Toggle LED's between blink and solid

Hello, I am looking to flash 36 LED's, actually want to use a switch to flash them with the other state being solid. I've gotten a start from this forum and have a 555 timer. I'm not skilled with this but it appears I need a SPDT switch where one side will send the power through the 555 timer and on to the LED's, and the other side of the switch will bypass the 555 timer all together? (I thought the other side should go thru another 555 timer in monostable mode but this seems to only be for a timed period)

The LED's are 5mm, 3.8v and 30mA. I've figured out the parallel setup and resistors to use.

Next issue is hooking up the 555 timer for astable state - any suggestions on where I can learn how to do this. I've looked at the schematic but need some training.

Thank you,
Frank

36 leds @ 30ma each wired in parallel is a little over an amp, the 555 won't run that directly. Reaching over into a parts drawer labeled "drivers" beside my desk and blindly grabbing what's on top, I come up with an NDT3055L. This is an N-channel MOSFET that can handle up to 4A at up to 60V, so it can easily handle the power, and I think it must be pretty cheap or I wouldn't have bought so many of them. It also has a low Vgs, so it can easily be turned on by simply connecting the gate directly to the output of the 555, which I happen to remember is pin 3. The "source" of the NDT3055L will be connected to ground, the "drain" will be connected to the cathode (-) side of your led/resistor combination (the anodes would be connected to your positive supply voltage). To bypass the driver and just keep the 36 leds "on" you would wire your switch across (in parallel) with the Source and Drain. It's safe to do it here, but don't try it on the 555 output, it has a "totem pole" drive that will be shorted.

You say you have a schematic of the 555, you know you want astable (blinking) mode, but need some help. What is it you need? Suggestions on assembly? I prefer to use "perf" board, the kind that has the little copper donuts around each pad, which you can get at Radio Shack in various sizes. Bend the wires on the components, poke them through the holes, solder according to the diagram, etc.

Is it the frequency you are confused about? In the spec sheets there's a chart with some diagonal lines, it will have a name like "timing combinations of R and C". Follow the lines along that chart till you find a frequency you like (4Hz makes a nice blink) and it will show you some combinations of the timing resistor and capacitor to get the 555 to run at that speed.

Hi Frank,

here is your circuit.

You might connect three LEDs in series and get a total of 36 LEDs making 12 strings.

3.8V forward voltage per LED will require a total of 11.4V per string. Providing 15VDC at a total current of 360mA is used by the LEDs. Add at least 40mA for the timer and the darlington transistor arrays which you will need two of them. (A15VDC/500mA wall wart should suffice.)

The circuit contains three strings out of U1 connected to LEDS and one string out of U2 also connected to LEDs.

Fill the gap with identical numbers of LEDs + the current limiting resistors of 18Ohm.

The astable multivibrator circuit around IC1 provides frequencies of 1 to 10Hz. Flash rate can be changed by RV1.

Boncuk

Why create such a complex circuit????
Yellow LEDs don't drop 3.8v.
30mA per string is too high. LEDs are designed for 17mA to 20mA.
A 555 on 15v can deliver 400mA via a simple transistor buffer.
18R is far too low.
Add a sw across the collector-emitter to create FULLY ON.

I spread chaos where ever I am.

Click to expand...

So true.

colin55 said:

Why create such a complex circuit????
Yellow LEDs don't drop 3.8v.

Click to expand...

When simulating LED circuits any colour will do! It's just a matter of assigning properties to a part.

But may be this is beyond your horizon.

30mA per string is too high. LEDs are designed for 17mA to 20mA.

Click to expand...

Where did you learn that?

Superbright LEDs might use a forward current of up to 50mA. Just look into http://www.kingbright.com

A 555 on 15v can deliver 400mA via a simple transistor buffer.

Click to expand...

Very interesting! I bet all forum member would like to learn from you how to do that.

18R is far too low.

Click to expand...

You're so right! The current will be 30.1mA per string - much too high!

Add a sw across the collector-emitter to create FULLY ON.

Click to expand...

A switch across 12 collectors and the (common) emitter will certainly do the expected action.

So true.

Click to expand...

You certainly don't want me to tell the truth I found out from the University of Sydney, do you?

Sitting in a green house and trowing rocks is not the best advice!

I have been a teacher of electronics for more than 30 years.
You have made so many mistakes in the above reply.
Firstly, you are not simulating a LED circuit. You have specified a yellow LED as 3.8v. This is not true. Secondly, you have specified the dropper resistor as 18R.
It is very dangerous to use a resistor less than 100R as LEDs have a wide margin for their "characteristic voltage-drop" and any variations will be easily "eaten" by the small voltage you have allowed by the 18R.
I use 20,000 LEDs at a time and for long term performance, most common LEDs are best run with a current of about 17mA to 20mA. 25mA shortens their life and 30mA is damaging.
You can see my website has dozens of circuits driving a set of LEDs via a 555 and a buffer transistor.
You circuit is grossly over-designed and providing ill-information.

colin55 said:

I have been a teacher of electronics for more than 30 years.

