# 77cm x 50cm LED Signboard

Discussion in 'The Projects Forum' started by ElectricStatiallistic, Sep 19, 2013.

1. ### ElectricStatiallistic Thread Starter New Member

Sep 6, 2013
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Hey there,

This is my very first project where I would like to build a LED sign board with a total of 210 superbright LEDs making up two different words.
Hence, the circuit is basically separated into 2 parts, which I shall name these parts A and B respectively.
Tho, both parts are connected in parallel to the same power supply of 24VDC, 2.1A, also in series with a toggle switch.

5 to 6 LEDs are connected in series with a resistor to form a strand where at the final stage, the strands are connected in parallel to the power supply.
This configuration applies for both A and B.

Circuit A is a simpler one and it is made up of 26 blue superbright LEDs
which I have connected using 4 strands of 5 LEDs and 1 strand of 6 LEDs.
Each strand is connected in series to a resistor of 180ohm.
This part is lit constantly as long as there is power supply.

Circuit B is made such that the remaining 184 red superbright LEDs are blinking by connecting the strands connected in parallel, to a NE555N IC in series.
There are 32 strands of 5 LEDs and 4 strands of 6 LEDs, with each strand connected in series to a resistor of 380ohm.

I did some simple calculations with the assumption that each LED requires 3V and 20mA.
Therefore, a strand of maximum 6 LEDs, will use a forward voltage of 18V with a total of 0.82A to support the total of 41 strands.

Now here is my problem.
When I supply the power, one of the strand of 5 LEDs in part A does not lit, unlike the remaining strands which are lit brightly in blue. I have checked the connections but to no avail.

Next a major problem resides where at part B, all red LEDs are lit dimly with several unlit strands. However, part B still blink accordingly.

Some steps I have made including testing each strand using 2 9V batteries and also testing A and B separately using 3 9V batteries. In both condition, they are working well.

Thank you!!

Last edited: Sep 19, 2013
2. ### ronv AAC Fanatic!

Nov 12, 2008
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It's a little hard to follow in written form, but it sounds like it should work. If you have a volt meter I would start with that and measure down the unlit strings. I'm not sure how you have the 555 hooked up but it is not good for 24 volts.
A little sketch would help.

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3. ### ElectricStatiallistic Thread Starter New Member

Sep 6, 2013
10
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Hi Ronv,

Sorry for not attaching the schematic earlier but here it is.

Since 555 is connected in series with other strands of LEDs, will it not only take a suffice voltage of around 5V leaving the remaining 19V for the strands?

Last edited: Sep 20, 2013
4. ### ScottWang Moderator

Aug 23, 2012
6,589
1,021
NE555 → N-type MOSFET x 2 ← PartA,PartB

NE555 → G N-type MOSFET D ← PartA ← +24V
NE555 → G N-type MOSFET D ← PartB ← +24V

+24V → 78L12 → NE555

PartA : LED * 5 + R, I_LED=20mA x 80% = 16mA, for the LED brightness and using life.
PartB : LED * 5 + R, I_LED=20mA x 80% = 16mA, for the LED brightness and using life.

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5. ### BobTPH Senior Member

Jun 5, 2013
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334
You have some serious misconceptions about electronic circuits.

The diagram you have shown, has all of the strands and the '555 in PARALLEL, not in series. Parallel circuits across a power supply all see the same voltage.

Furthermore, you do not put circuits in series to reduce the voltage, except in cases where they always draw the same current.

You will need a voltage regulator to reduce the voltage for the '555.

You should calculate a resistor for each string of LEDs based on 24V as the input, your string of 5 and 6 will have different resistors, and red vs blue will need different resistors.

And finally, the blinking LEDs must be driven by a transistor switch (MOSFET preferred) driven by pin 3 from the '555.

Edit: I think ScottWang was trying to say basically the same thing, but even I don't understand his post.

Bob

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6. ### ElectricStatiallistic Thread Starter New Member

Sep 6, 2013
10
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Hi Bob,

This should be the correct schematic. Sorry for the previous one as I have forgotten to check.

I will try reworking on the resistors and add in a voltage regulator.
Will update as soon as possible.

Thank you very much!!!

7. ### BobTPH Senior Member

Jun 5, 2013
1,449
334
That is a lot better. But, in addition to lowering the voltage on the NE555, you will have to use a transistor to drive the blinking LEDs since they will no longer get 24V.

