controlling a circuit using a changing current

[ericgibbs]

The only ones I can see in the Maplin's catalogue at 20mA or below are 5mm extreme brightness @ 50p each, or 5mm standard violet or turquoise @ £2.50 each!

Perhaps limiting resistors or 2 LEDs in series might be the answer - I've just had an exciting few minutes on my breadboard and 2 LED's in series drop the current! (I told you that I am a newbie )

With the two leds in series on the breadboard how were they connected and what series resistor.?

 

[DaveK]

Sorry, I didn't realise that it would be an issue! I seem to be adding more constraints and problems for you, but my original spec was the (perhaps naive) ideal. I am very happy to compromise where necessary - the school is getting a bespoke, handcrafted product after all!

Hmmm - 12 metres is the maximum - if I have two LED's at a lesser angle than 90 degrees to each other the students could get to within 5 metres and still be looking within the cone of the viewing angle (they aren't actually allowed in the courtyard without a teacher because it has a pond plus a lot of expensive weather measuring equipment

In future I shall write myself a detailed spec of exactly what is needed before I start!

Anyway, the effort you have put in has not been wasted - I will place the LED's where they can be seen rather then making them visible from everywhere the kids might look from. They are pretty excited by the project and will be happy to have a "viewing point".

Dave

 

[ericgibbs]

hi Dave,

Its a question of cost, if your budget can stand it, I can do a display design that you can see 'miles' away.

Also I was trying to keep the loading of the battey/gen down, so the display dosnt hog lots of power.

You decide what the spec should be and I'll come up with a design.

 

[DaveK]

4.5V on the (regulated) power supply. 300Ω resistor and one or two standard 5mm red LED's in series. I measured the current between the LED and the positive terminal.

Dave

 

[DaveK]

I will think this through and get a spec that doesn't waste your time altering the design, makes the job worthwhile and effective, but doesn't break the budget. (And hopefully doesn't go beyond my practical skills)

Dave

 

[ericgibbs]

4.5V on the (regulated) power supply. 300Ω resistor and one or two standard 5mm red LED's in series. I measured the current between the LED and the positive terminal.

Dave

With one led and a Vsupply of 4.5V and a 300R in series, using a standard 2V red, the current would be 8mA.

Adding another led = 2mA.!

 

[DaveK]

According to the spec, the LM3914 can handle 30mA maximum at each output so 2 LED's in series (would an extra resistor be needed?) could bring the current below 20mA

Since the LED's are ultra bright (>1cd) then dropping the current and hence brightness by 30+% would still give far more light than a typical 2 - 50 mcd LED
(more a question than a statement!)

Dave

 

[ericgibbs]

According to the spec, the LM3914 can handle 30mA maximum at each output so 2 LED's in series (would an extra resistor be needed?) could bring the current below 20mA

Since the LED's are ultra bright (>1cd) then dropping the current and hence brightness by 30+% would still give far more light than a typical 2 - 50 mcd LED
(more a question than a statement!)

Dave

Dave,
As I have pointed out, the LM3914, 2K0 sets the LED constant current, not an external series resistor.
Also as stated two LED's in series will pass the same current,, the LM3914 LED drivers are Constant Current outputs.

The 30mA in the datasheet is the absolute maximum rating, which I would not run a device at.

I cannot comment on the apparent brightness, its too subjective.
Another point to consider with the LED, is the radiation angle.
Some are wide angle and some narrow angle.
Image a standard hand torch, with a highly focussed beam it can be seen further away but its not suitable for general illumination.

At 12mtr a narrow angle would be best.

 

[DaveK]

Sorry, sometimes I can be dense until it clicks! Plus learning via email is a new experience for me and (with my experience as a now ex-teacher), I imagine it must be frustrating for you when someone doesn't get the point very quickly! So please bear with me and let me ask some direct clarifying questions . . .

Nearly all the LED's @ Maplin's are rated at 30mA - Is there any way we can use these by setting the LM3914 to give a 20mA current?

Is 25mA too close to the 30mA max? (it's just that there are some rated at 25mA)

With two LED's I can increase the field of view in a section of a corridor where the kids could see the display clearly. Would each LED would be as bright as one LED by itself since the current will not have dropped because the chip is compensating and maintaining the current level?

Dave

 

[ericgibbs]

Sorry, sometimes I can be dense until it clicks! Plus learning via email is a new experience for me and (with my experience as a now ex-teacher), I imagine it must be frustrating for you when someone doesn't get the point very quickly! So please bear with me and let me ask some direct clarifying questions . . .

Nearly all the LED's @ Maplin's are rated at 30mA - Is there any way we can use these by setting the LM3914 to give a 20mA current?

Is 25mA too close to the 30mA max? (it's just that there are some rated at 25mA)

With two LED's I can increase the field of view in a section of a corridor where the kids could see the display clearly. Would each LED would be as bright as one LED by itself since the current will not have dropped because the chip is compensating and maintaining the current level?

