A super-bright marking strobe that pushes the boundaries of its LED.

Strobe warning devices are pretty common around us once you're aware of them. Many are just steady flashing lights with an on-time that's in the half to one second range, and a similar off time. Think of the red flashing lights on construction site cranes or communication towers anywhere that an aircraft is likely to be flying: Many of those are just a slow, steady flash. The most famous is probably 'Blinky Bill', the red flasher on top of the Sydney Harbour Bridge.

However, in some situations, a much brighter strobe is needed. Some time ago, I was on holiday on the Sunshine Coast in Queensland. There were plenty of red flashing lights on top of taller buildings and communication towers, and a lot of light generally. There was a hospital helicopter flight path as well as a regional airport for light aircraft, both of which create a need to mark hazards.

However, I noticed many of the construction cranes instead had bright white or red strobes on them, much brighter than the regular flashing or blinking lights, but with a much shorter flash and a longer gap between flashes. The purpose here is to stand out from the mass of regular lights, and to be visible from a greater distance. This is important when a change is made, like a temporary construction crane, when regular pilots may be accustomed to the location of hazards they have had to avoid before. I have also seen them on communications towers in some situations, like when a tower or two are set away from a cluster of others, or where there might be lots of urban lights behind them from a pilot's view.

Short, bright strobes are also used in other situations. The military use them for personal locating, and anyone who has seen a modern war film may be familiar with their use to mark locations for air support or helicopter pickups. Life jackets for use in open water often have such a strobe, too, as its short, bright flash is visible over greater distances and stands out more than a slower, weaker flash would. Hikers going into remote places may also carry similar strobes. The photo here is of a life jacket strobe in a rescue training mission, not a real rescue.

Regardless of their use, strobes like this can also just be fun to make. We will use a familiar circuit, the NE555-based astable multivibrator, but alter its behaviour and make use of some of its handy features. We will use this to teach about LEDs and how they are normally used, compared to how they can be used carefully to gain much more brightness than normal. You can use your strobe just for fun, or for more practical stuff like marking guy ropes on a tent or some other hazard.

SOME HELPFUL HINTS

We encourage you to read all the way to the end of the article before you build. Not only will you then have a better feel for the overall picture as you build, but we sometimes discuss options or alternatives that you will need to have decided on. You will need some basic hand tools for most builds. Small long-nosed pliers and flush-cut side cutters meant for electronics are the main ones. Materials like tape or glue are mentioned in the steps, too. We always produce a tools materials list if you have to go shopping, but anything that is lying around in most homes is just stated in the steps.

As always with Kids' Basics, we avoid soldering to make the build more accessible to more people, but having an adult around can still be helpful. You won't need any particular skills besides being able to identify components at a basic level, and even then, we help as you go along. If, for example, you don't already know what a resistor is, you'll probably be able to work it out from the photos and description in each step.

We do provide a schematic or circuit diagram but this is just helpful if you already know how to read one. Don’t stress if you have never learned, but take the chance to compare the digital drawing of the breadboard layout (which we call a 'Fritzing' after the company that makes the software) to the schematic and see if you can work some things out. You can make this project from the Fritzing and photos alone. You might also like to check out our Breadboarding Basics from Issue 15.

The Build:

Parts Required:	ID	Jaycar	Altronics	Pakronics
1 x Solderless Breadboard	-	PB8820	P1002	DF-FIT0096
1 x Packet Breadboard Wire Links	-	PB8850	P1014A	SS110990044
1 x 36Ω Resistor *	R5	RR0537	R7523	DF-FIT0119 %
1 x 62Ω Resistor *	R3	RR0543	R7529	DF-FIT0119
1 x 220Ω Resistor *	R4	RR0556	R7542	DF-FIT0119
1 x 2.2k Resistor *	R2	RR0580	R7566	DF-FIT0119
1 x 56kΩ Resistor *	R1	RR0614	R7600	DF-FIT0119 %
1 x 100nF Capacitor *	C2	RM7125	R3025B	DF-FIT0118
1 x 47µF Capacitor *	C1	RE6100	R5102	DF-FIT0117
1 x NE555 Timer IC	IC1	RE6100	Z2755	-
2 x Very High Brightness LED, see text. #	LED1, LED2	ZD0292	Z0865C	-
1 x 9V Battery Snap	-	PH9232	P0455	DF-FIT0111
1 x 9V Battery	-	SB2423	S4870B	PAKR-A0113

