The target audience is young pups who don't know what they're doing but have boundless enthusiasm and "Insatiable Curiosity, which means they ask ever so many questions" [Kipling].

 

MagPi is a way to get them started.  Like all magazines and wikis that depend on unpaid contributors, quality varies and at some point a magazine needs to decide if it should follow a strict periodic publication schedule or only publish when they have something worth saying.  Personally, I find most printed matter becomes obsolete pretty quickly and a well-edited wiki with an enthusiastic community behind it is a much better use of 21st Century resources.

 

Googling things is problematic.  If you know exactly what you're looking for and have enough knowledge to filter out the high noise content it can be very effective.  If you're a newbie, it's hit and miss -- mostly miss.  I generally start tech searches with Wikipedia, which often has excellent technical information though not always -- quality depends on whether there's someone (preferably plural) fanatically interesting in the topic who takes the time to polish the article.  I go to Wikipedia to fill in gaps in my knowledge, not to learn something big like logic design or circuit theory from scratch.

Hello John,

I don't think we're too far apart on this :-) I think the Pi is a great catalyst for those of an inquisitive nature and has a great number of possibilities. But those with that sort of mind normally like something to get their teeth into straight away. Whilst I'm all for delayed gratification in most cases, I just think that £25 spent on one of the current Pi books and a starter book on electronics would be £25 better spent, with more immediate rewards, even for a newbie.

If the magazine continues as a free enterprise, then more power to their elbow, time is precious for everyone and they are giving up their own time for the benefit of others. If it does become more of a commercial enterprise though, I think they need to up their game a bit to ensure success.

Steve

Hi

This is Meltwater, one of the MagPi members, after spotting this thread, I thought I would pop my head over the parapet and see if we can get to the bottom of it (I just hope I don't regret doing so).

If we've over looked something (which is more than possible - there is not a vast amount of time available split between the team, while every effort is made where we can to check things) then I would like to ensure that suitable corrections are made.  The magazine is community based, and we accept that we can't get everything right on our own.

Firstly, the circuit is aimed to provide a direct link to the manual switches for the readers understanding, and also to aid with the basics of transistor selection.  While I am aware there is a question around the min gain value for the transistor used (RS quoted it as 210, but I've seen that as the typical value in other places, so RS may have this wrong and caused that problem).  Also, the calculation for the base resistor may incorrectly ignore the 0.7voltage drop, although I believe this results is less current drawn by the GPIO than expected.

I know there are people who do such electronics every-day, and probably can do stuff like this in their sleep, so I would welcome clear answers about this specific circuit.  I am well aware there are better options out there, but as mentioned, it was selected to allow a clear parallel between the circuits.

Right, I'm off to hide!

PS. The price of the printed edition is purely at cost price and to ensure we don't make loss on printing more than we sell.  The magazine will remain free online, just as it always has.  Printed versions were what people asked us to provide too, and we hope that allows more people to see it and hopefully become interested in the RPi and engineering (as per the foundations own goals).

Thanks meltwater.

Hi meltwater, I'd just like to say that minor mishtakes crop up in publications all the time - even after the most vigorous of applications of the blue pencil... At least with an online publication any issues can be caught within hours, rather than having perhaps a month's turnaround on corrections / clarifications.

That said I can appreciate the value of the option of hardcopy - squinting at a pdf is a soul destroying task for an old giffer like me and I find that the remoteness of said document distracts me from it's actual content.

Anyway, keep up the good work, I think that the mag addresses topics that would be of interest to the average Pi owner very well. My only tiny quibble with the publication is that I'm not a huge fan of the handwritten font, but that's just my personal taste. Good luck with the hardcopy launch!

@ melt water,

Congrats re. raising your head over the E14 parapet !

I'll try to be nice (or at least helpful).

I've read the article a bit more carefully now and it contains so many errors that I hardly know where to start !

The transistor chosen is not suitable, RS don't publish a proper data sheet but you can find one on the web. To get the transistor to turn on hard enough to meet the published saturation CE voltage you need 60mA of base current (which they say is pulsed). It's usual that Vcesat is measured with much larger base current than you might expect from Hfe calculations because the Hfe approximation doesn't hold in saturation. Even with the massive 60mA base current the typical Vcesat will be about 250mV at 2.5A resulting in a power of 625mW in the transistor which is perilously close to the spec maximum of 700mW.

If you drive the transistor with 3.3V through 270R you can expect a base current of about (3.3 - 0.9)/270 = 8.8mA and Vce of at least 3V (yes 3V) at 2.5A, which will give you 6.25W into the transistor which will fail almost instantly.