Dave

hi Dave,
Dont get confused with the 'rating' of devices, that usually means the recommended maximum value.

eg: a LED rated at say, 30mA will give out 'x' amount of light, but at say, 20mA will give out less light, but it will work just fine.
Problem is, some of these parameters are not linear, so half the the current dosnt necessarily mean half 'apparent' intensity.
Its best to determine by trial which works best for your application.

Ref the angle, you could use a wide angle led [emitter] or narrow angle with a diffuser [ of course the visibilty 'range' will be reduced]

I would suggest that you use 2 leds in series at about 20mA, once we have determined the performance,
then we can calculate a series resistor value from the 12Vdc to the anodes of the LED.
This resistor will warm up instead of the LM3914,, but still allow the 20mA current.

I know you are an old guy, but I can give you a few years, so dont think age is a barrier to picking up new technology...

Lets see if we can put this one to bed today.

EDIT:
when you decide the LED type, post a link or the datasheet.

 

[ericgibbs]

more....

Another important point that you should consider is the panel on which the leds are mounted.
Its colour can effect the 'apparent contrast' of the leds.

eg: a black matt faced panel would give a better contrast for the leds than say
a light coloured panel.

If you plan to mark each led with a legend indicating the charging current for that led, then for 12mtrs it needs to be a large legend.

Also a simple cowling over the top/sides of the panel will reduce ambient reflected light.

 

[DaveK]

Hi - sorry that I disappear for a while, but as I said, I'm only semi-retired!

Do you have access to a Maplin's catalogue? (page 764)

Kingbright 10mm super bright 2.8cd: L-813SRC-E
or Kingbright 10mm super bright diffused: L-813SRD-E

both have the same spec;

http://www.datasheetcatalog.org/datasheets2/51/513436_1.pdf

I will mount them on a matt black panel - no legend, just the idea that more light = more charging!

Dave

 

[ericgibbs]

Hi - sorry that I disappear for a while, but as I said, I'm only semi-retired!

Do you have access to a Maplin's catalogue? (page 764)

Kingbright 10mm super bright 2.8cd: L-813SRC-E
or Kingbright 10mm super bright diffused: L-813SRD-E

both have the same spec;

http://www.datasheetcatalog.org/datasheets2/51/513436_1.pdf

I will mount them on a matt black panel - no legend, just the idea that more light = more charging!

Dave

Morning Dave,
The datasheet shows a led forward voltage drop of 1.85V ay 20mA.
This means you could have 4 leds in series on each LM3914 output.!!

I would have thought that would produce a highly visible 'line', one 'line' for each LM3914 pin[ 10 pins in total]

Note what the datasheet says about the absolute mA and running mA.
Abs = 30mA and Test = 20mA.

Use the circuit already posted, but for every pin where one led is shown you connect four in series.
If you use 4 in series you can forget the series resistor to the leds.

Do you follow OK.?

BTW: I would choose the 'water clear' they have a narrower emission angle.

 

[DaveK]

Thanks - yes I think I follow - 4 LED's in a line (or a block) will give a nice bright "image" and won't cause the LM3914 to overheat. I thought about the diffused LED's because with one or two LEDs it would increase the fov- but with four, that won't be a problem! Therefore 4 water clear it is!

Dave

 

[ericgibbs]

Thanks - yes I think I follow - 4 LED's in a line (or a block) will give a nice bright "image" and won't cause the LM3914 to overheat. I thought about the diffused LED's because with one or two LEDs it would increase the fov- but with four, that won't be a problem! Therefore 4 water clear it is!

Dave

hi,
Goto to this link and download the free drawing and pcb program.
If you do that, I can post the charger1.sch [ the drawing of your circuit] so that you can change it to suit.

 

[DaveK]

I've downloaded and installed it.

Dave

 

[ericgibbs]

I've downloaded and installed it.

Dave

I'll pretty it up and post, the file extension will be *.txt change it back to *.sch

EDIT:
Attached sch drawing, check it thru.
Ive had to redo it, couldnt find the original.!

 

[DaveK]

Thanks - The schematic is nice and clear - I'll now have fun transferring the design to stripboard Dave

 

[DaveK]

Hi Eric

I haven't disappeared - I've just been very busy now the weather has brightened up a bit. I'm still working on the project and I'll let you know how it pans out!

Dave

 

[ericgibbs]

Hi Eric

I haven't disappeared - I've just been very busy now the weather has brightened up a bit. I'm still working on the project and I'll let you know how it pans out!

Dave

hi,
The weather has been awful for weeks, nice to be getting a little sunshine at last.

Let me know when you are ready to switch ON, I'll close my eyes, put my fingers in my ears and hum very loudly...