Construction Diary -- Part 1, The LED matrix/display

Original build of a televisor by a complete novice.

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Construction Diary -- Part 1, The LED matrix/display

Postby Andrew Davie » Sat Jan 27, 2007 2:17 am

Well, here we go -- here I hope to record the things that happen as I embark on my attempt to build a mechanical TV. I should start by pointing out that I have absolutely no electronics skills whatsoever -- I'm very much a software guy, so this will be interesting.

I have been reading everything I could find on the 'net, and in particular I will be referring to the excellent construction tutorial on Peter Yanczer's site, along with the information on the NBTV.ORG website in the 'Handbook' section.

I have already purchased a Nipkow disk from Peter, along with a hub as shown on his website, and I believe I'm getting a motor and drive belts along with it. I'm based in Australia, but these parts are in the USA at the moment. I expect to see them in a month... or three.

There's a gorgeous mechanical TV that I'm using as my 'fondest desire' target. If I can make anything like this, I'll be thrilled...

Image

The above by Alan Rosser, as recently reported on the NBTV site.


Here's how I understand the system will work, in summary.

A CD player providing stereo input via an RCA jack, with the left channel holding NBTV 32-line video, and the right channel holding sound. I'll use a club CD for test patterns initially, and develop my own software for conversion of arbitrary video later.

The sound channel is fed through a separate sound amplifier controlled by a volume knob.

The video signal consists of synch pulses of 0.3V 'height' (and please bear with me, I know nothing about electronics), merged with an actual picture signal that ranges from the top of the synch pulse at 0.3V up to 1.0V. That is, the range of the video itself is 0.7V.

The synch pulses are stripped out, or separately handled -- these are used to synchronise the speed of the Nipkow disc (which has 32 holes around the circumference to assist with timing). A separate IR detects these holes and generates a separate signal that is somehow compared with the synch pulse from the video signal and that comparison is used to either speed up or slow down the motor that drives the Nipkow disk.

I do have a desire at some stage to have colour, but I'm thinking that at my first attempt I should just stick with a stock-standard greyscale monitor.

OK, so the video signal (with the synch pulses removed?) is amplified and this is then used to drive an array of so painfully bright that you can't even look at them LEDs ganged together in an array. It's the scanning of the holes in the Nipkow disk over this pulsing LED array that gives us our actual pictures.

Now I've seen a lot of stuff written about the LED array, and it seems that as few as 6 ultra bright LEDs are sufficient. I note that the light needs to be diffuse, so these LEDs have a diffusion barrier (glass/plastic??) between them and the Nipkow disc.

My first step, of course, is to understand just a little electronics so I can build myself a reasonable light array. I'm OK with the idea that LEDs run on current, and that there's a voltage drop across each LED -- so I need to make a decision based on the rating of the LEDs I use that will mean that I don't exceed the power supplied. See how I'm randomly pulling up electronics terms? This is because I don't really have a clue.

Step 1. Understanding how power works...

I'm going to need to provide power that I can't kill myself with. Various sources I've read suggest that 15V or 12V are reasonable output ratings for transformers suitable for this project. I have handy access to computer power supplies, and they output at least one 12V supply, probably more. So my first decision is to use a readymade computer power supply, which will be something like 200W.

If that's a bad idea, please stop me right there before I do any damage!

I will need to power the sound amplification circuit (and speaker that I assume is powered from the same circuit's output?), the video amplification (and I assume the LEDs are powered again from that circuit's output?) and of course the motor control/synch circuit (again, the motor being controlled from that?).

Finally, once that is all working (easy, right?) I'll need to have a pretty case, much as the above. I've already scouted out a few woodworking/cabinetmaker friends so that will be fairly straightforward, I think. I'll make a barebones first, then worry about the pretty bits.

Now, looking at http://www.nbtv.wyenet.co.uk/syncsep.htm this seems to me to be something that I'd like to use. What I understand this will do is that it will amplify the video part of the input signal (although I'm a bit confused because the club standard suggests to me that a standard signal has a black to white range of 0.7V, yet this circuit block diagram says 3x amplification and has an output of 1.4V (ie: 2x the standard). So there's something I"m not understanding there (and what's pp? pulse to pulse?).