Bob

8. ### ElectricStatiallistic Thread Starter New Member

Sep 6, 2013
10
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Gotcha.

So in a nut shell, resistors, voltage regulator and a transistor.
May I know one of the possible transistor product codes?
Sorry I am quite a newbie in these stuff.

9. ### ronv AAC Fanatic!

Nov 12, 2008
3,657
2,817
A little 2N7002 would work good for the switch. Tie the resistors at the top to 24 volts and put the transistor between ground and the bottom side of the LEDs. The gate of the FET now goes to the output of the 555.

Ooops, my bad, I forgot there were 30 strings. Let me find you a bigger one.

How about an IRF510. You can get them almost anywhere.

Last edited: Sep 20, 2013
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10. ### wayneh Expert

Sep 9, 2010
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Yup. I use the beefier IRF540N but the 510 is at the shack.

Pin 5 of the timer should not be left floating open. I believe a small capacitor (0.1µF) to ground is the typical connection.

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11. ### wayneh Expert

Sep 9, 2010
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Have you verified that the bad string can be made to light up when it's not part of the system? A single bad component in a series string makes the whole string go dead. As suggested, tracing voltage down the string will show where the break is.

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12. ### ElectricStatiallistic Thread Starter New Member

Sep 6, 2013
10
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Thank you ronv and wayneh for transistor suggestions.

About that, I have yet to find the problem source.
Will have to try it out again later as I have left my workshop. Hahas.
Will let you guys know as soon as possible.

Much appreciated!

P.S. Have to hunt some new components too~

13. ### ElectricStatiallistic Thread Starter New Member

Sep 6, 2013
10
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1. Part A is settled... The cause was some accidentally burnt LEDs.
2. Part B is now blinking after I added a voltage regulator, L7812 and a power transistor, IRF540, with decent brightness however I have another problem.

The circuit is as shown.

I have connected a 47ohm, 5W resistor to the IRF540 because previous resistors with lower Watts heat up to a very high temperature. Using only 47ohm, I managed to supply a suffice power to lit up all the strands. However, this resistor still heats up to a temperature of 50 degree Celsius.

May I know if this is normal or is there something wrong with the circuit?

14. ### wayneh Expert

Sep 9, 2010
15,872
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The "ground" end of the LEDs should be attached to the drain pin of the MOSFET, and the other end should be attached to 24V via the current-limiting resistor. As drawn, the MOSFET is in parallel with the LEDs. When it conducts, it shorts out the LED strings. It works to turn them off, but is very wasteful and makes the resistor hot.

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15. ### ElectricStatiallistic Thread Starter New Member

Sep 6, 2013
10
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Thank you Wayneh!

If I am not wrong, it should be looking like this.

Am I correct? The 5W resistor should be gone right?

Edit : 1 - G, 2 - D, 3 - S

16. ### wayneh Expert

Sep 9, 2010
15,872
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Leave the resistor in for safety, to limit current to the LEDs, unless and until you are certain it is not needed. Very simple to remove once you are sure. If you are wrong, though, you'll burn up all your LEDs.

Otherwise looks good. That #5 control pin still needs a capacitor.

17. ### ElectricStatiallistic Thread Starter New Member

Sep 6, 2013
10
0
I have applied the configuration including a capacitor to pin 5 of NE555.

However, the circuit does not work now.
No LED are lit nor blinking.

Could it be the IRF540?
Or is it that the drain voltage is insufficient?

Last edited: Sep 21, 2013
18. ### ScottWang Moderator

Aug 23, 2012
6,589
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The LED should be in series with current limiting resistor.
The watts of current limiting resistor should be great than 5 times of calculation value.

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19. ### wayneh Expert

Sep 9, 2010
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It's easy to get the pins wrong. Check and check again. And then assume you did it wrong and check it again.

Also for a quick check, make sure the LEDs still light if you bypass the MOSFET. Another check is to watch the voltage on the gate pin (the timer output) to make sure it's still on and off.

You can also check the MOSFET by touching the 1k gate resistor to +12V instead of to the timer output. Should turn on the LEDs.

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20. ### ElectricStatiallistic Thread Starter New Member

Sep 6, 2013
10
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Erm, each LED strand has 1 current limiting resistor in series with 5 LEDs maximum.
Hence, the "LED strands" in my schematic shown above are actually all these LED strands connected in parallel.

Okay, Wayneh. I will try to do that and update again. Thanks a lot!