Click to expand...

You have made so many mistakes in the above reply.
Firstly, you are not simulating a LED circuit. You have specified a yellow LED as 3.8v. This is not true.

Click to expand...

Please have the kindness to let me know about the mistakes I made. I can't improve being left in the dark by just stating I made mistakes.

Please refer to the original post:

The LED's are 5mm, 3.8v and 30mA.

Click to expand...

.

In a simulation I can "promote" any LED for those specifications.

Secondly, you have specified the dropper resistor as 18R.
It is very dangerous to use a resistor less than 100R as LEDs have a wide margin for their "characteristic voltage-drop" and any variations will be easily "eaten" by the small voltage you have allowed by the 18R.

Click to expand...

Have you taken into consideration that darling transistors cause additional voltage drop?

I use 20,000 LEDs at a time and for long term performance, most common LEDs are best run with a current of about 17mA to 20mA. 25mA shortens their life and 30mA is damaging.
You can see my website has dozens of circuits driving a set of LEDs via a 555 and a buffer transistor.
You circuit is grossly over-designed and providing ill-information.

Click to expand...

Maybe it is grossly over-designed, but that would be the way to use those 36 LEDs. You are welcome suggesting your design instead of talking high tones here!

Before going any deeper into the subject you should reread the original thread.

The OP stated these LED data:

The LED's are 5mm, 3.8v and 30mA.

Click to expand...

If you're stuck with an ancient LED forward current you should take a look into high current LEDs designed for room (even street) illumination. They are rated up to 10W!

5mm LEDS are not rated at 10W.
When designing a string, you need to allow at least 1v to 2v drop across the dropper resistor to allow for the variation in tolerance of the "characteristic voltage-drop" and the possible increase in supply voltage.
The OP is simply specifying maximum values. He does not understand the recommended or preferable current for a 5mm LED.
You simply need a buffer transistor to drive the LEDs. You are replying to a beginner and offering 3 IC's whereas a single IC and a transistor is all that is needed.
"Have you taken into consideration that darling transistors cause additional voltage drop?" The discussion has nothing to with Darlington transistor voltage-drops.
You have specified a 15v DC plug-pack. Do you realise the no-load output voltage of these devices is about 5v higher than 15v. You are simply zenering the LEDs with the supply voltage and they are sure to be damaged.

I'm confused, is it a minimum of 100Ω which gives a 3V drop at 30mA or is it a minimum of 1V which requires a 33Ω resistor at 30mA?

Mike.

colin55 said:

5mm LEDS are not rated at 10W.

You have specified a 15v DC plug-pack. Do you realise the no-load output voltage of these devices is about 5v higher than 15v. You are simply zenering the LEDs with the supply voltage and they are sure to be damaged.

Click to expand...

Sure!

It puts out the nominal voltage at nominal current. No matter if the LEDs are blinking or steady on - the current will be the same as soon as power is required.

It puts out the nominal voltage at nominal current. No matter if the LEDs are blinking or steady on - the current will be the same as soon as power is required.

Click to expand...

I don't know what you are talking about.

A 15v plug pack can deliver anything from 13v to 17v when 300mA to 400mA is being delivered.
You have absolutley no idea what current will be taken by the LEDs when you have allowed only an 18R current limit resistor.
An 18R resistor only drops about 0.5v while the supply can vary by as much as 4v.
In this case you need to allow 5v across the resistor to prevent excessive current being drawn.

Surely, everyone now buys SMPS plug packs. These give a very regulated voltage.

BTW, I'm still confused about my earlier question.

Mike.

Gentlemen, Thank you for your comments to this post. I have a lot of respect for your knowledge and strong opinions on how best to accomplish what I'm trying to achieve. May I ask for a bit more focus on how best to achieve my objective vs. challenging each other's skill sets. All are clearly strong.

My light design is for a hobby project I'm developing for some nephews. I will actually need four identical circuits each with their own color LED. They will be driven by a 12v, 16amp power supply from an older personal computer. It's been modified to run 12v and 5v. And it works fine running regular low voltage lights. I've burnt a few led's gaining understanding about needing resistors. I am a novice (clearly), that's why I'm here (for some homework).

I am looking at superbright leds and have read where running full 30mA will shorten their life, so it is my expectation to run the superbrights as 20-25mA (if 25 is even possible). Superbrightled.com has a nice wizard that tells me using source voltage of 12v, diode fwd voltage of 2v (for the red and yellow leds), 20mA and 36 LED's, I should set up a 6x6 array with a 1ohm resistor. Same result if I enter 30mA. For Blue and Green the diode fwd changes to 3.8v and a 3x12 array is presented with 33ohm resistors for 20mA and 22ohm with 30mA leds.

