A small 555-timer based DIY strobe for your room-sized party or some cool experiments.
Build time: 30 minutes
Strobes are a favourite of parties and dance venues, but they have many uses besides that. We are going to revisit our favourite Integrated Circuit (IC), the NE555, to create our own strobe using high-brightness LEDs. Then, we’ll explore some of the things it can do besides light up a party.
An Important Note About Strobes
Strobes can trigger epilepsy. Flash rates of between 3Hz and 60Hz (three to sixty flashes per second) are known to trigger photosensitive epilepsy. Epilepsy is a condition that affects around one in one hundred people. Photosensitive epilepsy, the form that can be triggered by flashing light, only affects around one in thirty of those who have epilepsy. That means only around one in three thousand people are affected. However, use caution, especially when showing your friends whose medical history you may not know.
Additionally, many people find strobes disorientating and they can make some people feel sick. This does not mean you have epilepsy, but it is something to be aware of. They do affect the way your brain interprets movement, so you can crash into things (or miss things you intend to catch) quite easily.
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.
|1 x Solderless Breadboard
|1 x Packet Breadboard Wire Links
|2 x 47Ω Resistors *
|1 x 10kΩ Resistor *
|1 x 91kΩ Resistor *
|1 x 100kΩ Potentiometer
|1 x 47nF Capacitor *
|2 x 100nF Capacitors *
|1 x 2.2µF Capacitor *
|1 x 470µF Capacitor *
|4 x White High-Brightness LEDs *
|LED1 to 4
|2 x NE555 Timer IC
|1 x 9V Battery Snap
|1 x 9V Battery
* Quantity used, item may only be available in packs
# 91kΩ not in pack, use 100kΩ
Place the breadboard in front of you with the outer red (+) rail furthest away from you, and the outer blue (-) rail closest to you. Add two wire links, one to join the two blue (-) rails and one to join the two red (+) rails.
Insert the two NE555 ICs. Have a look at the row numbers in the photos and line yours up the same, because spacing is important later.
Install the wire links shown to connect the ICs to VCC, GND, and Reset.
Place four wire links above the left-hand NE555. Count the rows carefully and notice that one of the links is bare uninsulated wire. We have coloured this with marker so you can see it but the ones in the wire link kits are silver.
Insert three wire wire links below and beside the left-hand NE555, and one more uninsulated link above the right-hand NE555, between pins 6 and 7. Again, we have coloured ours for visibility.
Install a 10kΩ (BROWN BLACK BLACK RED SPACE BROWN) resistor between the upper red (+) rail and the first row of the breadboard. Add a 91kΩ (WHITE BROWN BLACK RED SPACE BROWN) resistor between the upper red (+) rail and pin 7 of the right-hand NE555.
Add a 100nF capacitor between the upper blue (-) rail and pin 5 of each NE555, and a 47nF capacitor between the upper blue rail and pin 6 of the right-hand NE555. We used MKT capacitors but ceramic and greencap types will work fine too.
Place a 2.2µF capacitor with its negative striped side to the lower blue (-) rail and its other leg to pin 2 of the left-hand NE555. Also place a 470µF capacitor between the upper power rails, with the negative striped side to the upper blue (-) rail and its other leg in the red (+) rail.
Install a wire link from pin 3 of the right-hand NE555, off to the right. Place a white LED with its long leg in the same row as the wire link, and its short leg in the row next to that. Add another white LED so its long leg is in the same row as the short leg of the other LED, and its short leg in the next unused row.
Insert a longer wire link between pin 3 and a vacant row to the right. Install another two white LEDs as before, with the long leg of the first to the wire link, the short leg of the first and long leg of the second together, and the short leg of the second on its own.
Place two 47Ω (YELLOW VIOLET BLACK GOLD SPACE BROWN) resistors. One goes from the short leg of the second LED of the left-hand pair, to the lower blue (-) rail. The other goes from the short leg of the second LED of the right-hand group, to the lower blue (-) rail.