* Quantity used, item may only be available in packs. # LED requirements vary depending on build choices. % 56kΩ and 36Ω not in pack, use 62kΩ and 39Ω

We will build the basic circuit in steps 1 to 6, then add a few more steps to make some alterations. These are optional. The first will be to add another LED at its normal brightness, so you can compare the two. The second will be to add another LED in the strobe section so there are two flashing really bright together. Therefore, steps 7 and 8 are not one after the other, but independent. In other words, choose step 7 or step 8, but not both at one time. You will see in step 8 that we removed the components from step 7. Also, we have high-brightness LEDs in the parts list, which are water clear, but we swapped these in the build for coloured case low-brightness ones so you can see them on camera.

After the build, there is a section about testing and what to expect. Please read this before actually building. Also, there is a section about choosing LEDs. This is important because you might need different resistor values to the ones we used. In the 'How It Works' section, we explain how to calculate the resistor you need, and give a table. However, if you don't feel like the technical explanation of the whole circuit is for you, look for the heading 'Ohm's Law' and just read that bit.

Step 1:

Place the breadboard in front of you with the outer red (+) rail facing away from you and the outer blue (-) rail closest to you. Add the wire links which join the upper and lower red (+) rails together, and the upper and lower blue (-) rails.

Step 2:

Install an NE555 timer IC into the board with its notch or dot marking pin 1 facing to the left. Add three wire links. One goes from the upper red (+) rail to pin 8. One goes from the lower red (+) rail to pin 4, and the last goes from the lower blue (-) rail to pin 1.

Step 3:

Place three wire links around the NE555 to join pin 2 to pin 6. Also place one more link, from pin 6 off to the right. Also add the battery snap now, with its red wire to the upper red (+) rail and its black wire to the lower blue (-) rail.

Step 4:

Insert a 56kΩ resistor (GREEN BLUE BLACK RED SPACE BROWN) between the upper red (+) rail and pin 7 of the NE555. Add a 2.2kΩ resistor (RED RED BLACK BROWN SPACE BROWN) from pin 7 to the end of the wire link shown.

Step 5:

Add a 100nF capacitor (104 or 100n) from the upper blue (-) rail to pin 5 of the NE555. We used an MKT capacitor but any capacitor of the right value will work. Also, add a 47µF Electrolytic capacitor with its negative striped leg in the lower blue (-) rail and its other leg to pin 2 of the IC.

Step 6:

Place a 62Ω resistor (BLUE RED BLACK GOLD SPACE BROWN) from pin 3 of the NE555 off to the right, and a high-brightness LED with its short cathode (-) leg to the resistor. Add a wire link from the lower red (+) rail to the long anode (+) leg of the LED.

Step 7: NORMAL BRIGHTNESS LED OPTION

Insert a 220Ω resistor (RED RED BLACK BLACK SPACE BROWN) between pin 3 of the NE555, and the left of the board. Add another LED, with its long anode (+) leg to the resistor and its short cathode (-) leg into the next row. Finally, install a wire link between the short anode leg and the lower blue (-) rail.

Step 8 TWO HIGH BRIGHTNESS LEDS OPTION:

Remove the 62Ω resistor (BLUE RED BLACK GOLD SPACE BROWN) and replace it with a 36Ω resistor (ORANGE BLUE BLACK GOLD SPACE BROWN). Remove the wire link at the other side of the LED. Add another LED, with its short cathode (-) leg to the long anode (+) leg of the first LED. Replace the wire link, this time from the long anode (+) leg of the new LED, down to the lower red (+) rail.

TESTING AND WHAT TO EXPECT

All you need to do is connect a 9V battery to the snap, and you should see light. The first flash will take around three to four seconds, while the flashes after that should happen just short of every two seconds. If there is no light, disconnect the battery and carefully check the positions of all the components and wire links. Also make sure the LED and electrolytic capacitor, and NE555 for that matter, are the right way around.

