Introduction: 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.)

Step 1: 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

Step 2: Designing the Circuit

I wanted to learn CadSoft Eagle, so I started by creating a schematic. I planned to use a generic prototyping board with hookup wire rather than ordering or etching a PCB, but I still laid out the components in Eagle's board editor to get a sense of how they might fit in the small space. (2014 - See Note Below)

If you're not familiar with 555 timers, I recommend Charles Platt's excellent book Make: Electronics. My quick summary is that a 555 is a couple dozen transistors and resistors packaged together so that when the voltage on pin 2 (the trigger pin) dips low / turns off, then pin 3 (the output pin) will go high / turn on for a certain amount of time. The time is determined by the resistors and capacitor attached to pins 6 and 7 (the threshold and discharge pins).

In this case, I've connected the output of each timer to the input of the other, so that when timer 1 turns off, timer 2 turns on, which turns on the LEDs. Timer 2 has a relatively small resistor (820 ohms) and small capacitor (4.7uF), so its countdown only lasts about 5ms. It then turns off the LEDs, which causes timer 1 to start its countdown, which is the time between each flash. Timer 1 has a capacitor that is ten times larger (47uF) plus a fixed resistor of 1000 ohms and a variable resistor (potentiometer) that ranges from 0 to 10,000 ohms, so the total resistance is between 1 - 11K. This makes timer 1's countdown last between about 50ms and 550ms, or from 20Hz down to 2Hz. (The 1K resistor is there to make sure that even if the knob is turned all the way down, there is still enough resistance to protect the 555's discharge pin.)

I wanted the flashes to be bright enough to illuminate a dark room, so I used four 3-watt LEDs, each drawing about 600 milliamps. Since the 555 timer can only source about 200mA, I used the timer's output to turn on an NPN bipolar junction transistor connected to each LED. I used PN2222A transistors since they can collect up to 1 amp, whereas the similar 2N3904 is only rated for 200mA. Likewise, the 6.8 ohm resistors are relatively large 1-watt versions because they need to handle more than half an amp. (With 7.4V connected to a white LED and a 6.8ohm resistor, each resistor is actually dissipating over 2 watts, but since they are only on for 5ms at a time, the resistors don't get hot even at the fastest blink rate.) I used a 2S LiPo battery because they have a high C-rate and can easily supply the >2 amps needed.

Finally, after some frustrating trial-and-error followed by extensive web research, I realized I needed to add a capacitor and pull-up resistor between the output and input pins to create a trigger network. Doctronics has an excellent tutorial on this, but essentially the problem is that many 555 timers, mine included, will only turn off if the input pin is no longer low. Because Timer 1 is still off when Timer 2's countdown finishes, Timer2 never turns off, and thus Timer1 never turns back on. So I tied the input pins to positive voltage using a 10K pull-up resistor and then I inserted a 0.01uF capacitor between the output and input pins so that only the off pulse (or falling edge) gets passed to the input pin.

I also needed to add a button to manually kick off the process. I had hoped that turning on the power would trigger one of the timers at the right moment and start the cycling, but that never seemed to happen. The only way I could guarantee the cycle started was to ground one of the input pins for an instant, which is what the external button does.

(2014 EDIT - I've replaced the original schematic and board file because as you'll see in the comments, I mistakenly showed the outer two pins of the potentiometer used rather than the middle pin and one outer pin. Also note that blue and red traces in Eagle are supposed to indicate top and bottom layers, but in this case I'm using them arbitrarily since I made this on a protoboard using wires rather than PCB traces that can't cross.)

Step 3: Building the Circuit

Because the input and output pins are both on the left side of the 555 chip, I placed timer 1 upside down so that the two left sides of the chips would be adjacent to each other.

I first soldered in the two 8-pin sockets and then the resistors, capacitors, and wires that connect and support them. Next I installed the transistors and large resistors that connect to the LEDs. I cut holes in the right side of the Altoids tin and inserted the mini power switch, the potentiometer, and the start button.

I covered the bottom of the tin with electrical tape so nothing would short and then added tiny rubber feet to the bottom of the board just in case. Because the battery and board fit snugly into the tin, I didn't need to mount either one. In fact, the fit is so snug that I had to cut a notch into the board to make room for the base of the potentiometer.

Step 4: Putting It All Together

I spaced the four LEDs on the lid of the tin, drilled two holes for each LED, and inserted eight 4-40 screws with matching nuts on the inside. I then drilled two smaller holes next to each LED for the positive and negative wires. The positive wires are all connected underneath and go straight to the battery. The negative wires each connect to a different 1-watt resistor which is in turn connected to a transistor and then the output pin of timer 2.

That's it. The final result is very bright - unfortunately too bright to capture with a iPhone video - but it easily creates the jittery, staccatic effect popular in techno dance clubs 20 years ago.

