MintyStrobe2 - an adjustable strobe light in an Altoids tin

Inspired by the many amazing Altoids projects here and at Jameco and in Make magazine, I decided to try building a simple strobe light in a standard Altoids tin.

This project consists of four 3-watt white LEDs connected to a 7.4 volt (2S) LiPo battery and two 555 timers. A 10K potentiometer on the first timer adjusts the flash rate between about 2 - 20Hz. The second timer controls the duration of the flash and is fixed at about 5ms.

(I originally breadboarded the circuit using a single timer in astable mode rather than two timers in monostable mode, but the resistor values I needed to get the correct flash rate didn't seem to allow the short flash duration I wanted, even with diodes separating pins 6 and 7. Hence MintyStrobe2. The downside of this second method is that I needed to add a mini pushbutton in order to trigger one of the timers and initiate the oscillations. If this is all sounds like gobbledygook, read on! I am fairly new to electronics myself, especially last September when I made this.)
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Step 1: Materials

Picture of Materials
Below are the parts I used, including links to the exact versions at the stores I purchased them from: Adafruit, Jameco, SparkFun, Mouser, RadioShack, and eBay. However, these are all generic parts and equivalent versions can be purchased from almost any electronics store. These are just the parts I had lying around from building previous projects. Total materials cost is under $20.

Altoids tin - $2.50
2" proto board - $2.95
2S LiPo battery - $4.95
3W white LEDs (4) - $6.99 for 5 (I used warm white but cool white might've looked better.)
6.8 ohm 1-watt resistors (4) - $0.09 each
PN2222A transistors (4) - $0.06 each
LM555CN timers (2) - $0.25 each
8-pin IC sockets (2) - $0.13 each
10K panel mount potentiometer - $0.95 
JST battery connector - $4.98 for 10 pairs
standard 1/4 watt resistors - 120 ohms, 820 ohms, 1K, 10K (2)
standard capacitors - 0.01uF (2), 0.1uF (2), 4.7uF, 47uF, 100uF

Not shown:
Submini  toggle switch - $3.69
Submini push button - $2.50 for 2 (now missing from RadioShack website)
4-40 screws & nuts
hookup wire
PhilsTj6 months ago
This looks awesome. I'm going to build this into my backpack vs Altoids tin. Thanks.
dudes6 months ago
it's me again. :-) I have another question. what components would I have to change if I wanted to use a 5v power supply instead of a 7.4v?
markmoran (author)  dudes6 months ago

It should work as-is with a 5V power supply, the LEDs just won't be as bright. When the 2S battery is fully charged, it's over 8v, but it should keep working all the way down to 4v or so. The white LEDs require about 3.5 volts to turn on, so any voltage above that needs to be dropped by the resistors. Assuming a nominal voltage of 7.4V, the resistors are dropping about 4 volts. I chose 6.8 ohm 1 watt resistors so that about 600 milliampls (4V / 6.8R = .588 amps) would flow into each LED. The 3W LEDs are rated to handle about 850 milliamps (3W / 3.5V = .857).

With a 5 volt supply, the resistors would only be dropping 1.5v (5V - 3.5V), so the current through each LED would be about 220 milliamps (1.5V / 6.8R = 0.22amps). If you wanted to keep the same brightness, you'd need to use smaller resistors, e.g. 2.5 ohms rather than 6.8 (1.5V dropped / .6amps = 2.5 ohms).

Hope this helps!

dudes markmoran6 months ago
yes it does. thank you very much. my minty strobe is now USB powered!
markmoran (author)  dudes6 months ago

Terrific! Hope you have fun with it. Working on #2 now. Please post a pic if you get a chance.

dudes markmoran6 months ago
I will, it's still in the breadboard right now, but when I put it all together I will definitely post it.
dudes8 months ago

what would I have to do if I wanted it to flash faster than 20 hertz?

markmoran (author)  dudes8 months ago

Use a smaller resistor or capacitor on the first 555 timer. A 555 timer's delay is what's called an RC (resistor capacitor) circuit and it's based on the fact that it takes time to charge up a capacitor through a resistor. So the smaller the resistor, the faster the capacitor reaches it's threshold charge to trigger the 555's comparator that the "delay" is over, and the smaller the capacitor, the quicker it reaches that threshold charge as well. The first 555 creates the delay between flashes and the second 555 creates the delay for how long each flash is. Or thought another way, the first 555 turns the LED's on 2-20 times per second and the second capacitor turns them off after the burst time.

