[sdiy] Plug in-breadboard power supplies, non-CV simple audio signal gens, and more!

Tue Jan 11 03:54:43 CET 2011

I know it is best to keep things simple - but if you are having indicator 
leds on the PSU rails, you might as well make them bicolour & have a battery 
level detector chip saying when the batteries are flat.

Incidentally, I remember the first time I ever got to grips with a 
transistor - a circuit in physics prac in 1965, everything powered from a 
battery, and the experiment (on negative feedback in a two transistor 
amplifier) was buggered because of effectively positive feedback via battery 
internal resistance.

On the bright side, how many lessons do i remember from 45 years ago? not a 
lot ;-D

paul perry Melbourne Australia

----- Original Message ----- 
From: "nicolas" Sent: Tuesday, January 11, 2011 12:28 PM
Subject: Re: [sdiy] Plug in-breadboard power supplies,non-CV simple audio 
signal gens, and more!

> For experimental power supplies it is necessary that they can handle being 
> short-circuited and also good if they supply only a very limited current 
> in that situation and even better if they indicate their distress in some 
> way other than smoke signals.  A pair of 9V batteries through a dpdt 
> switch with a couple of LEDs to indicate the state of each rail is not a 
> bad setup.  I also recommend to my students the use of cheap low-power 
> batteries as then the max current is limited to well under 100ma and the 
> consequences of wiring mistakes is less destructive.  Batteries also avoid 
> a lot of difficult noise and hum issues.  The downside to the use of 
> batteries is that sometimes circuit behaviour is changed by the gradually 
> dropping and sometimes assymetrical voltage, which can obviously make 
> problem solving more challenging.  I have more than once been asked to 
> help debug circuits that had nothing wrong except that they didn't work 
> properly on +4/-7 volt rails
> or whatever the half flat batteries had sagged to :)
>
> For CV sources a couple of opamp buffered pots is a great idea.  And a 
> standard minimal two opamp triangle/square LFO with a smallish timing cap 
> so that it can operate up into the audio range would be the simple for 
> them to build as early-in-the-course-soldering-exercise.
>
> For that I often get my students to start by building a simple LM386 based 
> amplifier.  It gives them the soldering practice and then they can use it 
> to listen to everything else they build.  With that and a multimeter you 
> can do a lot of analysis before you need to turn on the scope.
>
> Cheers,
> Nicolas
>
>
> --- On Thu, 6/1/11, lanterma at ece.gatech.edu <lanterma at ece.gatech.edu> 
> wrote:
>
>> From: lanterma at ece.gatech.edu <lanterma at ece.gatech.edu>
>> Subject: [sdiy] Plug in-breadboard power supplies, non-CV simple audio 
>> signal gens, and more!
>> To: synth-diy at dropmix.xs4all.nl
>> Received: Thursday, 6 January, 2011, 3:38 PM
>> I get to teach my "Electronics for
>> Music Synthesis" class again this semester. Yay! And yes, I
>> do plan on taping the lectures again... ;)
>>
>> Anyway, having gone through four semesters of helping
>> people with their final projects, I've been thinking of ways
>> of trying to streamline the process of Synth DIY
>> experimentation (both for my students, and for myself as
>> well.) I've been thinking about the various fiddly bits that
>> take up time in lab, like dialing in the desired supply
>> voltages, hooking up power, etc. I've also been trying to
>> think of ways that students can do more building &
>> testing & debugging on their own, in their dorm room or
>> something, without necessarily being in lab - i.e. if they
>> just have a cheap multimeter, but don't have immediate
>> access to a bench power supply, signal generator, scope,
>> etc.
>>
>> [First off: yes, I know breadboards suck, for myriad
>> reasons. Moving along...]
>>
>> So I'm thinking about:
>>
>> 1) Power. I'm intrigued by the little breadboard plug-in
>> power supplies,.
>>
>> ---> One example is this one the MakerShed, that gives
>> +3.3V or +5V depending on a switch setting from a somewhat
>> arbitrary typical DC wallwart. I notice the switch changes
>> the resistances around a LM317; that's how it can get the
>> two voltages. It uses a slide switch which makes sense; the
>> voltage is something you wouldn't want to be able to
>> accidentally change from bumping the switch, so it's
>> something you'd want to make the user have to put a bit of
>> effort into doing.
>>
>> http://www.makershed.com/ProductDetails.asp?ProductCode=MKSF5
>>
>> http://makezine.com/images/store/Breadboard-PowerSupply-v2.pdf
>>
>> Here's one from Sparkfun, using the same trick:
>>
>> http://www.sparkfun.com/products/9319
>>
>> http://www.sparkfun.com/datasheets/Prototyping/General/Breadboard%20Power%20Supply%20-%20SMD%20v11.pdf
>>
>> Here's another variation on the same them, this time by
>> Curious Inventor:
>>
>> http://store.curiousinventor.com/media/upload/bread_board_power_supply_instructions_vc.pdf
>>
>> And here's another unipolar one:
>>
>> http://www.daqstuff.com/ebaydata/400039/400039-rev3-PDF.pdf
>>
>> http://store.curiousinventor.com/media/upload/bread_board_power_supply_instructions_vc.pdf
>>
>> And here's a skinny one: 
>> http://gadgetgangster.com/find-a-project/56?projectnum=358