But, assuming that it magically amplifies the input signal and there's a simple video out on it, there are two other ouptuts -- a line and clamp pulse, and a frame pulse. Obviously the line and frame pulse are used to synchronise the disc rotation with the signal -- another circuit I'll get to later. And the clamp pulse? I think I read something about this being used to generate true blacks (like giving the base-level of black, perhaps???). Ok, that will have to wait till later, too.

But it looks to me like I'm going to want this particular board, to start with, right? A pity it's unpopulated, as I'm sure my soldering skills are pretty much like trying to solder with a hot housewife-type-iron.

THe next thing that looks like I might be interested in is http://www.nbtv.wyenet.co.uk/vidamp.htm a video amplifier circuit. Clearly this is intended to be connected to the 'video separator' circuit, and I note with the block diagram it also seems to amplify the signal by a further 2x (giving 2.7V not 2.8 -- can't electronics circuit designers multiply? :? ) I have just noticed in the block diagram on the above page, the synch pulses still seem to be there in the signal, so the LED array is actually going to 'display' these, and trying to understand what I'm seeing here, it looks to me like the 'clamping' gets fed to the transistor at the right, which seems to merge the clamped value and the video value before driving the LED array.

I note the LED array is 'powered' at 100V, so presumably we can whack some pretty large number of LEDs (or power hungry) at this point. Clearly I need to understand LEDs a bit better. I do think I understand that if you string them together in serial, you sum up the voltage for each, and you have to provide enough V to 'get through' the whole string, or none will light. I also think I understand that you have to put a limiting resistor in series with the LED string, so that they aren't fried by excessive power. I guess I just don't understand what's the real-world difference between a string of LEDs with a single resistor, and a parallel array of LEDs, with a resistor for each. The LEDs still light up, right?

Anyway, more ignorance later....

So I'm pretty sure I need to get the above two circuit boards (but I'm a bit concerned that now I'm dealing with 100V, not 12, so need to really investigate what this means in reality).

That leaves the speed control circuit, and the sound amplifier.

I would have to assume that the sound amplifier is a pretty stock-standard electronics thing, and that I can find that pretty much anywhere.

And the big thing I've been thinking about (particulalry since the article in the last NBTV newsletter about 'Phase Locked Line Synch' seems to suggest that synchronisation is by no means a simple issue) is the synchronisation of the disk rotation with the signal.

I'm aware that the standard technique is to mask off one of the synch holes on the Nipkow disk, and that corresponds to the frame synch. So the idea, I guess, is that we have (from the synch circuit above) a line synch corresponding to each one of the signals from the holes in the disk, and a frame synch corresponding to the absence of a synch hole in the expected position on the disk. Somehow this has to 'fire up' in a state that will cause the motor to start, regardless of what position it is in, and once started get some idea of if it should speed up or slow down. Interesting. I guess I'd do it with some sort of state machine, if I were programming. I wonder how it's done in electronics. More stuff to learn!

OK, that will do for now. Please please feel free to comment on the above. Obviously my understanding will be riddled with mistakes, but one does have to start somewhere.





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Last edited by Andrew Davie on Sat Apr 07, 2007 11:15 pm, edited 1 time in total.
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1st Monitor

Postby MagicAnim8R » Thu Feb 01, 2007 6:54 am

Hi Andrew,
Wow, it's really nice to know that my monitor has inspired some inspiration for your project. Thank you for the compliment. I think it was the most difficult project I've ever undertaken, but the most rewarding. I have moderate electronics skills but learning more all the time. What helped me tremendously was purchasing the video driver and sync seperator printed circuit boards from the club shop. If you have not done that yet for your project, I can highly recommend it. The boards do need to be drilled so just take extra care. Good luck with your monitor. It's a tremendous thrill to see images appear on the disc for the 1st time! I can understand why there is such enthusiasm amongst club members for this hobby. They are a great group of people and very helpful. I hope someday to go to a convention.

Best regards,

Allan Rosson
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Printed Circuit Boards and CDs

Postby Klaas Robers » Fri Feb 02, 2007 2:37 am

Sooohh... Allan, you used my Circuit Boards.

I am pleased to read that you like the results. That is the reason that I took the effort to design real PCBs of the circuits, and not stop at the single experimenters board version that I started with. Stepping into this strange hobby should not be too difficult for newcomers.

