Electronic Circuits and Projects Forum

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.

So true.

Originally Posted by colin55

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

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.

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.

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

18R is far too low.

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.

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

So true.

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!

Originally Posted by colin55

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.

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.

.

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.

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.

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.

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.

Originally Posted by colin55

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.

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.