It seems like a sweet wizard for setting up a simple on/off string of 36 leds. But I desire to toggle them to a blinking state on when needed, so their results may be all for naut when introducing the 555 timer and various other components into the circuit.

I'd love to learn how to make my design, but I'm not sure I'll have the time to fabricate a working circuit in the near term (next 30 days), but am willing to learn over the longer term - and that is why I'm here. It'd be nice to find someone to pay to make the circuit, but that seems to be a challenge as well. Although I'd like to learn some things going along.

Duffy, your reply seems the simplist but (for me) and I shall pursue the information further.
Boncuk - your circuit diagram is elegant and I am very thankful you shared as it looks very informative and functional.
colin55 - i'd need more specifics to follow much of what you're commenting on.

If any of you would like to PM to further assist I'd appreciate your guidance. And thank you for taking such an interest.

Regards,
Frank

Nearly all 5mm LEDs have their lifetime and brightness spec'd at 20mA, not 30mA.

Hi all,

this is the information given in post #1:

The LED's are 5mm, 3.8v and 30mA.

Click to expand...

Am I wrong taking that info for LED specification?

Frank88, it would really be helpful to get the real specification of the LEDs you will be using.

With a forward voltage of 2V up to 5 LEDs could be used for one string and a supply voltage of 12V.

Boncuk

The LED's are superbright: Green 3.3V fwd voltage with max fwd voltage of 3.8V; pwr dissapation 120mW; continuous fwd current 30mA with peak fwd current of 100mA
The blue have same spec's. The red are 2V / 2.4V / 85mW / 30mA / 100mA and the yellow is same as red. I may have suggested using 3.8V for blue and green earlier, where 3.3V s/b used.

I've used the LED wizard from LED Center to get a 3x12 array with 120ohm resistors for the blue and green using 20mA instead of 30mA. I keep hearing to use 20mA? And an array of 6x6 with 1ohm resistors for the red and yellow lines, also using 20mA.

Also, I already have a 12v, 16 amp power supply from a personal computer dedicated for the four light strings. And each light will be spaced about 3 feet apart in the application.

Frank

And an array of 6x6 with 1ohm resistors

Click to expand...

You are going back to the very point that I tried to tell you was a dangerous condition.
Didn't you read my discussion??????

You calculated a current-limiting resistor as 33 ohms for three 3.8V LEDs in series with a 12V supply which is 18.2mA.
But the LEDs are "typically" 3.3V, not 3.8V, the "12V" might actually be 5% high at 12.6V and the 33 ohm resistor might be 5% low at 31.4 ohms. Then the current will be 86mA and the LEDs will burn out in seconds.

The problem is that you do not have enough voltage across the current-limiting resistor to smooth the variables.

Now try only two LEDs in series which allows more voltage across the resistor:
1) If they are 3.8V and have a 12.0V supply then the resistor value is 220 ohms for 20mA.
2) If the LEDs are actually 3.3V, the power supply is actually 12.6V and the resistor is actually 209 ohms then the current will be a little high at 29mA and the LEDs will not burn out.

The value of the current-limiting resistor should be calculated for the "typical" forward voltage, not for the maximum forward voltage.

The last two posts are correct. More series leds and a smaller resistor seems very efficient, and convenient, but in practice you have a "tolerance stack" problem.

Thanks AG, I initially used the wrong voltage for the diode, as you point out it is 3.3V. The P.C. power supply is probably 12.6V. When I run through the wizard I get the following:

Blue and Green superbrights:

Source voltage - 12.6V
diode forward voltage - 3.3V
diode forward current (mA) - 20mA
number of LEDs in your array - 36

Solution 0: 3 x 12 array uses 36 LEDs exactly
R = 150 ohms for each of the 12 parallel series

The wizard says: In solution 0:
•each 150 ohm resistor dissipates 60 mW
•the wizard thinks 1/4W resistors are fine for your application
•together, all resistors dissipate 720 mW
•together, the diodes dissipate 2376 mW
•total power dissipated by the array is 3096 mW
•the array draws current of 240 mA from the source.

Red and Yellow superbrights:

Source voltage - 12.6V
diode forward voltage - 2.0V
diode forward current (mA) - 20mA
number of LEDs in your array - 36

Solution 0: 6 x 6array uses 36 LEDs exactly
R = 33 ohms for each of the 12 parallel series

The wizard says: In solution 0:
•each 33 ohm resistor dissipates 13.2 mW
•the wizard thinks 1/4W resistors are fine for your application
•together, all resistors dissipate 79.2 mW
•together, the diodes dissipate 1440 mW
•total power dissipated by the array is 1519.2 mW
•the array draws current of 120 mA from the source.

Thanks,
Frank

Calculations for source voltage should consider the 5% tolerance then?