That is also the reason that I made the CDs. If you have to make the signal first and after that make the display, that is the way I went because CD-R was not yet available at that time, then it is a long road to go. I first built a signalgenerator and programmed some still pictures in EPROM. Then I had an NBTV signal.

Now with the CDs you have nice signal material available, which is exactly according the standard, and your development path is overseeable. Doug Pitt once said that the CDs are boosting the activity in NBTV. That's nice said, isn't it.
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Printed circuit boards and CD's

Postby MagicAnim8r » Fri Feb 02, 2007 6:59 am

Klaas!!
So you're the man to thank directly!
Well thank you indeed! Your boards were certainly responsible for helping to move my project forward. Also, the pdf documentation was outstanding and helpful as well.
I have a question for you and all the members as well regarding CD players. I noticed what appeared to be "sag" in the black levels of my images until I tried a different and older CD player. From what I'd read, I thought at first it was a problem with a resistor leaking too fast or too slow. With this other player however, my black levels are perfect with regards to contrast and brightness etc... It also sounded to me like my first player is slightly leaking audio from one channel to another. Possibly introducing unwanted noise to my LED driver? I was wondering if you or other members have encountered this problem. Also, it sure seems like it's more difficult to find portable CD players with a line out these days.
Thank you again for your wonderful boards. I look forward to my next project which will be a 32 line Mihaly-Traub Scanner. I've been doing quite a few drawings for the design of the cabinetry. Hopefully very stylish! :) Best regards!
Allan
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Postby Andrew Davie » Sat Feb 03, 2007 1:02 am

Still awaiting the actual delivery, but I'm slowly putting together some of the components I think I'll need. Here's what I 'have' so far

Nipkow disk, sourced from Peter Yanczer
hub, belt for same
cassette motor (I think)

8 ohm speaker salvaged from a photocopier my kids disassembled as part of a school holiday project

50 'ultrabright' white LEDs, from eBay
Source Material:InGaN !
Emitting Colour:White
LENS Type:Water clear
Color Temperature: 8000K
Luminous Intensity-MCD: Avg: 40000mcd Max: 55000 mcd
Reverse Voltage:5.0 V
DC Forward Voltage: Typical: 3.4 V Max: 3.8V
DC Forward Current:25mA
Viewing Angle:20±10 degree
Lead Soldering Temp:260oC for 5 seconds
Intensely Bright
Viewing Angle: 20 Degree

The above LEDs costing me roughly US$18, delivered.
I'll be very keen to see how these perform.

Meantime, I'll have to learn how to design a LED array :)

The next thing I'm thinking about getting is the LED/Neon driver, and Synch Separator circuits from the NBTVA.

I'm thinking of scavenging a computer power supply. My understanding is that these typically output 12V DC, so this seems a pretty safe and straightforward way of powering my monitor. Please stop me if I'm being silly.

Somewhere in the garage I have a portable CD player sent to us by CocaCola company for sending in 100 coke bottle wrappers, or some silly thing like that. It probably has a line-out on it, and if I can find it I will use that as my video signal 'generator'.

I am assuming that the sound signal (right channel) goes through a stock-standard audio amplifier circuit, sufficient to drive the 8 ohm speaker. So I'll need to find something for that 'stage'.
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Postby Klaas Robers » Wed Feb 07, 2007 2:10 am

Andrew, learn from Allan.

You should have a CD-player with a line output. I always use a home-type CD-player, never a portable one. Home CD-players have an output (line) that goes from 2 Hz to 20 kHz straight as a line, i.e. the same amplitude for all frequencies. Although it looks that 12,5 Hz for the lowest frequency is enough, this is not true. 12,5 Hz as a low pass cutt-off frequency will say that it is fallen down to -3dB. But it should be still at 0dB at 12,5 Hz. This coincides more or less with -3dB at 2 Hz.

Portable CD-players have an output for earphones, that should NOT be fed with frequencies below say 30 Hz. So the amplifiers are layed out to go not below that frequency.

On the other hand, for the real HiFi line output of a CD-player there is a polarity described. Our CDs are recorded such that white is the more positive and sync is the more negative signal. For earphones this doesn't bother, so you might get there video signals in the reverse polarity. So don't use earphone outputs, don't use CD-ROM drives, only use a stationary CD-player of well known brand.