I'm new to this hobby and this is my first instructable, so I look forward to any comments, corrections, or questions.


philesra (author)2015-12-17

This is fantastic, as a project and as a source of info. (Cute baby and hat too.) I have just started playing with electronics and a 555 timer. For a project like this, would it be possible to use a single 555, and use default-closed transistors ("PNP"?) to reverse the on/off response to the 555 output, to achieve the short-duration strobe burst? (I'm hoping to mount a strobe on the underside of a disc-golf disc for night play, for aesthetics but also to use it to estimate the disc's rpm in flight...)

markmoran (author)philesra2015-12-17

Thanks, philesra! Glad you liked it. Yes, you can use a single 555 timer configured in astable mode, which is what I originally did for my MintyStrobe1. But I wasn't able to find resistor or capacitor values that allowed me to both adjust the flash rate within a range I liked (~2-20Hz in this case) while also only being on for a brief enough period to be visible as a strobe flash (~5ms). If I had reversed the electronics logic and used PNP transistors I might've found a duration value that looked strobey, but I suspect it would've only looked right at certain flash frequencies, not over the whole 2-20Hz range. Using two separate timers allows you to separately configure the flash duration from the flash frequency. To simplify things, you can pick a specific frequency and thus get rid of the pot, just using a fixed resistor to set the frequency like I did with the flash duration. You can also use a 556 chip, which is just two 555 timers packaged into a single IC.

philesra (author)markmoran2015-12-17

Ooh, hadn't considered how the adjustable resistance would come into play on that front--tricky. I'm tentatively planning to use only fixed resistors in hopes of having a more concrete idea of the actual computed flash rate (though I note your point about the wide tolerance...) than I would with the unlabeled, continuously adjustable knob.

It was also useful to read your note about 555 chips wanting >1k ohm resistors. Might have to check out that book you mention too.

What purpose does an IC socket serve--is it to protect the chip from cooking when you solder it?

markmoran (author)philesra2015-12-17

I'm a big fan of Make:Electronics by Charles Platt. I have more than a dozen other maker or DIY electronics books, but that one is by far my favorite.

I've read the resistor that connects the discharge pin (7) to the positive supply is supposed to be at least 1K in order for the timing calculations to be predictable and accurate.

The IC socket is just there to let you swap out the IC without desoldering. If a chip ever fails (or you even suspect it might be flaky), it's very convenient to try replacing it just by popping it out of the socket. I've done this multiple times after accidentally burning out one or more pins on opamp and microcontroller chips. The sockets only cost a few cents, but I don't always use them, especially when I want the PCB to have a flatter, sleeker look.

philesra (author)markmoran2015-12-17

Ah, makes sense about the socket. I will order the book tonight! Thanks for the recommendation.

dudes (author)2014-05-25

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

markmoran (author)dudes2014-05-27

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.

DaveC72 (author)markmoran2015-11-18

I know I am a little late on this :-) How can I slow the time down to about 1 flash every minute? How big of a capacitor would be required?



markmoran (author)DaveC722015-12-17

Sorry for the slow reply. As dudes said, using a 4700uF capacitor with the same 1-11k resistance in place would give you a delay that ranges from 5-57 seconds. If you instead use a 47,000uF capacitor, the delays would be 10times longer, i.e. 57 seconds with the resistance turned all the way down to 1K and 570 seconds (almost 10 minutes) with the POT turned up so the resistance is 11K.

dudes (author)DaveC722015-11-18

If I understand correctly, using a capacitor of 4700 uf and a resistor of about 11k ohms should give you about 1 minute delays. Using 1000 uf and 54k ohms should work as well. Perhaps markmoran could verify whether I am right or not.

markmoran (author)dudes2015-12-17

Yes, sorry for the late reply. That's exactly right - a 4700uF capacitor with 11K of resistance will result in a delay period of 57 seconds. (Since these amounts are never exact, you'll want to use a variable resistor (two legs of a POT or a rheostat) so that you can tweak the resistance up or down in order to get exactly a minute between blinks.

dudes (author)markmoran2014-06-14

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)dudes2014-06-16

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 (author)markmoran2014-06-22

wow, yes that's helps a ton! thank you! I plan on building one as soon as my 555's get here.

dudes (author)dudes2014-07-08

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)dudes2014-07-09

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 (author)dudes2014-07-08

I meant pin 6+7. not 3

PhilsTj (author)2014-07-15

This looks awesome. I'm going to build this into my backpack vs Altoids tin. Thanks.

dudes (author)2014-07-08

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)dudes2014-07-09

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 (author)markmoran2014-07-09

yes it does. thank you very much. my minty strobe is now USB powered!

markmoran (author)dudes2014-07-09

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

dudes (author)markmoran2014-07-13

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

avlasveld (author)2013-09-23

Dit is echt leuk

KROKKENOSTER (author)2013-03-24

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)KROKKENOSTER2013-03-24

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.

domdomgin (author)2013-03-20

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)domdomgin2013-03-21

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.

domdomgin (author)markmoran2013-03-22

will try what you suggested.Thank you so much...Cheers....

sjroth (author)2013-02-27

Could I use 2N3904 NPN transistors instead of PN2222A's?
Great Instructable by the way

markmoran (author)sjroth2013-02-27

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.

sjroth (author)markmoran2013-02-27

Okay, thanks. I'll have to order some of those.

arnefl (author)2013-02-11

In your schematics I can not see the potentiometer has any function. Solder leg 1,2 together?

arnefl (author)arnefl2013-02-11

ie. the brd file

markmoran (author)arnefl2013-02-12

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.

arnefl (author)markmoran2013-02-12

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 :)

aliyevzaur1989 (author)2013-02-01

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)aliyevzaur19892013-02-01

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!

aliyevzaur1989 (author)markmoran2013-02-02

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)aliyevzaur19892013-02-02

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.

PaxDesigns (author)2013-01-29

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)PaxDesigns2013-01-29

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.

About This Instructable




Bio: Film producer and former video game programmer. Electronics enthusiast and amateur maker.
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