On Apr 11, 2012, at 6:15 AM, Rob Jarratt wrote: It's not universally true, but in *most* transistor circuits a higher gain shouldn't hurt. Just looking at those specs, the PNP equivalent to a 2N2222A should do the trick (I think it's a 2N2907A). Make sure it's the A variant, though, because the regular one only has a 40v Vce max. ...and, looking at what you've linked, that's exactly the part you picked. It's probably fine. - Dave

On 04/11/2012 06:15 AM, Rob Jarratt wrote: It's pretty unusual for a circuit design to rely on a specific current gain (Hfe), because it's not constant. It varies with the voltage across the transistor, and varies a good amount from transistor to transistor of the same model. Don't try too hard to match the Hfe. -Dave -- Dave McGuire, AK4HZ New Kensington, PA

On 2012 Apr 11, at 12:48 PM, David Riley wrote: Probably something like Motorola vs Fairchild. I'm just guessing, but in preponderance of personal observation of equipment made back when they were new, I think 440x has been more Fairchild and 390x more Moto or TI. Everyone cross-supplies of course, but it would be nice if there were a resource available to look back at which manufacturer a particular type originated with. (Why is there a 40 min. delay for messages getting through distribution?)

Looks like there is an MPSA55 and an MPSA55G. The MPSA55 is a Darlington transistor, according to Farnell; I recently learned a little about those repairing my RD53 motor control board, and my understanding is that the gain is even higher with those. Given that it seems I need less gain than the MPSA55G provides, the straight MPSA55 would seem unnecessary. However, Farnell will charge me a lot because it has to ship them from the US to the UK. An Ebay seller here in the UK seems to have the Darlington variety, so I could get some of those sooner and cheaper. The alternative seems to be an MPS2907AG from Farnell http://uk.farnell.com/jsp/search/productdetail.jsp?sku=1611212. What would people advise? NB This transistor is used to switch in a resistor that controls the timing of a 555 which drivers the PSU chopper transistor. If anyone wants to look, it is Q21 on p88 (in the PDF) of the H7140 printset, which is here: http://bitsavers.org/pdf/dec/pdp11/1124/MP01018_1124schem_Aug80.pdf. Thanks Rob

about than to Thanks, hadn't thought about that! I won't use the MPS055 then for sure. Regards Rob

that's a typo as will Thanks for that. I am left wondering about one thing though. I believe the c-e resistance of the transistor will have a material effect on the frequency of the oscillation of the 555, which is controlling the chopper transistor of a switched mode PSU. I don't know how this resistance might vary between different transistors and how this might affect the chopper and the PSU as a whole. The reason I think it affects the oscillation is as follows: The transistor switches in a resistor that affects the 555's timing. Before the failure of the transistor in question the 555 oscillated at about 28KHz. After the failure it oscillates at about 14KHz. As a test, using just a 9V battery, I shorted the C-E terminals of the transistor with a piece of wire just to bring the switched resistor back into the circuit; when I do this the 555 oscillates at 33KHz, so the transistor must be adding resistance that influences the timing. Regards Rob

On 2012 Apr 11, at 12:00 PM, Rob Jarratt wrote: It appears Q21 (the A55 at issue) is working in conjunction with Q21 to form a constant-current supply for the timing capacitor. (That should mostly linearise the charge ramp on the cap, but I'm not clear yet as to what the objective is there as the output from the 555 is still off-on and I haven't examined the entire power supply schematic enough to figure out where this fits in the big picture of the whole power supply. Stability perhaps, or, is that 300V through the 3.9M R a kickstart or is it regulation feedback?) The critical components in the Q21/Q22 circuit are D34, R41, R42 & R45, the transistors in operation will 'adapt' around (as determined by) the values of those components. Transistor specs vary widely (between units and with temp) and (again, generally) one doesn't design around a specific value of transistor spec - one chooses a transistor for various mins and maxes and the 'design' proper is done with more precise and stable components (R/C). (Flippantly, that's also an explanation of why digital has won: so you no longer have to sit down with a slide rule to accomplish something.)

Flippant reply : o, you have to sit down with a CAD system to accomplish anything (or at least a computer running a cross-development system) My expeireence is that analogue and digital design are about equally hard, but in different ways. Some people take to one, some to the other. Yo ucan do simple digital design (a handful of gates and flip-flops) in your head. You can do simple analogue design in much the same way. When you start making complex state machines, or using microcotnrollers, prgrammable logic devices, analogue circuits dring 'nasty'' loads (like a chopper transsformer), or have many frequnce-ydependant analogue feedback loops, then you start neediong to use a calculator or a computer. -tony

I think this depends -- a lot -- on how the designer thinks. Certain;y I find hat for myself going over to the digirtal domain for what are essentially analopgue problems makes them harder to understnad. Sometimes it's the only sensible way to solve the problem, but I certain;y don't think it makes it easier _for me_ to find the solution. -tony