Be aware that we use the audio system beyond the specification that is needed for good quality sound. An MP-3 file is unsuitable for NBTV, although you will HEAR absolutely no difference between the CD-quality and the MP3-quality. In the past I did some experiments that showed the inferiority of MP3-like systems for NBTV.
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Postby Andrew Davie » Wed Feb 07, 2007 12:32 pm

Klaas Robers wrote:You should have a CD-player with a line output. I always use a home-type CD-player, never a portable one.

(snip)

Be aware that we use the audio system beyond the specification that is needed for good quality sound. An MP-3 file is unsuitable for NBTV, although you will HEAR absolutely no difference between the CD-quality and the MP3-quality. In the past I did some experiments that showed the inferiority of MP3-like systems for NBTV.


Thanks for the comment/suggestions. My plan has been to use a portable player with a line-output. I very much want to have a small portable CD as the signal generator, and there are many available (eg: eBay) with line-out connectors. I will not use headphone-out! Your information about the on-board amp and signal degradation are understood.

I would be very interested to see actual images from MP3 playback compared to WAV/line-out playback. Do you have any examples, by chance?

Cheers
A
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MP3-video

Postby Klaas Robers » Fri Feb 09, 2007 5:21 am

No, but I have video signal wave forms. I will send them to you, so please glue them to this message.
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Postby moderator » Fri Feb 09, 2007 9:07 am

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Postby Andrew Davie » Sun Feb 11, 2007 11:41 am

I've just arranged a purchase of a few components from the NBTV club shop ( http://www.nbtv.wyenet.co.uk/clubshop.htm ) -- in particular, I've ordered the sync board, the clamp + LED board, 5 x CA3140 op-amp ICs, CD #1. It's all very convenient, as one can use PayPal for purchasing items (Vic Brown handles the details). I expect these things will be in my hands in a few weeks.

I'm also waiting on my recent eBay LED purchase to arrive (the package is en-route from Hong Kong and should be here Tuesday). This consists of 50 ultra-bright LEDs ( http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 0077701677 ). I expect I'll have a number left over, so if anyone needs some of these I'll be happy to do a deal.

My Nipkow disc and cassette motor is sitting in the USA, and it might be some time (months) before I get around to arranging for that to be shipped to me. I can work on the other bits before worrying about this.

The first thing I'll tackle is building a LED array. I have bought myself a 'wishboard' so that I can play around with circuits without having to solder things. As I said, I'm a complete novice with electronics, so the LED array will probably be a good starting point.

So I need to wait for a while. At the end of next week I hope to have a LED array, and I'll have started to gather the components needed for the boards that I've ordered from the club.

Cheers
A
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Postby Andrew Davie » Sun Feb 11, 2007 10:38 pm

I went electronics shopping today -- Jaycar Electronics is an Australian electronics store -- www.jaycar.com.au -- the guy serving me just had pictures of the circuits from the nbtv.org website's info about the two boards I've bought, and I left him to sort it out whilst I browsed the store. Considering the time it took to find all the bits and pieces, I can't see how they make a profit on electronics stuff!

They had all but one capacitor, which I'll go back and get next week.

Whilst browsing, I found a lovely magnifier for only AUS$9 (about US$5) a very nice heavy and easily-removed glass of 11cm diameter. Very very nice, and *exactly* what I've been looking for to put in front of the display.

I also bought myself a digital multimeter on special for about US$15 -- this one has auto-ranging and more to the point actually works, which my other didn't. Anyway, it's a nice little toy which put itself to use straight away when a power suply blew at home, and also figuring out resistors...

I added a small mono amplifier kit (mini champ) to my purchases and put it together (yes, soldering!) when I got the lot home. It's my first soldered circuit, so it will be interesting to see how it all went when I finally get around to testing it. I need to find that old CD player first, so I can jook up some sound.

I connected the speaker that the kids pulled out of the old photocopier they disassembled as their school holiday project.

In my eagerness I didn't see that the power LED on the thing was to be mounted 10mm above the board -- I mounted it flush. I guess I'll leave that for now and see how it works.

That's the audio side of things handled, I guess. I'm now waiting on the boards from the club, and then I'll go wild soldering it all together. As noted, I have all the components (but for one capacitor).

