First Project help, please.

[JibTeenuc]

Hi everyone,

First time poster (please be gentle).

I’m a computer professional of some 30 years (software and hardware repairs since the BBC Model B, Vic20 and Spectrum 48K LOL).

I have a small(?) LED project and not the slightest idea where to start except in that I have researched what I believe I need and am armed with Soldering Iron (which I am used to) along with stuff I’m not used to as follows: Breadboard and cables, battery holders, AC/DC power adaptor, a shed-load of 5mm Ultra-Bright LEDs, Resistors, Capacitors, Diodes, Ne555 and Ne556 chips, Crocodile clips etc., etc.

I currently have no idea how to read an electronic schematic diagram (but am reasonably confident that I can learn).

I want to produce an up/down, flashing, LED sign and have attached my own (non-schematic) diagrams of what I hope to achieve.

I have no idea whether this is a simple or a difficult project, I am so “green-behind-the-gills” and do not even understand the advantages or otherwise of running the proposed LED project in either Series or Parallel mode.

I would wish to power the project from either AS/DC adaptor or a 12 volt car battery and only for very short (on-demand) periods of time, similar to how often the button is pressed on a Pelican Crossing, say.

Is anyone able to offer any help and advice, please?

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[pebe]

Should be fairly simple to do.

Have you got the forward voltage drop and recommended current for both color LEDs?

[JibTeenuc]

Should be fairly simple to do.

Have you got the forward voltage drop and recommended current for both color LEDs?

Thanks for replying.

The forward voltage for all LEDs is 3.2 - 3.8v.

Hope that helps

[pebe]

And the recommended current for each colour?

[JibTeenuc]

And the recommended current for each colour?

According to the supplier's literature, 20mA (typical) - 30mA (max).

Ideally, I wish to run from a 12v car battery and have been experimenting with an AC/DC adaptor at everything between 3vDC and 12vDC with some success and appropriate resistors.

[pebe]

OK. I'll do you a circuit.
What is the flash rate to be? 1sec on and 1sec off?

[JibTeenuc]

OK. I'll do you a circuit.
What is the flash rate to be? 1sec on and 1sec off?

Sounds perfect, pebe.

Can't believe someone could be so kind

[pebe]

No problem.
But can you recheck the forward voltage for the red LEDs? They are usually quoted as between 1.8V and 2V.
It makes a difference to how many LEDs can be wired in a string.

[JibTeenuc]

No problem.
But can you recheck the forward voltage for the red LEDs? They are usually quoted as between 1.8V and 2V.
It makes a difference to how many LEDs can be wired in a string.

Yes, the reason for the difference is that I've had a change of heart with regard to the colours and would prefer the red to be replaced by blue and the yellow to be replaced with white.

The Ultra-Bright white and blue LEDs I purchased both state a forwarding voltage of 3.2 to 3.8.

Many thanks.

Jib T.

[pebe]

One more question. You said it would be powered by an AC/DC wall adapter OR a car battery, so I assume it will be used near a mains supply. Working with 12V may be a bit tight, so can it be increased to, say, 15V?

[pebe]

The design is almost finished. Can you tell me about the 15V?

[JibTeenuc]

The design is almost finished. Can you tell me about the 15V?

Sorry pebe, for not replying earlier (prodigal son returned from 2 year around the world trip and not been collecting my emails or otherwise visiting the PC).

A couple of things:

1) I really want the option to run on a 12v car battery (for odd location purposes).

2) If it helps, the LEDs don't actually have to be Ultra-Bright as, whilst they should show up in daylight as well as in the dark, there should be no direct harsh sunlight problems.

Thanks for you help, thus far, which is really appreciated to a newbie such as me.

[pebe]

OK. I'll give you a circuit in the next few days.

[pebe]

Here is the circuit. I suggest using two 11x12 matrices because if the 11x17 display were used, the 5 rows of LEDs that are common to each lit display would require 40 isolating diodes that would be neither practical nor economical.

The circuit to the LH side of the green line is for matrix A. All the components need to be duplicated for matrix B. IC1 and IC2 are both normally reset (off with outputs low). Sw1 then selects which matrix is to be lit by raising the voltage on the reset pin of the wanted one.

The 555 feeds its output to the FET that switches the LEDs. The FET is needed because a 555 can only switch 200mA but the LEDs will take almost 1A. The 555 switches the FET at a rate of 1sec on, 1sec off. That timing period is determined by the values of R2 and C1. Relays could have been used to switch the LEDs but in this application FETs are probably more reliable.

To be economical on battery current, the LEDs are grouped into strings, with the surplus voltage being dropped by R* to stabilize the current. As the blue and white both have the same voltage drop and current, they can be mixed in the strings. I have shown 3 LEDs per string with a 15V supply, because if a 12V supply were used it would be necessary to have only 2 per string, resulting in a battery current increase of 50%.

It would be advisable to test and sort each LED for forward voltage so that R* for 20mA can be calculated for each string, ranging from R* = 220Ω for 3.2V LEDs to R* = 150Ω for 3.8V

To give a good display that doesn’t dim as the battery discharges, then the supply should be stabilized. The easiest way to get 15V from the battery is to use a boost adapter like these. They can be set for a steady 15V, whatever the state of the battery.

http://www.ebay.co.uk/itm/LM2577-DC-DC- ... 53fb047d31

http://www.ebay.co.uk/itm/DC-DC-Convert ... 565bccac29

For mains use you could use any suitable power source to drive the converter.

I hope that is what you wanted.

[JibTeenuc]

Thanks for devoting your time and experience on this for me, pebe (I just wish I knew visualise the physical layout of your schematic, as I don’t currently know how to read a schematic......I better get learning, fast, I guess!!).

Two matrixes as opposed to one is no problem as the total number of LEDs employed are only increased by one row, in this instance, so your advice certainly does seem to make good sense to me.

A couple of questions, or so, if I may:

1) What is a “FET” and where can I purchase them?
2) What effect, if any, on additional LEDs would the failure of any one LED have?
3) One matrix of 11x12=132 /3=44 Resistors. Does each individual Resistor have to be directly linked to a string of 3xLEDs or can the Resistors be placed in-parallel to each other on the control board and the whole row of Resistors then linked to a single point of the light-matrix (if that makes sense)?

All the best.

Jib T