As a rough and very general guide you should only ever expect to get about 50% of the published headline performance out of a transistor (except for voltage ratings where you can expect them not to fail at rated voltage but should only work at  a max of 80%).

The RPI outputs are so dammed feeble that MOSFETS or motor drive chips are your only hope - I suggest a motor driver chip because you get the full H bridge an dsome protection in one easy to use DIP package.

Michael Kellett

Hi

@MK

I tried to suggest on the RPi forum that there might be a few things wrong with the circuit. I regret I got a bit carried away but, as you says " it contains so many errors that I hardly know where to start" and I lost it. 

Anyway the rejoinder was that there is nothing at all wrong with the circuit - it works perfectly and that's that.

All electronic theory thrown to the wind. Shame really.

Hi again

@MK Sorry I don't understand the 6W25. If the bottom transistor is in saturation at 2A OK a big IF) then the power dissipated would be approx 0V4 x 2A5 = 1W, which is too much but not 6W5.  If power rail is 4V5 (I understand the modern BT uses 3 x 1V5) the top transistor would seem to be dissipating  (4V5 - 2V76 x 2A5 = 4W5- way, way too much but still not 6W5.

As I read the datasheet the hFE(min)  is quoted as 140 at Ic=2A. It is pulsed to avoid the thing over dissipating. AFAIK the value of hFE is not affected by pulsing (except in as much as the transistor does not go up in smoke).

Unless anyone can tell me I am wrong the top transitor has its collector at supply voltage and its emitter is never above 3V3 - 0V7.

Hi Graham,

I didn't do the math (btw this transistor seems very obscure), but from a mental simulation! : the top transistor will have a few volts across it, so

even if the motor only takes a few hundred mA, the to92 package will be dissipating way more than it is designed for.

At 2.5A, the dissipation will be of the order of watts, so even other physically very large BJTs would be running hot. The bottom one would be hot too.

@Graham,

Not sure if we have the same data sheet but mine shows that the transistor will not saturate at 2.5A with the 8.8mA base current it will get - that's why you'll get 2.5V across it. I was assuming the power rail issue was fixed since the article talks about putting 2.5A into the motor. (We're back to the "so many things wrong" issue again )

I'm attempting to attach the data sheet I found.

(attempt failed !)

Google for BCU81 Magnatec

Michael Kellett

@Graham,

Not sure if we have the same data sheet but mine shows that the transistor will not saturate at 2.5A with the 8.8mA base current it will get - that's why you'll get 2.5V across it. I was assuming the power rail issue was fixed since the article talks about putting 2.5A into the motor. (We're back to the "so many things wrong" issue again )

I'm attempting to attach the data sheet I found.

(attempt failed !)

Google for BCU81 Magnatec

Michael Kellett

Hi

It seems I have just been banned from the RPi forum - at least the topic is locked - maybe not the same thing.

I closed the datasheet a few minutes ago but it is here http://products.semelab-tt.com/pdf/magnatec/BCU81.pdf

Yes, I now see your point, I had not before, I apologise. With this small base current the thing probably (possibly, maybe, certainly ) won't saturate, the motor will stall and the current will be limited only by hFE x Ib and the thing will glow.  3 alkalines in the power supply have a fairly low resistance.

This is only for the lower transistors - the upper ones glow all the time when switched on, at least they will until they burn out.  If they worked as the designer thinks the lower transistors might be provided with Vce=4V5.

Actually it is hard to discuss this because all the various issues get mixed uo, the lower transistors should never feel the 3V as the top ones will die first , but if they didn't.....

I did offer to discuss all this with the designer by PM but he prefers to keep it all in the open. God knows why. He is trying to defend the indefensible and doesn't even know it.

Rant now completed, thank you for reading - if you did.

Regards

Graham

Date: Sun, 9 Dec 2012 17:58:41 +0000

From: messages@element14.com

To: grahamedriver@hotmail.com

Subject: - Re: The MagPi Magazine - Aimed at learners - Printed edition Kickstarter

    Re: The MagPi Magazine - Aimed at learners - Printed edition Kickstarter

    created by Michael Kellett in Raspberry Pi - View the full discussion

@Graham,

Not sure if we have the same data sheet but mine shows that the transistor will not saturate at 2.5A with the 8.8mA base current it will get - that's why you'll get 2.5V across it. I was assuming the power rail issue was fixed since th earticle talks about putting 2.5A into the motor. (We're back to the "so many things wrong" issue again )

I'm attempting to attach the data sheet I found.

(attempt failed !)

Google for BCU81 Magnatec

Michael Kellett

    Reply to this message by going to element14

     Start a new discussion in Raspberry Pi at element14

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