If you chose to stop at step 6, you will see a short, sharp, bright flash almost every two seconds. If you went to step 7, you will have one LED which is on at standard brightness for most of the time, then it will turn off while the other LED flashes very quickly. If you move these two LEDs around a bit in the breadboard (just tip them, don't unplug them), then you should be able to get both LEDs to shine on one part of the roof in a darkened room. This will let you see the difference in brightness because the spots overlap - during the short flash, the light should be much brighter.

If you chose step 8, then you have two LEDs which should flash together. This will mean double the brightness of a single LED, and is getting to the point where you should be careful not to look straight into the top of the LEDs where the light is the strongest. Looking at them from the side will be on for most people, because LEDs focus most of their light into a narrow(ish) beam.

CHOOSING LEDs

The LEDs we used are slightly unusual, which means you might need different resistors for yours. All LEDs have a 'Forward Voltage' or voltage drop, to make them work. Many start emitting light below this figure but it is really considered the operating voltage. However, LEDs do not have enough internal resistance to stop too much current passing through them. So, if you have a 3V LED and put it across a 3V battery, it will burn out. It may do so straight away, or it may take many hours, but it will eventually destroy the LED.

Most red and yellow LEDs have a forward voltage of 1.6V to 2V. Most green, blue, and white LEDs have a forward voltage of 3V to 3.5V. They also have a current limit. Most examples of any colour are between 20mA and 30mA. This is normal, but low-brightness standard diffused types are often 10mA or so. We even have some very old high-efficiency types of panel-indicator brightness that run on 2mA. We'll go into detail about why this matters in the 'Ohm's Law' heading in the next section.

While the main retailers have a good selection of LEDs, if you really want choice, be sure to check out
LEDsales.com.au. Run as an independent business from Tasmania, this company has a huge array of LEDs of all types and price ranges. You can get some very specialised or very bright LEDs in a colour of choice. There is a minimum order of $10 but it won't take you long to find things worth buying to that amount!

HOW IT WORKS

THE DRIVE CIRCUIT

The drive circuit is based on the NE555 timer IC, one of our favourite devices in Kids' Basics. It can run in either monostable mode, where it is triggered externally, times onces, then resets for the next external trigger; or it can be used in astable mode, where it triggers itself over and over. For our flasher, we're using astable mode.

Before explaining astable mode, we need to know what's inside the NE555. There are three equal resistors, connected in series from the supply voltage to ground, forming a voltage divider. A voltage applied across resistors in series divides across them depending on their value (the amount of resistance). These resistors are equal, so the voltage is divided equally across them. That means one third (one part out of three equal parts) across the first, one third across the second, and one third across the last one.

The junctions of the resistors, giving a voltage of one third and two thirds of the supply voltage, are connected to comparators. These are devices which compare two levels, one at each of their inputs. One comparator has an input connected to the one third voltage point on the voltage divider, and its other input connected to pin 2. The other comparator has its input connected to the two thirds voltage point on the divider, and its other input connected to pin 6.

The outputs of the comparators are connected to another device, called a flip flop. This has two inputs and one output. The output is either high (on) or low (off) depending on what state the inputs are in. The output of the flip flop is connected to a transistor, and an inverter, An inverter just changes its output to be whatever the opposite of the input is. The transistor is connected between the discharge pin 7 and ground.

In astable mode, both pin 2 and pin 6 are used. Pin 2 is the trigger voltage, and pin 6 is the threshold voltage. When power is first turned on, current flows through R1 and R2, to charge the capacitor C1. The voltage across this capacitor is measured by pin 2 and pin 6 feeding their comparators. At first, the flip flop inside has its output low. This means the discharge pin 7 transistor is off, but the inverter means the pin 3 output of the NE555 is high.