>>
>> And finally, here's an adjustable supply from adafruit
>> industries: http://www.ladyada.net/make/bbpsup
>>
>> ---> OK, that's enough examples... moving on... It would
>> be great to have one that plugged into a breadboard and put
>> ground on blue rail, +15/12/9 on the right red rail, and
>> -15/-12/-9 on the left red rail, and have a separate set of
>> breadboardy-type connects on the PCB with +5 V or maybe +3.3
>> V to power a microcontroller or whatever else digital
>> logic.
>>
>> A three-way switch could be used to select 15, 12, or 9
>> volt bipolar. I require my students to conform to the
>> MOTM/Blacet power format, which is 15 volts, but sometimes
>> they choose to adapt a 12 V synth module design or 9 V
>> guitar pedal design. I tell them to get things working with
>> the original rail voltages first, and then we can fiddle
>> around with adapting it for 15 volts. A second switch could
>> turn everything off and back on. It could also have an MOTM
>> style connector (and maybe connectors for a few other
>> manufacturer types) for providing power to a *single* module
>> on the bench, so don't need to run another power cord from
>> your big fancy MOTM Power One that's powering the rest of
>> your synth.
>>
>> Does such a beast exist? I'm thinking probably not, but it
>> sure sounds useful.
>>
>> If I were to design such a think I'd probably want to start
>> with a +/- 15 V DC (or maybe +/- 18 V DC? I have seen 18 V
>> used in old Oberheim designs, I recall) wall wort (or
>> preferable wort-in-the-middle to avoid taking up space on a
>> power strip), rather than having the students fiddle with
>> transformers and mains voltages. Anyone have a favorite wall
>> wort-type supply?
>>
>> The wattage wouldn't need to be very high - just enough
>> current handling to juice up typical "single" modules. No
>> one will be trying to power a whole wall of modules with
>> this.
>>
>> Anyway, I doubt such a three-voltage beast as I describe
>> above already exists, so I'd need to design one. Would it be
>> better to try to use a 7815, 7812, and 7809 "in parallel"
>> (same for the 79xx side), and use a dual-pole switch to
>> switch between their outputs? Or perhaps use the 7812 to get
>> 12 v from the 7815, and then use the 7809 to get 9v from the
>> 12 volt (i.e. cascading them), and use a dual-pole switch to
>> pick the voltages? Or would it be better to use a LM317 and
>> use the switch to switch in different resistors to choose
>> +9, +12, or +15 (that's the trick used in most of the
>> circuits listed above), and similarly use an LM337 for the
>> negative side. I could use a dual-pole switch to pick the
>> different resistors (since the positive and negative sides
>> would need to switch together.) The more characters I type
>> in this paragraph the more it sounds like the LM317/LM337
>> approach would be simpler and less costly. (Even if I am
>> using a bipolar 15V DC wall wort, I was thinking regu
>>  lating it - with the 7815, or appropriate resistors for
>> the LM317 - would be a good idea in case someone was using
>> an extra crappy wall wort).
>>
>> For the +5 and +3.3 volts - again, is it better to say get
>> the +5 volts from the +9, +12, or +15 supply, or get it
>> directly from the original DC coming in?
>>
>> 2) Test signals. It would be great to have some sort of
>> uber-rough, minimal parts count sawtooth, square, and maybe
>> also triangle wave generator (or generators), that output
>> say -5 to 5 volts (MOTM oscillator standard), not CV but
>> maybe with a knob that changed the frequency in a
>> super-rough untuned exponential sort of way. Any favorite
>> non-CV oscillator circuits? There are probably hundreds of
>> such designs out there...
>>
>> Maybe also a lamp-based oscillator for sine waves (I recall
>> Grant R. was a fan of these).
>>
>> 3) CV sources. For easily emulating typical CV sources, I'm
>> planning on just using post and an op-amp buffers - one
>> could run from 0 to 10 V (emulating MIDI-to-CV pitch info),
>> and another from 0 to 5 V (emulating EGs), or something like
>> that. The student can just turn the knob up and down and be
>> a human LFO or whatever. ;)
>>
>> 4) Output. Students often look at the scope traces long
>> before they actually plug in a speaker. Generally, there's a
>> few PC-type speaker sets in lab, and students swap these
>> around, but they tend not to stay penned down for very
>> long... also it would be good for students working in their
>> dorm or whatever to be able to hear their circuit working
>> (or not as the case may be) without needing a scope. Maybe
>> something like this?
>>
>> http://www.sparkfun.com/products/9151
>>
>> I've often seen students try to stick their module output
>> straight into a tiny speaker like that, which messed with
>> the circuit in all kinds of ways. I then explain that they
>> need some sort of appropriate way to drive the speaker,
>> which is *not* part of a standard synth module, and which
>> most computer speaker sets have built-in... so I'm thinking
>> of sticking in an LM384, although maybe that's overkill.
>>
>> Anyway, any thoughts are very welcome - and if I come up
>> with any cool PCBs or something I will make them available.
>>
>> http://www.national.com/mpf/LM/LM384.html#Overview
>>
>> - Aaron
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