I absolutely agree with you. Far too many so-called designers today try to solve problems by throwing ever more transistor at them. Yes, there most certainyl are times where microprocessor, a DSP, A microcotroller or an FPGA are appropraite. There are also times when they most certainly are not. Far too few people spot the latter. I think I mentioend some time back that I bought some educational kits that were being sold off cheaply over here. They make toy vehicles that do a reverse-and-turn wthen they bang into something. One was designed in the UK, the other was designesd soemwhere in the Far East. The UK one contians a PIC 12C508 microcontrolelr and a motor driver IC on a pre-soldered PCB. You connect that to a pre-assembled motor/gearbox units. There are no scheamtics of program listings in the instructions either. The Far Easten one is a bare PCB and a bag of components. And there's not an IC in sight. It's done with 10 discrete transistors. And you get to put the gearbox together... I regard the second one as more educational _and_ a better design. It does the job properly, it's easier to repari, it's probably more reliable. Oh, those are wonderful. I am unable to play one, but I have repaired a couple over the years. Great pieces of machinery., I regard that as one of the all-time great designs. I would love to get one. Unforutantely, they are very rarew in the UK, we didn't have DTMF-capable exhanges until the 1980s AFAIK by which time the DTMF generator in a telephhoe used one of the standard ICs. I have managed to obtain a couple of ratary-dial US telephones (One of which is a Bell 500 set), and a Bell 2500 set DTMF telehoone, but of course with an IC on the PCB under the keypad. Als the companies that sell old telephones on the web say more aobut the colour and styling (which don't really bother me) than the insides. I can't find any compnay that can guarantee to supply a single-transistor+pot_cores DTMF generator. I was recently looking in some 1970s UK electornics magazines. The projects back then, using mostly discrete transistors with a few op-amps or TTL parts sometimes seem to do everything you really need. Adding complex ICs might add a few bells-and-whistles, but I am not sure that's an improvement. -tony

On 15/04/2012 21:15, Mouse wrote: I wanted a slave flash trigger which would be compact, and smart enough to work for ordinary flashes (from an SLR with a conventional flashgun) or for a compact (which has a pre-flash that can't be disabled). My design used a small PIC, a phototransistor, a few resistors, a small thyristor, and a CR2032 lithium cell. It's turned on and off by a momentary pushbutton, or can be made to go to sleep or wake up by the PIC's internal timer. It runs for months on a single CR2032, because the current draw is very low. So a properly-designed uP solution needn't be a problem. -- Pete Peter Turnbull Network Manager University of York

Please don't use paragraph-length lines. True. I'm sure it's possible to get lower-power than a 555, too, in the non-processor solution space. I was really comparing the first-reaction "use a processor" solution to the first-reaction "use discretes and maybe simple LSI" solution. The camera drew enough power that a milliamp this way or that in control circuit draw was pretty much ignorable, though, so there was little incentive to put more human time into improving it, especially in view of the limited amount of improvement that was even possible. /~\ The ASCII Mouse \ / Ribbon Campaign X Against HTML mouse at rodents-montreal.org / \ Email! 7D C8 61 52 5D E7 2D 39 4E F1 31 3E E8 B3 27 4B

True. That actually was intended for the person who produced the paragraph-length line (in this case, by converting a perfectly reasonable paragraph into a single quoted long line), not the person (you) who quoted it. I should have made that clearer. Yeah. It's a fun little chip. Very true. Different goals -> different solutions.... /~\ The ASCII Mouse \ / Ribbon Campaign X Against HTML mouse at rodents-montreal.org / \ Email! 7D C8 61 52 5D E7 2D 39 4E F1 31 3E E8 B3 27 4B

On Apr 16, 2012, at 3:02 PM, Tony Duell wrote: Well, yes. It is a figurative Swiss Army Knife of interesting bits. It's kind of a shame that it's nearly universally referred to as a 555 *timer* when in fact it's really an interesting, connected assembly of comparators, a BJT, a flip-flop and a resistor ladder. I think the "timer" moniker keeps people from really employing their wetware when designing things with it. You could even use it as a simple PLL core, though I'm not sure how much quality you'd get out of it. - Dave

On Mon, 16 Apr 2012 20:02:50 +0100 (BST) ard at p850ug1.demon.co.uk (Tony Duell) wrote: Here is a step up DC-DC converter with current regulation for high power LEDs build around a ICM7555. (Enhanced NE555) Beware, German, but the schematics is language independent: http://www.ledstyles.de/fpost184856.html -- \end{Jochen} \ref{http://www.unixag-kl.fh-kl.de/~jkunz/}

On 16/04/2012 02:56, Mouse wrote: This should be the way the gods intended. I've set it to not flowed for sending plain text messages. Somewhere along the path of ridiculously frequent Thunderbird upgrades, a default has changed, and I didn't notice. -- Pete Peter Turnbull Network Manager University of York