So, as the voltage on the capacitor rises as it charges through R1 and R2, it goes past the one third mark, and on to two thirds. When it reaches two thirds, the comparator connected to pin 6 changes its output. This triggers the flip flop to change. Its output goes high, turning on the discharge transistor and connecting pin 7 to ground. But, the inverter after it means that now the pin 3 output is low. Now, the capacitor discharges through R2, through pin 7, to ground inside the IC. As it does so, the voltage across the capacitor falls. When it reaches one third of the supply voltage, the comparator connected to pin 2 activates and changes the flip-flop back, turns off the discharge transistor, and turns on the pin 3 output, starting the process again.

Because R2 is so much smaller than R1, and the capacitor only discharges through R2, it discharges very quickly. That's why we see such a short flash. Charging is much slower because it happens through R1 and R2 together. However, we have just said that pin 3 is high during charging, the long bit, and low during discharging, the short bit. So why do we see a flash? Shouldn't it be the other way around?

The NE555 has an output pin that can source and sink current. That means that when high, it can output current from the supply voltage (sourcing) and when low, it can carry current back the other way to ground (sinking). Not many ICs do that. Many have an active high output, where the output sources current when high, but is otherwise a dead-end. There are some active low ICs too, where they sink current when active but otherwise do not source it, they are just a dead end again.

That is why we connected the LED to the lower red (+) rail, so that current flows from the supply, through the LED and its current-limiting resistor, and then to ground through the NE555 when its output is low and sinking current. The NE555 can sink and source about 200mA of current.

In step 7, we connected an LED with a bigger current-limiting resistor the other way, where current flowed from the output pin 3, through the LED, to the lower blue (-) ground rail. This shows the much longer high time for the output when charging, compared to the short discharge low time.

If you want a deeper explanation of all of this, check out Classroom 58, where we took a deep dive to celebrate the 50th birthday of the NE555.

LEDs AND CURRENT LIMITS

As we mentioned above, all LEDs have a forward voltage at which they should be operated (they can run below this but not above) and a rated forward current. These figures are for maximum brightness, and you can run an LED on less voltage or current for reduced brightness. However, the rated forward current is for continuous operation. It is actually possible to allow much more current through the LED, for short periods of time. In this circuit, we are running the LED with a forward current of 100mA. That is the maximum 'Peak Forward Current' from the datasheet for our LED.

There are challenges to this. The LED cannot be on for long. Generally, the LED is pulsed at over one thousand times per second, and it is on for only one tenth of the pulse cycle, and off for the other nine tenths. That is called 'Duty Cycle', the term for how much on time there is compared to off time in a cycle. This is a 10% duty cycle, meaning the LED is on for 10% of the time, and off for 90% of the time. If you are a younger reader and have not done percentages at school yet, you might like to check out our Classroom on voltage dividers in issue 51. In that, we start by explaining division and fractions, then move on to percentages before explaining voltage dividers.

Each LED's datasheet has this information in it, if your LED has a datasheet. Unfortunately many available in retail stores or online do not, so the information is not always available. However, we have found very few LEDs which differ from this. Most seem to be around 80mA to 100mA Peak Forward Current, regardless of if their constant current is 20mA or 30mA and no matter their forward voltage.

The problem is, we're pushing the boundaries here. Our duty cycle is under 10%, but the on time is still much longer than it really should be. It's around seventy-two milliseconds, whereas at 1000Hz (one thousand cycles per second) at 10% duty cycle, the LED would be on for one hundred microseconds, or 0.1ms (milliseconds). The longer the LED is on at high current, the more heat is generated, and heat is the enemy. Heat is what determines the current limits in the first place. Still, we ran our prototype continuously for over a week and had no failure. It will reduce the life of the LED, but not enough to worry. A normal LED has a rated life of around 100,000 hours. Even if running it hot reduced this to 10,000 hours, it will still outlast your need for the project!

If you built step 7 with the other LED and resistor for the 'high' part of the NE555's cycle, you will have noticed that the resistor was much larger than the LED we fitted for the short flash. This is because the second LED is running at its regular forward voltage and current.

SERIES VS PARALLEL LEDs

If you built step 8, we reduced the value of the resistor. Any resistances connected in series add up. The voltage drops of the LEDs do, too. But in series, the current stays the same through the whole line, which we call a branch. Because of that, instead of a 2.7V drop, we have a 5.4V drop.