Insert a 100kΩ potentiometer to the left of the left-hand NE555. Its first leg goes to the first row. Its other two legs should line up with the wire links. Also install a 9V battery connector, with its red wire to the upper red (+) rail and its black wire to the lower blue (-) rail.
Turn the potentiometer to half its rotation. Connect a 9V battery with only one of the studs connected. Swing the clip so the remaining two studs touch, and look for light from the LEDs. If you don’t see any, swing the battery snap away immediately and disconnect it. Then, go back over your connections one at a time and look for wire links in the wrong rows, components loose in the breadboard, and component legs in the wrong rows. Also, check to make sure the electrolytic capacitors are the right way around.
If you do see light, lift the unconnected stud for the battery snap, slide it over the battery terminal, and press it home. Now, as you turn the potentiometer, you should see the flash rate change. We’ll use this feature later on for some exploring.
HOW IT WORKS
There are two halves to this build. The first will function alone, but the second will not function without the first. The first half is based around IC1, the left-hand NE555 on the breadboard. This is the conventional Astable circuit, which we have used a few times in Kids’ Basics. The second half is a monostable section, which we have only used once before with Issue 39’s Doorway alarm. It is built around IC2, the right-hand NE555. We’ll explain both in detail anyway.
The pins on Integrated Circuits (ICs) in Dual Inline (DIL) or Dual Inline Plastic (DIP) packages are numbered anticlockwise in a U-shape. Pin one is shown by a dot on the surface of the package above pin one, or more commonly today, a notch at the top of the package. The notch shows the top of the case, and then pin one is always to the left when viewed from above.
THE ASTABLE HALF
Looking at IC1 first, Pin 8, VCC, is connected to the 9V supply rail. This powers the internal circuitry within the IC. Current also flows through R1, a 10kΩ resistor, and R2, a 100kΩ potentiometer wires as a variable resistor. These are connected to pins 7, 6, and 2. Pin 2 is the trigger pin, which monitors the voltage on it and triggers the internal flip flop when the voltage reaches one third of the supply voltage. Pin 6 is the threshold pin and triggers the flip flop when the voltage reaches two thirds of the supply voltage. This voltage being measured comes from the two resistors, R1 and R2, charging the 2.2µF capacitor C2.
At first, the flip flop inside the NE555 is high, holding the output transistor inside it in saturation and conducting. The output is now very close to the supply voltage and able to source 200mA. As C2 charges, the voltage across it rises. When it reaches two thirds of the supply voltage, the flip flop ‘flops’, and changes state to low. This drops the output circuit to 0V, and also allows the output to ‘sink’ to ground the same 200mA current. That is why, in some circuits, you can flash two LEDs with the timer: One is supplied by the output, the other is grounded through the output.
While the output is low, the internal transistor connected to pin 7, the Discharge pin, is active. The charge built up in C2 now discharges through this transistor. Because there is no resistor between C2 and the discharge pun, the current drains very quickly, limited only by the internal resistances of the capacitor and the NE555 itself. The voltage across C2, therefore, falls very suddenly, down below one third of the supply voltage and triggering the flip flop to ‘flip’ once again for the output to go high. Because of this, the output is low only for a short time. In fact, we measured it on the oscilloscope at about 35 microseconds. Not milliseconds, thirty-five millionths of a second!
The low period is fixed. It is determined by the internal resistance of the capacitor and IC, and so will vary only with heat, and then only a little under normal conditions. The output high time, however, is variable. The current to charge the capacitor flows through R1 and R2 and so is limited by them. The 10kΩ resistor R1 sets a minimum charge time, resulting in an upper frequency of close to 86Hz (85 cycles per second) measured with the oscilloscope.
That’s too fast for the human eye to see. As you turn R2, however, the resistance increases and causes the current flowing to be reduced, in turn increasing the time it takes C2 to charge to two thirds of the supply voltage. The period of the high time increases, making the low pulses further apart, and slowing down the rate to as low as 8.5Hz. We had to measure that with an optical tachometer, because our oscilloscope won’t compute frequency below 10Hz.