On 04/16/2012 02:57 PM, Tony Duell wrote: I'm right there with you on that...but I think that, with Arduinos in particular, there is a net gain. FAR FAR more people are into electronics and microcontrollers than would've been without that platform, and SOME of those people "take off the training wheels" and move past the hand-holding of that platform. Some don't...and that's ok, because in the end a larger number of people still have come into the field "for real". Note that I'm not dissing Arduinos here, not at all. I think it's a great platform, both the software and the hardware, and I design and build "real" microcontroller-based systems commercially all the time. But the Arduino was (as far as I'm aware) *intended* to be training wheels, and it does make a great set. One can "graduate" from the Arduino software platform and start programming the microcontroller directly, or one can use the Arduino software platform on a scratch-built board, just as easily. Basically someone can "graduate" into more advanced hardware OR more advanced software whenever they feel comfortable, and do the other later. -Dave -- Dave McGuire, AK4HZ New Kensington, PA

On 04/16/2012 05:33 PM, ben wrote: Huh? WTF are you talking about? Low-pin-count devices are very much in the minority in the microcontroller world...but how is that even relevant? -Dave -- Dave McGuire, AK4HZ New Kensington, PA

I wonder just how many extra people 'take off the training wheels' (that is, take an Arduino and then go further) over and above those who would get itno electronics/microcontrollers/whatever no matter what. My guess is that it's small. -tony

Good!. I am not so sure that is the case over here.... Butm FWIW< the only people I know who seriously uses the Arduion boards are electronic enngieeers who are quite capable of making their own stuff, but use them to test out ideas. I am not sure I see the point, but.... I am not at all convicned it's 'not their fault'. There are plenty of books,m articles, etc that mention the 555. It's not some secret device. Personally, I feel youshould read up o na subject before working in it. Oh yes, many, many, mant. Problem is, as I've never heard of them, I can';t tekk you waht they are... This is one problem with the lack of paper databooks. At one time I'd read every new datavbook I could get my hands on, just to see what was aviaalble, Of course I mised some deivevs, I couldn't see all the books, but... It's a lot harder, at least for me, to do that on a web page. One day, I really must learn html... But more seriosuly, why would they look at such a web page. I can't believe there aren;'t already web pages about the 555. The data sheet is certianlly on the web, I've looked at it there. You do relaise that I routinely introduce people to electroncis (my flavour of electronics), etc, at HPCC meetings... That's not truye. I use microcontrollers. Perhaps not as much as I should do, or as much as others do, but I sue them. I have no problem with them being used when they're the 'right solutuon', for a suitable defintion of 'right'. As I said in a previous message, what is the 'right device' to use depends o na lot of things. It depends on what you, the designer, is used to. If you can calcualtee 555 timer components in your head, and have a good stock of reissotrs and capacitors, that would push a 555 solution. If you have an Arduino board on the bench, the devlopment tools on the PC, and know how to write the code, then that would push that solution. Whether you're making one unit or a production run matters too. For the fomrer, design time tends to dominate, for the latter, it's construction time. So ofr the former, using extra components to get a design that works canb b the right thing to do. For the latter taking the time to get it into a microcontrolelr can be the right thing to do too. What I am really moaning aobut is not people who use microcontrollers (you wound't find me complainign there's on in a DiskFerret, or that that there's one in a PS/2 keyboard to soemthing-else interface or...), but people wh say 'Oh you cna do that with a microcontorller (or an Arduino) without (a) actually doing it (they might well find there are problems), and (b) without considering other options at all. -tony

On Wed, Apr 18, 2012 at 02:42, Toby Thain <toby at telegraphics.com.au> wrote: if you download the OS X Arduino package (the latest doesn't have PPC included of course, you have to grab 0009) it has all the avr tools in the app package. The IDE is a java package but all the backend is separate executables, standard precompiled gcc toolchain and avrdude. The Arduino board is nothing but the ATmega chip with a USB-serial interface, but the ATmega has a small bootloader preprogrammed. It is documented, and is what avrdude talks to. IIRC, after reset it looks if anyone is trying to download new code into the chip, if not, it'll execute what is in the chip already. There are plenty of tutorials on command-line use of the Arduino tools, geared towards C programming. I think going to assembly would not be a big step from there, if you want to. Joe. -- Joachim Thiemann :: http://jthiem.bitbucket.org

I beleive it's part of the regulation loop of the bias supply. This PSU, overall, is rather complciated, the part in question is a small (fairly conventional) SMPSU tthat runs all the time that mains is connected to porvide power to the chopper control ICs for the nmain outputs, and also to the control logic. The associated 555 timer drives the chopper transistor for this little PSU. Q21 is some part of the regulation loop, it controls the duty cycle of that 555 based on one of the voltages in the PSU. I would be very suppirsed it this was originally a darlington, and as such it shouldn't be replaced by one. -tony