It was interesting getting the resistors home and then realising I had to interpret the colours. I couldn't for the life of me tell the difference between the browns and reds. Besides the difficulty with the colours, they're also so small, I can barely see them. Am I getting old? Fortunately, the multimeter came in handy, and it clearly sorted out which was which.

To power the thing, I bought a transformer that converts AC240V to a variety of 6, 9, 12, 15V AC at 1A, I think it was. The guy at the store told me that if I used a bridge rectifier and a smoothing capacitor, which he supplied, I'd get close to 17V which is what the diagram on the PDF accompanying the LED/neon driver board shows. I noticed that to calculate which tab on the transformer to use, he multiplied the tab voltage by 1.414 (ie: root 2), suggesting that if I used the 12V AC tab, then the bridge rectifier/capacitor would end up outputting 16.968V

Well that's about it. I learned a little today, built a circuit, bought some supplies, but the best find, really, was that magnifier.


Costs so far (US$)...

Nipkow disc & stuff from P.Yanczer ~$150
Boards, components, CD, postage from NBTV.org $32
Components, transformer $30
Audio kit, components US$15
Magnifying lens US$6
Ultrabright LEDs from eBay US$18

Total US$251
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Postby M3DVQ » Mon Feb 12, 2007 12:11 am

Andrew Davie wrote:To power the thing, I bought a transformer that converts AC240V to a variety of 6, 9, 12, 15V AC at 1A, I think it was. The guy at the store told me that if I used a bridge rectifier and a smoothing capacitor, which he supplied, I'd get close to 17V which is what the diagram on the PDF accompanying the LED/neon driver board shows. I noticed that to calculate which tab on the transformer to use, he multiplied the tab voltage by 1.414 (ie: root 2), suggesting that if I used the 12V AC tab, then the bridge rectifier/capacitor would end up outputting 16.968V


this is because AC voltages are measured as RMS voltages, which stands for Root Mean Squared, rather than peak voltage. Obviously an AC signal is only at it's peak value for a very short time, so RMS is used to give a sort of 'average' when you rectify and smooth it, you will end up with the peak voltage (minus some voltage dropped on the rectifier diodes, but that's small) so as you say, it is multiplied by root 2.
That probably didn't explain it very well.. http://www.kpsec.freeuk.com/acdc.htm that probably explains it better :-)

I'm attempting to build a televisor too (although school work is getting in the way at the moment)
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Postby Andrew Davie » Tue Feb 13, 2007 11:24 pm

Well, a bit of progress! I finally managed to track down a portable CD player with a line-out. All the stores thought I was crazy wanting one -- MP3 players are all the rage these days. I'd sent a few emails to friends, and a close friend had a real beauty (a high end model, I'm sure) that not only has line-out, it also handles MP3s. So, when I get a chance, I'll easily be able to test both 'pure' CD type, and MP3-type video.

I think I mentioned earlier that I'd soldered together the kit amplifier (CHAMP from Jaycar). Well today I hooked up the potentiometer (sound control) and the CD player. I used some CD-player bud-type earphones, cut off the ends, and took one ear's wires as input to the sound amp. My first test was to ground my fingers on the inputs, and sure enough I heard the promised 'blurt' as mentioned in the construction notes. Next thing was to try a CD playing something (it happened to be Dee-Lite) and voila!!!!! it worked perfectly. This was with the speaker from the disassembled photocopier, too. I'm pleased about that.

So, that pretty much covers the electronics side of the sound system. I have a nice, reasonably loud, sound output from a CD through an 8 ohm speaker.

I bought a few more electronics 'things' today -- a set of wire strippers, a stand for my soldering iron, 8 alligator-clips (so I don't have to solder/sticky tape everything) and a connector for the power-connector on a multi-switch plug-pack transformer. I'll try and do everything with the kiddy transformer first, before switching to mains and the 'real' transformer. All-up, I spent another $25 or so.

Also today, my 50 ultra-bright LEDs arrived! They came with 50 resistors too, so I figured, hey, 1 resistor, 1 LED -- couldn't be simpler, right? So I hooked up a resistor in line with a LED and a transformer DC supply, and...nothing. The resistor, by the way, was 360 ohms... and the rated Vf (forward voltage, I think) of the LED was 3.0 - 3.5. Now I really need to hunker down and learn how LEDs and LED arrays work. I'm aware of V=IR, and you know what I probably just fed the wrong voltage for the given resistance. I gave it a low 4V or something like that.