CHALLENGES WITH ASTABLE
Why not just set up the NE555 to give a short on time and change the frequency to vary the flash rate? The IC doesn’t support that. The high time has to be longer than the low time (but you can get extremely close to half and half). So, we set it up to give a short low pulse and have to invert it. Now, the 35µs pulse is so short that it will not light an LED, at least not in a way the human eye will notice. That means we can’t just put an inverter circuit on the output. The way to lengthen the low time is to add a resistor between pins 6 and 7, so that C2 charges via all three resistors, and discharges via only the new resistor into pin 7. However, we went via a different route, partly to enable another feature, and partly as a chance to explain a monostable NE555 circuit.
THE MONOSTABLE HALF
While 35µs is very short, the output of IC1 is low long enough to act as a trigger for IC2 which is connected as a monostable circuit. Notice there is no connection between pin 2 of IC2 and the capacitor connected to pin 6: The triggering of this IC is entirely external. As soon as the low pulse arrives from IC1, pin 2 senses the voltage having fallen below one third of supply, and triggers the flip flop.
The output goes high, and the LEDs connected to it are supplied with current. At the same time as the output goes high, the internal transistor connected to the discharge pin and which is keeping that pin connected to ground, is deactivated, allowing the 47nF capacitor C5 to charge via the 91kΩ resistor R5. As it charges, the output pin 3 is still high, until the voltage across the capacitor reaches two thirds of the supply voltage as sensed by pin 6, the Threshold pin.
When this happens, the internal flip flip resets, the output goes low, and the discharge transistor reactivates, allowing the charge in C5 to drain to ground. Because the capacitor is not connected to pin 2, it has no bearing on the timing as it discharges. There is no sensing of the voltage across it falling to one third of supply, as with the astable IC1. Also, it discharges fully through pin 7, whereas in the astable circuit, it only ever discharges to one third of the supply voltage before the flip flop is triggered and charging starts again.
Having the circuit set up this way carries some advantages. Having the monostable circuit of IC2 driving the LEDs means we can give them a high pulse, or on time, that is fixed in duration. The values of C5 and R5 determine this, no matter the length of the incoming trigger pulse. This means we can choose a value that gives good apparent brightness, because if a pulse is too short, the human eye does not see its full brightness, if indeed it is seen at all. If the pulse is too long, the strobe effect is lost, because fast flash rates would mash together with no gap and just be ‘on’.
The other advantage is that it behaves as an inverter: The short low pulse from IC1 is turned into a longer high pulse to drive the LEDs. Because there is no connection between threshold and trigger functions in IC2, you can trigger the output as close together as you like, having high times much shorter than the low times. This is just not possible, at least not in a Kids’ Basics-friendly way, with an Astable NE555 circuit.
In addition to that, the setup gives you freedom to change the length of the flash from IC2 without altering the timings from IC1, or vice versa, change the timings in IC1 without affecting a flash length in IC2 that you are happy with.
On that note, changing either R5 or C5 or both will give you a different flash length. Change these values to see what you get. Change incrementally though, don’t make huge changes at first. Swap the 47nF capacitor for a 39nF or 22nF, for example, not a 4.7µF. You can also fiddle with R1 and C2 the same way.
SOME OTHER LITTLE BITS
The remaining components are C1 and C4, which are both 100nF capacitors used to keep the unused pin 5, the control voltage pin, stable. C3 is a buffer capacitor used to cope with sudden current draws, which could cause the supply voltage to dip momentarily and cause stability issues.
R3 and R4 are 47Ω resistors for limiting the current into the LEDs, and can be changed to suit whichever LEDs you are using. For example, if you are replacing them with red LEDs which typically have a lower forward voltage, this value needs to increase.
The LED arrangement also needs mention. We have two parallel pairs of two LEDs connected in series. See last month’s Kids’ Basics for more on that. The output of the NE555 can source 200mA. Most white LEDs draw around 3.2V to 3.5V forward voltage with a current draw of 20mA or 30mA. However, the current figure of 20mA to 30mA is for constant current.