If using lots of LEDs for an application, it is more efficient to run them in series from a higher voltage with one resistor. All voltage dropped across a resistor is wasted as heat. It's still power used, it's not like turning a tap on slowly where the water is just held back. So, if connecting LEDs in series, add up their voltage drops and use a resistor as small as possible with as many LEDs in series as the supply voltage allows. You can also add lots of series groups in parallel. This diagram is for 3V LEDs at 20mA each on 12V. Note that because they will not limit their own current, we still need a resistor and three LEDs even though four LEDs would equal 12V.

You can also have LEDs in parallel with a resistor on each. In parallel, the currents add up. So, if you have four 30mA LEDs in parallel, you will need 120mA. This is less efficient but sometimes has to be done, like if the supply voltage is too low to allow series connections. The one thing you cannot do is have lots of LEDs in parallel and one resistor for the lot. Yes, you can add up the current used by each resistor, and calculate a resistor to drop the correct voltage and supply the needed current. The trouble is, the LEDs do not share equally. We still have the problem of them not current-limiting themselves. The first LED will absorb much of the current and burn out, then the next. On top of that, the voltage drops are never quite the same, so one LED will always take more of the current even if it is not first in line.

OHM'S LAW

If your LEDs are different to ours, you are going to need to calculate the correct resistor values for them. We do that with Ohm's Law. Ohm's Law describes the relationship between voltage, current, and resistance, and it's a fixed relationship. For a given resistance, if the voltage dropped across it increases, the current going through it decreases. We can use this to limit the current through the LED to stop it self-destructing.

The voltage we need to drop across the resistor is the supply voltage, minus the LED forward voltage. This is why we need to recalculate: As noted before, LEDs can have a forward voltage of anywhere from 1.6V to 3.7V depending on the colour and manufacturer, and that makes a big difference to the current for the same resistor. Start by subtracting the LED forward voltage from the supply voltage. If you do not have the datasheet for your LED, then the supplier's catalogue/product listing should still state the forward voltage and forward current.

In our LED's case, the forward voltage is 2.7V. We need to subtract this from the 9V battery supply voltage to leave us with 6.3V.

Now, we need to divide voltage to be dropped (6.3V) by the desired current to go through it. For our short. Bright flash, that's 100mA. Calculations in electronics formulae, including Ohm's Law, are always done in 'base units'. That means Amps, Volts, Ohms, Farads, and so on. Any milli, micro, kilo, mega attached are multipliers. For example, a milliamp is one one-thousandth of an amp, so 100mA becomes 0.1A. If we divide the numbers, we get 6.3V ÷ 0.1 = 63Ω. Whenever we divide by less than one, we end up multiplying. We would love to explain that but it's a pretty big concept that a few lessons are dedicated to across your schooling years, so we can't fit it here.

63Ω is not a standard value. If you look at the resistors available from your local electronics store, you'll see we have 62Ω or 68Ω, at best. Some ranges have even less values. It is normal practice to choose the next bigger value, so that you don't exceed your calculated current. However, the difference is only 1Ω and there are small losses inside the NE555 that we did not calculate for, so we went for 62Ω.

There is an easier way to remember and use Ohm's Law if maths isn't your natural thing, and that is by putting it in a triangle. You can remember that V goes at the Very top, and R goes to the Right, then that leaves only one place for I. We use I for current even though it is written in Amps, because A means something else in formulae.

To use it, take away the thing you want to find, and you will have the maths you need to perform. One letter over another with a line between them means divide the top number by the bottom number. That's where we get the ÷ symbol: The dots represent numbers. So, V ÷ R = I, or V ÷ I = R. If you want to find the voltage drop for a given current through a known resistance, we're left with V = I x R. If two letters are next to each other in a formula, they are multiplied.

For the record, it doesn't matter what order I and R go in the bottom row but having a saying like that to remember two of the three letters just makes people who are not confident with it feel more in control.

For our other LED, the one that is on most of the time in step 7, we need to recalculate for 30mA. So, we have the same 9V - 2.7V to get 6.3V, but this time we have 6.3V ÷ 0.03A, leaving 210Ω. 210Ω is not a standard value and this one runs almost constantly, so we definitely go with the next value up, 220Ω. It won't hurt the brightness visibly.