How much faster than 20Hz do you want it to flash? Much faster and it will just appear to be continuously on because it is flashing more quickly than your eye/brain can process. That is, old hand-cranked movies shot at 15-20 frames a second appear to flicker (hence the old name "flicks"), but once the camera gets up to 24 frames per second, they appear to us as smooth motion.

dudes markmoran7 months ago
thanks! I was hoping to get somewhere around 60 hertz, for use in strobe photography. any idea what the values should be for that range?
markmoran (author)  dudes7 months ago

Sure, the formula for the delay of a 555 timer's resistor-capacitor (RC) circuit is 1.1 * R * C (in ohm and Farads). To get 60Hz, you need a delay that is 1/60th of a second, or 16.666 microseconds. Which means, you either need to lower the resistance or the capacitance in my circuit to get a delay that short. (A smaller resistor lets more current flow in which charges up a capacitor more quickly, or a smaller capacitor will charge up more quickly with the same amount of current.)

If you keep the 47uF capacitor from the original circuit, you would need the resistance to be 321 ohms. R = .01666 seconds / (1.1 * .000047 Farads)

I used a 1,000 ohm fixed resistor in series with a 10,000 ohm variable resistor, so my circuit allows resistance from 1,000 - 11,000 ohms. You would need to replace the 1,000 fixed resistor with a 330 ohm one or smaller to have resistance low enough to charge up the 47uF capacitor quickly enough. However, the 555 manufacturers recommend always having at least 1,000 ohm resistance on pins 6 and 7 to protect the timer, so it would be better to use a smaller capacitor instead:

Keeping the 1,000 ohm minimum resistance, you would need a 15uF capacitor to get a delay of .0165 seconds. Since most cheap capacitors have sloppy tolerances (more than +/- 20%), I'd use a 10uF capacitor, which should give you delays from .011 seconds (90Hz) with the pot turned all the way off, down to to .121 seconds (8Hz) with the pot turned all the way up (11,000 total ohms).

Hope this helps!


dudes markmoran7 months ago
wow, yes that's helps a ton! thank you! I plan on building one as soon as my 555's get here.
dudes dudes6 months ago
I just built the it, and it works! first time! the schematics were very clear and easy to follow. just one thing, where pin 3 of the 555 connects to a potentiometer, it needs to connect to the middle pin of the pot, not the end pin. otherwise, it works flawlessly. thanks for a great instructable.
markmoran (author)  dudes6 months ago

Right, that was a mistake in the schematic (and thus board file) that I showed it using the two outer most pins of the pot (which would just make it a fixed 10K resistor) rather than the middle pin plus one outer pin (which makes it a variable resistor from 0 - 10K). I should've corrected the files over a year ago when it was first pointed out, but I finally did.

I'm really glad it works for you and that you enjoyed making it!

dudes dudes6 months ago
I meant pin 6+7. not 3
avlasveld1 year ago
Dit is echt leuk
Fine circuit but what is the voltage of the battery? This that the only thing standard is that ziltch is standard drive me mad Otherwise it looks excellent
markmoran (author)  KROKKENOSTER1 year ago
I used a 2S LiPo battery, which is nominally 7.4 volts. It will actually be over 8 volts when fully charged, and drop down to just over 6 volts when close to empty. The components I used (LEDs, transistors, resistors) can all handle this range. Each cell of a LiPo battery is nominally 3.7v, but starts at 4.2 or 4.3 volts and drops down to 6.2 or 6.3 before stopping. I used a LiPo battery because they have a high C-rating - that is, they can supply much more current than a typical alkaline battery, which has a much higher internal resistance.
domdomgin1 year ago
just completed the circuit and it does not work even after pressing the switch..Please help.Also the middle leg of the potentiometer i did short with the side leg..Please help...
markmoran (author)  domdomgin1 year ago
Hmm, I'm sorry it isn't working for you. Can you post a picture of the front and back of your board? I'm assuming the LEDs aren't flashing at all? My guess would be that the capacitor trigger network is not working and that both timers are stuck in a low state, rather than taking turns triggering each other. You can test this by shorting either one of the 555 timer's inputs (pin 2) to ground, which should start the oscillations unless something is grounded and forcing one of them to stay low. Do you have a multimeter you can use to see which parts of the circuit are high and low?

