The MagPi Magazine - Aimed at learners - Printed edition Kickstarter

I'm not qualified to comment on the technical details but I think this all highlights a couple of things.

Even amongst people who are qualified to comment, there seems to be some dissent over the measurements and components involved. Perhaps this highlights that this is not quite such a beginner's topic as it may seem on first viewing. I stick to what I said earlier, in that this needed to be a longer article, with perhaps the first part being done more as a theory section, and then an actual project with a motor driving chip. I'll restate that as a possible customer of the magazine, I'd have been happier to have seen this, along with a nice BOM with perhaps links to the appropriate parts on Farnell and drop the articles on CECIL and ADA. Even if it is just a flashing LED, anyone inquisitive wants to see something happen !

With regards to the Kickstarter, I'd have been happy to contribute but I think there wasn't much of an offering from the £2 mark up to £25. I'd have liked to see something around the £5-10 mark, with perhaps the reward being a special copy, or just the box so I could print them all out and store like that. Going over what I've already said, £25 to me will get me a couple of books and I couldn't justify the outlay against that. I know it has been stated that there have been requests for hard copy but, leaving the successful kickstarter aside, I hope the demand continues and doesn't evaporate after a few issues.

I have also seen that the thread discussing the H bridge has been locked over on the Rpi forums. People who use this forum (normally as they have bee banned at the Rpi one) are well aware of the likelihood that anything negative about the Pi mentioned on the forums (not the troll kind, but people genuinely pointing out valid items) is likely to end in a thread lock at best, thread deletion or banning at worst. It is best not to use feedback solely obtained on the Pi forum as a guide to the accuracy or success of anything.

I'll also comment again, that anyone giving up time on a voluntary basis is doing something worthwhile, as time is such a precious resource these days. However, to make the jump to something financially successful, I think a number of things need to be addressed. In 6-12 months i'd love to be reading a nice, shiny copy of the magazine purchased from a local newsagent. (And I don't normally buy magazines !)

Steve

I'm not qualified to comment on the technical details but I think this all highlights a couple of things.

Even amongst people who are qualified to comment, there seems to be some dissent over the measurements and components involved. Perhaps this highlights that this is not quite such a beginner's topic as it may seem on first viewing. I stick to what I said earlier, in that this needed to be a longer article, with perhaps the first part being done more as a theory section, and then an actual project with a motor driving chip. I'll restate that as a possible customer of the magazine, I'd have been happier to have seen this, along with a nice BOM with perhaps links to the appropriate parts on Farnell and drop the articles on CECIL and ADA. Even if it is just a flashing LED, anyone inquisitive wants to see something happen !

With regards to the Kickstarter, I'd have been happy to contribute but I think there wasn't much of an offering from the £2 mark up to £25. I'd have liked to see something around the £5-10 mark, with perhaps the reward being a special copy, or just the box so I could print them all out and store like that. Going over what I've already said, £25 to me will get me a couple of books and I couldn't justify the outlay against that. I know it has been stated that there have been requests for hard copy but, leaving the successful kickstarter aside, I hope the demand continues and doesn't evaporate after a few issues.

I have also seen that the thread discussing the H bridge has been locked over on the Rpi forums. People who use this forum (normally as they have bee banned at the Rpi one) are well aware of the likelihood that anything negative about the Pi mentioned on the forums (not the troll kind, but people genuinely pointing out valid items) is likely to end in a thread lock at best, thread deletion or banning at worst. It is best not to use feedback solely obtained on the Pi forum as a guide to the accuracy or success of anything.

I'll also comment again, that anyone giving up time on a voluntary basis is doing something worthwhile, as time is such a precious resource these days. However, to make the jump to something financially successful, I think a number of things need to be addressed. In 6-12 months i'd love to be reading a nice, shiny copy of the magazine purchased from a local newsagent. (And I don't normally buy magazines !)

Steve

Hi

To be honest I think there is little dissent amongst those who do know about these things (yes that does include me) about the demerits of this circuit.What little there is stems from "don't know where to begin - it's so bad" to paraphrase someone's comment.

Hi Graham,

Genuine question, what would your design be ? Like I say, i'm not as technical as i'd like to be, so eager to look at an alternative and learn from the differences.

Steve

Hi Steve

Ok genuine answer.

Someone posted http://letsmakerobots.com/node/9450 as a defence of the circuit in question. If you look down to the post by Weiss you will see a standard, proper, efficient, safe, well protected, easily available, did I mention STANDARD design which freely available to anyone. Need I say more?

OK I will say more. It could be improved by replacing the output BJTs by complentary FETs - more expensive but better. And I have to admit I wouldn't fully trust any design until I had modelled it, tried it, etc though I don't think the values were shown anyway.