We are pulsing our LEDs which allows time for the internal parts to cool, and means we can drive them with more current. Keep this in mind if changing the flash length. The LEDs we used have a forward voltage of 3.2V, so two in series is 6.4V. Taking this away from 9V, we are left with 2.6V.
Using Ohm’s Law from the triangle above, we arrive at 0.055A, or 55mA. That’s plenty to increase the brightness without damaging the LEDs when the current pulse is short. However, we use two 47Ω resistors, one for each series pair. It is possible to use one resistor, but the forward voltage of each LED is slightly different. Whichever pair of LEDs has a lower total forward voltage will take more of the current, and the degree to which this occurs is difficult to predict here. It’s best avoided, and we avoided it by giving each series pair its own current limiting resistor.
WHAT TO DO WITH IT
Of course, most of you didn’t come here to find out how it works in that much detail, you came to play with it! The strobe is a bit more than something to light up the music in your bedroom. It’s a great experiment and scientific observation tool. In darkness, your eyes will only see while the strobe is lighting something up.
Try pointing the strobe at moving objects and watching them. If you have a ceiling fan in your room, try setting it to a low speed and adjusting the strobe. You should be able to find a point where the fan blades look like they aren’t moving at all! You can also make them look like they are moving slowly, and change the appearance of direction as well.
Rotating objects are great fun with strobes, but so are many other objects. With the strobe on a slower speed, try lobbing a ball in the air (gently, not too high) and see if you can catch it. The ball moves even while the strobe is not on, but your brain processes the ball’s location based on when your eyes last saw it.
This might be manageable if you’re practised with ball sports - you probably know from muscle memory where the ball will go when you launch it. However, try having someone else throw it to you.
Along the same lines, it can be fun to observe any moving object under strobe light. Try waving a pencil really fast with the strobe on a fast setting- suddenly you have a bunch of pencils.
The other main use of the strobe is stroboscopic photography. That sounds big but it really isn’t. With so many of us having access to advanced cameras in mobile phones now, stroboscopic photography is more accessible than ever. You might have your own phone or may need an adult to help you.
Manual modes are becoming more common, but most cameras have scrapped the ‘night mode’ because they’re so adaptive. With a light on in the room, set up the camera on a holder of some sort - a small tripod, or propped up on a support. We used Blu Tack and a ruler to hold ours vertical next to the edge of a table. Next, turn on the timer mode for ten seconds or so.
Make sure the camera is focussed near where your object will move (this usually just means holding the object still and touching the screen, autofocus does the rest). Then, start the timer by pressing the shutter, start the strobe, and turn off the room light. The camera should give a beep to let you know the timer is almost up. Start moving your chosen object. This could be waving something, or throwing a ball, anything you can think of.
The effects of this might take some experimenting. Some cameras try to refocus all the time, while others work with whatever was set before the timer. Some will try to take the photo for too long, or not long enough. Don’t be disappointed if things don’t work at first.
Better results are gained with manual or ‘pro’ mode. This is a feature on more and more phones, and allows you to set the camera the way you want it. You can choose to expose (take the photo for) one second, five seconds, a quarter of a second, or eight thousandths of a second. You can choose the ISO (sensitivity to light) and sometimes the aperture (the size of the opening in the lens). You can also choose to lock the focus on some. This would be a much better option if your phone has it.
The idea is to expose the photo for long enough that several flashes of the strobe light up a moving subject - it will be in a different position each time, with nothing in between. A black backdrop works well, and no backdrop at all works better - if you can find a way to set your subject up on a table edge and have nothing behind it until way off in the background, you will get the best results.
WHERE TO NEXT
Most of the things you can do have been outlined already: Changing the components in the circuit explained at the end of ‘How It Works’ if you skipped that section, and the experiments. Our main recommendation for what to do from here is to make the circuit just a little more user-friendly.
We suggest mounting the circuit on a piece of cardboard using Blu Tack or double-sided tape, and also using three plug-to-socket jumper leads to move the potentiometer off the breadboard. You can Blu Tack this and the battery down to the cardboard to stop everything sliding around.