It was quite interesting testing the resistors to see what actual values they had. THe range was from about 354Ohms to 361 Ohms. I was surprised at the variation.

OK, I've just come back from an experiment and sure enough, when I feed 12V+ into the LED with the given resistor it lights up. Boy, does it light up. I can't look into it the thing is so bright... it advertises 55,000mcd... a slight ultraviolet/blue tinge to it, but when you're head-on it's very cold-white. An array of these things is going to be awesome!

Well that's about it for this installment. In summary, I have a mono-amplifier working from a CD line-out source, with a second 'lead' ready to read the video off. I have my LEDs, and I'm starting to work on a LED array. I'm waiting for my boards from NBTV to arrive from England, and I have nearly all my components ready.

I might start thinking about what to put on a sample CD for test images/movies. Obviously I'll use club CD#1, but I'd like to have historical period-pieces running on the actual thing.


A bit more experimenting -- I hooked up 7 of the LEDs without a resistor, to the same 12V supply. Mmh, so that's what burning LEDs smell like. I watched them wink out of existence one by one. I can only assume that I've exceeded the voltage and once one blew the others had correspondingly higher voltage, etc., etc. I know now that a shorted/blown LED will still conduct. Interesting. Clearly I need to learn LED stuff before I run out of supplies.
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Burning LEDs

Postby Klaas Robers » Thu Feb 15, 2007 12:21 am

Andrew,

realise yourself that LEDs are diodes. First they conduct (work) only in one direction. If you reverse the voltage + and - they don't work.

More important is that they need a certain threshold voltage before they start conducting. For the white (= blue) LEDs this threshold voltage is about 4 volts. Below 4 volts they do nothing.

And then when you increase the voltage the current starts running, where a small increase in the voltage gives a large increasing current. It is not a good idea to set the correct current by cahnging the voltage.

The better idea is to set the current more or less and let the voltage be what it will be (about 4 volts). This is the aim of the resistor. With a resistor in series the voltage across the LED will be (about) 4 volts. The remaining voltage will be accross the resistor.
Example: Voltage = 12 volt, one LED and one resistor, across the resistor is 12 - 4 = 8 volts. If you want to have a current of 20 mA the resistor should be 8 / 0,020 = 400 ohm.
Now the resistor defines the current much more than the LED.

So if you want to connect LEDs in parallel, each LED needs a resistor. If you connect LEDs in series you can use one resistor. However you need a higher voltage to run them. I have connected 32 LEDs in series and use about 80 volts to power the total chain.
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Re: Burning LEDs

Postby Andrew Davie » Thu Feb 15, 2007 10:53 pm

Klaas Robers wrote:More important is that they need a certain threshold voltage before they start conducting. For the white (= blue) LEDs this threshold voltage is about 4 volts. Below 4 volts they do nothing.



Thanks Klaas for the excellent assistance. I have now successfully tested a small parallel array of LEDs, each with their own resistor. I did it this way because I only have the correct resistor for this particular arrangement. Tomorrow I will go and attempt to buy a different resistor value that will let me place a parallel array of multiple lots of (three LEDs in a series with a resistor).

The LEDs are rated at 3.4V, 25mA, so I figure that with a 12V supply, three LEDs in series (10.2V) leaves 12 - 10.2 = 1.8 / 0.025 = 72 Ohm resistor required for each row of 3. I understand to use the next highest 'standard' resistor value, which is 73.2 Ohms (1%), I think -- or 82... I will learn a bit more about what's available, from the store tomorrow.

My parallel array tonight consisted of 4 rows with a single LED/resistor in each row. Gosh, it's bright. So bright I can't look at it. It's awesome to think what an array of 20 or so will look like.

Anyway, thanks again for the assistance. It's starting to click, now, and I feel that I understand LED operation, how current and voltage across LEDs work, how to calculate the correct resistor values. I know it's all simple stuff, but one has to start somewhere.

For the next few days I'll be playing with creating that awesome blinding array that will become my 'monitor' display.

Cheers
A
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