Better? Well a BJT driven into saturation wastes a small voltage (Vce sat) - anything upto 0V5 whereas the FET looks like a small resistance (down to a few 10s of milliohms for the big expensive ones) so RI leads to a much lower voltage loss. Also FETs tend to turn off when abused - self-protection whereas BJTs tend to turn on - so called thermal runaway.

So this circuit is more expensive but -

Uses only 2 GPIO

Presents much less load to the GPIO

locks out turning on the source and sink on the same side

And is less expensive than blowing things

And will probably be the one shown in next months issue, so what was the point of issue 8 complete with gratuitous and wrong calculations. All that was needed was a block diagram and a comment that the resistors needed calculations.

If you need more feel free - since I retired I no longer charge £100/hr

Regards

Graham.

That second circuit is indeed already an improvement.

You could use darlington transistors for the power ones.

However, if you drive both inputs high, I think you will still see smoke.

Power fet's have a capacitive input, so to achive fast switching times, you need to be able to drive the gate to vcc and to gnd.

This makes the circuit more complex. Power mosfets usually don't saturate at a 3V Gate voltage. Most of them need something between 5V and 12V

The original circuit probably worked because the unit had low voltage motors. No idea how hot the transistors became, as they are only in TO92 housing.

Hi

You need to look for low-threshold or "logic-level" FETs which typically switch with 2V.

I wouldn't worry about turn times - the gate resistors will be only 100R or so and so the FET will switch in a uS or so.

The Weiss circuit is good enough really though.

If both inputs are driven high then smoke will happen. To avoid this you need a new topology - still two inputs, now called enable and direction. Or a couple extra transistors arranged to ensure that a turned on input transistor at one side disrupts the signal in the other side, and vice versa.

All more expensive but properly put together almost bomb-proof and much cheaper than an accident.

You might ask why professional designs don't seem to bother so much about bomb-proofing. Well in my experience the final product will be on a pcb and driven by well-tested software, and if there are a few disasters during experiment (sorry design) well what's a few components when you are paying/charging £100/hr for your labour? And proper lab power supplies tend to protect experimental hardware, and did I mention design software, modelling and simulation?

MagPi is a fine idea, however one doubtful article where I know something seriously damages credibility in those where I don't - areas which includes Linux (ok I know something), python, and one or two (well almost all really) other things.

Graham

Oh, and just found out that this forum does not like iPads so a couple of posts have disappeared including the one where I said I now understood and agreed with MK about his assertion of 6W dissipation in a non-saturated (the condtion I failed to note) bottom transistor.

I think you have probably shown why the simple circuit was aimed at, and by the time you have got to the bomb proof stage, an IC is the obvious answer (that is why they sell them).  However, you don't try to teach someone calculus by getting them to write out Fast Fourier Transforms, they have to understand the parts which make them too.  So yes, more circuits and solutions are needed to cover this topic fully - it is not something which can be covered all at once or over a few pages.

Of course we would welcome a clear and well written article which covers this topic in detail for a future issue from someone who has the skills and expertise to do so.  Graham obviously you do know this topic well and are keen for the right information to be presented on the topic and are keen to highlight where we have gone wrong so far, so I would welcome an article from yourself (or anyone else for that matter who is keen to help others learn).

There are many similar topics like this which would be a massive benefit to the community if clear stage by stage explanations can be provided to help beginners (and hobbyists) gain a deeper understanding of them.

An interesting point was made previously about why go through the steps rather than give the solution and be done with it.  There are a lot of reasons for this, firstly being a community magazine, we are happy for authors to take their own approach to their articles, afer-all what we think is a good way to present something may not be the right way for everyone, so the diversity which people bring to the table is very important.

Also, the articles (particularly this series) are a reflection of the process the author is going through (a step or so behind his current stage) so there is no full step of instructions for what he is doing.  I think more importantly though, this reflects the real world of engineering, real projects unfortunately don't usually come with a step by step guide and part of the role of an engineer is to turn the real world problem into such a guide so that others can make what they have made (it's not a perfect process as many of you know full well).

Finally, the reason why magazines do split such things up is because not everyone is interested in the same things.  If I'd brought the latest issue of Stuff, I would be very annoyed if it had all the pages filled with the iPad, I don't have one and although I don't mind reading about it, I also want to read things which interest me too.

We welcome useful input to the magazine, we maintain a live draft of the coming issue (linked from our site) to allow comments and feedback as we work on it, and are keen to have more people on the team of all levels of skills and knowledge to help us continue to improve and grow.  We are lucky to have a great team who work very hard and do whatever they can to help.

I think I'll draw my involvement with this to a close now, as I have a better idea of what the issues are, what actions we could take on the article and most importantly what the people here (a section of our readers) would like as we move forward.

Many thanks for your input on this,

Meltwater. (on behalf of the MagPi).