If that all hurts your head, then use this table we calculated. We have shown common LED voltages, and shown the resistor for 20mA, 30mA, and 100mA. The resistor values are the next highest in the E24 series (the one most commonly available from retailers) from whatever the calculated value was, unless the calculated value was an exact match. Always choose the closest higher-value resistor, never go backwards.

There is also a table here to show how decimal multipliers work. It shows the name of the multiplier, the symbol used, the value in either multiplication or division, what it looks like in scientific notation, how many times to move the decimal place, and what direction to move the decimal place. That means how far to move the decimal place to make a number with a multiplier into a base unit. For example, to turn a 100kΩ resistor value into ohms to calculate something, move the decimal place to the right my three, which adds three zeros. So, 100kΩ is 100,000Ω. The same goes for capacitor values. A 47µF capacitor in microFarads needs the decimal moved to the left six places. It is currently to the right of the 7 in 47, so that becomes 0.000047 Frarads for calculations.

LED FORWARD VOLTAGE (V)	LED FORWARD OR PEAK CURRENT (mA)	RESISTOR VALUE FOR SUPPLY VOLTAGE (V):
		5V	6V	9V	12V
1.7	20	180Ω	220Ω	390Ω	560Ω
	30	110Ω	150Ω	270Ω	360Ω
	100	33Ω	43Ω	75Ω	110Ω
2	20	150Ω	200Ω	360Ω	510Ω
	30	100Ω	150Ω	240Ω	360Ω
	100	30Ω	43Ω	75Ω	100Ω
2.3	20	150Ω	200Ω	360Ω	510Ω
	30	91Ω	130Ω	240Ω	330Ω
	100	27Ω	43Ω	68Ω	100Ω
2.7	20	120Ω	180Ω	330Ω	470Ω
	30	82Ω	110Ω	220Ω	330Ω
	100	24Ω	33Ω	68Ω	100Ω
3	20	100Ω	150Ω	300Ω	470Ω
	30	68Ω	100Ω	200Ω	300Ω
	100	20Ω	30Ω	62Ω	91Ω
3.3	20	91Ω	150Ω	300Ω	470Ω
	30	56Ω	91Ω	200Ω	300Ω
	100	20Ω	27Ω	62Ω	91Ω
3.5	20	75Ω	130Ω	300Ω	430Ω
	30	51Ω	91Ω	200Ω	300Ω
	100	15Ω	27Ω	56Ω	91Ω

Multiplier Name	Symbol	Value	Scientific Notation	Number of Decimal Places to Move	Direction to Move Decimal Place
Tera	T	x 1,000,000,000,000	x10^12	12	Right
Giga	G	x 1,000,000,000	x10^9	9	Right
Mega	M	x 1,000,000	x10^6	6	Right
Kilo	k	x 1000	x10^3	3	Right
Centa	C	x 100	x10^2	2	Right
Deca	D	x 10	x10^1	1	Right
BASE UNIT	-	x 1	-	0	N/A
Deci	d	1 10	x10^-12	1	Left
Centi	c	1 100	x10^-2	2	Left
Milli	m	1 1000	x10^-3	3	Left
Micro	µ	1 1,000,000	x10^-6	6	Left
Nano	n	1 1,000,000,000	x10^-9	9	Left
Pico	p	1 1,000,000,000,000	x10^-12	12	Left

WHERE TO NEXT

You can experiment with different combinations of LEDs if you like, to see if having, say, a bright red short flash and green long flash would make a difference to getting attention. You can also explore series-parallel combinations but be aware that the current limit for the NE555 is 200mA. The only other change we can think of would be to make the circuit as small as possible. We build Kids' Basics circuits on breadboards to make sure anyone can have a go, but if you can solder or know someone who can, you could try soldering components directly in an 'air wiring' arrangement around the NE555, and glue the circuit to the top of the 9V battery snap for a very compact, portable strobe. This would make it much easier to hang from hazards like a tent peg or guy rope, or anything else you wish to mark.