Note - The potentiometer's middle leg should not be shorted with one of the side legs. The middle leg should be connected to pin 7 of the 555 timer and either one of the side legs should be connected to the 1K fixed resistor which is in turn connected to Vcc. The other leg of the pot should be left unconnected. The idea is to create a resistor that ranges from 1K - 11K between positive voltage and the first timer's input, which controls how long the timer is high (how long the LEDs are dark between each flash). In my schematic, I mistakenly drew the middle leg not connected, which turns the potentiometer into a fixed 10K resistor and is the same as permanently forcing the knob to the highest position, which makes the longest delay or slowest flash rate (about twice per second). I meant to draw it with one connection to one of the side legs and the other to the middle leg, rather than the two side legs.
will try what you suggested.Thank you so much...Cheers....
sjroth1 year ago
Could I use 2N3904 NPN transistors instead of PN2222A's?
Great Instructable by the way
markmoran (author)  sjroth1 year ago
Thanks! Glad you like it.

The 2N3904's would probably burn out. I wanted to use them too (since I have many more of them than PN2222's), but the 2N3904 is rated for a max of 200mA whereas the PN2222 is rated for 1000mA.

Depending on how charged the 2S LiPo battery is, each transistor will be collecting between 600 - 700mA during the flashes.
Okay, thanks. I'll have to order some of those.
arnefl1 year ago
In your schematics I can not see the potentiometer has any function. Solder leg 1,2 together?
arnefl arnefl1 year ago
ie. the brd file
markmoran (author)  arnefl1 year ago
Good catch. In the board file, one lead should be coming from the middle pin of the potentiometer and the other from either side, rather than both from the sides. It's correct in the actual project, which was built on proto board with just the legs of various components soldered together (or when they didn't reach, hookup wire between them). I made the board file just to see how everything might fit together and be arranged, basically as a more literal schematic. But it is misleading the way it is now.
Thank you! I have drawn it as single sided now, and will build it when I get the resistors from Thailand, Cool project by the way :)
Hi. Can you please send me Eagle schematics of it? or some well-understandable circuit scheme? And please tell me, does it make for example, a spinning fan to look standing still? Thanks. and sorry for my bad English
markmoran (author)  aliyevzaur19891 year ago
I added the Eagle schematic and board file to Step 2. Remember, they are just a reference that I used when I was building the circuit, but the final version is hand soldered onto a proto board. So for example, the LEDs appear at the bottom of the board file, but they are actually larger LEDs screwed into the lid of the Altoids tin, and the bottom of the board is just where the ground wire for each LED connects.

As for freeze effect, it's more like the staccato effect seen in dance clubs where everyone seems to move in slow-motion. The flashes are 5ms, which is short, but not as short as the flashes a photographer uses to freeze a drop of water or a flying arrow. You can shorten the flash duration by using a smaller capacitor on Timer 2 (I used 4.7uF for C7, but if it was 0.47uF, it'd be half a millisecond. That would be better at "freezing" objects but wouldn't appear as bright to our eyes since the room would be darker for 10 times longer.) Hope this helps. Have fun!
Thanks for quick reply.
I cannot download the schematics :(
It looks like this when i click on link:

This XML file does not appear to have any style information associated with it. The document tree is shown below.

markmoran (author)  aliyevzaur19891 year ago
Interesting! I uploaded the schematic and board files (both from CadSoft Eagle 6), but when I download them from Instrucatbles in Safari, it adds an .xml extension. Just remove the .xml from the file extension so that the files are .sch (schematic) and .brd (board) and then they should open correctly in Eagle.
PaxDesigns2 years ago
If you are new to electronics you are kicking some major butt! I would love to see is a video of it in action!
markmoran (author)  PaxDesigns2 years ago
Thanks! Building it was a fun way to learn more about 555 timers, especially when they don't work as described in the book and need capacitors to just pass through the rising or falling edge of a signal.

I've tried taking several videos with my iPhone in both light and dark rooms, but since the LEDs are only on for 5ms, they almost never get captured by the 24 or 30fps sensors. And when they do get captured, they're bright enough that they just wash out the screen to white.

Here's a video of another LED project I made last year that does capture well with an iPhone.