Introduction: Fireflies in a Jar

This is a simple project that uses NE-2 neon bulbs in a group of 'relaxation oscillators.' In essence, an RC circuit charges a capacitor at a rate determined by a resistor and discharges the cap though a neon bulb to start the process over. The effect is a blinking lamp. With a group of these oscillators you get multiple blinking bulbs and the effect is similar to fireflies. Put them
into a jar and Bob's your uncle. The neon lamps operate at 90v (+/-) and so a half wave rectifier on 110-120vac works well without over-voltage to the neon lamp. The half wave rectifier is a fancy way of saying "put a diode on the wall socket wires."

I made this project using junk parts and so I don't have part numbers or retail sources to offer. However, these are all very common parts and are available from many sources. For instance, you can get the power cord from that lamp in the living room no one ever turns on. By the way, this project is oriented towards American house current @ 120vac. Other nations should use a transformer to reduce their voltage appropriately.

You will need:

10 1uf capacitors 150v
10 1 meg ohm resistors 1/4 watt
10 NE-2 neon lamps
2   Diodes (300v)
Shrink Tubing (1/8 in)
Power cord and male wall plug


Wire cutters
Soldering Iron
Hot Glue Gun (Epoxy OK too)
Rotary Tool (optional)
Thinking Cap*

* You will be dealing with AC line voltages, so think before you touch anything. Capacitors store a charge, so they can still give a shock when the unit is unplugged. Always 'ground out' the circuit before touching it. Electric shock has been associated with burns, organ failure, heart failure, stroke, nervousness, anxiety, indigestion, flatulence, epithet outbursts and finding religion.

Step 1:

The first step is to mount the capacitors to the lid of the jar. I chose a jar with a sprayed on plastic liner/seal. It wasn't really necessary but I wanted to avoid the possibility of a short circuit. The capacitors I had were rectangular, but they come as barrel packages too. It doesn't matter what they are so long as they are non-polarized 1uF caps. Glue these to the lid. You will want to join the legs of one side of the capacitors --this will be your "ground." I soldered the legs to a piece of stripped wire and then cut off the excess leg,

Step 2:

Next, we will solder the end of a resistor and a piece of insulated wire to the unused leg of the capacitors. We will also solder another length of matching wire to the "ground" leg of the capacitor. I used shrink tubing to cover the resistor/wire/leg junction to hold the resistor so it faced down, and to "pretty up" the solder junction. You will need to do this for all 10 of the capacitors. When done, each capacitor should have a pair of wires sticking up.

With that done, run a piece of stripped wire around the capacitors and solder the free end of the resistor to it. Cut off excess resistor leg. Solder them so that the wire is held up off of the jar lid. It should be stiff enough to be able to bend it into place the way you want it and stay there.

Step 3:

This is a view of the "back" side of the capacitors and resistors. Yours should look similar to this. Keep in mind that when done, there will be a glass jar over all of this, and so you don't have to worry about exposed wires shocking people or getting short circuited. From this picture, you can see the two "sides of the circuit to which you will attach power. You can also see that the NE-2 lamps are soldered with each of their legs attaching to one of the wire pairs rising up from the capacitors.

Step 4:

Solder each of the NE-2 lamps to a capacitor wire pair. It is wise to spread the legs of the NE-2 lamps to keep the wires apart from one another. Shorting these wires would likely pop the associated capacitor when power is applied, which might or might not pop your wall socket's circuit breaker or fuse. At any rate, it would smell bad and be quite unnerving, possibly implicating an undergarment change.

Step 5:

You can use shrink tubing to protect the NE-2 legs from shorting. I usually use it when I make these, but I left it off so that the connections would be obvious. This is a safety step --but given the protective jar, it's a step that can be skipped without a lot of risk of catastrophe. If you used a fairly stiff wire and separated the NE-2 legs, they should stay that way. 

Step 6:

It is now time to solder the diodes in place. Note the diodes show their polarity with a silver band at one end. Solder the "ground" side of the capacitors (center wire in the photo)  to the diode leg without the band so that the band is on the power cord end. Solder the other diode to the resistor connecting wire with the band facing the resistors.

Now, if you have a rotary tool with an appropriate grinding stone, drill a hole in the glass jar about an inch or so from the mouth. I suggest that you fit the jar onto the lid assembly first and make a mark about where the diodes are. That way you won't have extra wire inside the jar. It's already going to be tight for space. Anyway, make a hole and feed the power cord through it. Tie a knot in the power cord  about three or so inches from the end. This will be a strain relief so the wire will not pull on the diodes.

If you do not have a way to make a hole in the jar, then make one in the lid. If you use this method, you will need to find a few rubber "feet" to stick on the lid. This will allow the power cord to pass underneath without tipping the jar. Make sure to use a rubber grommet to protect the power cord from the sharp edge of the drilled hole!!  I suggest using five rubber feet if you use this method.

Step 7:

Here is the completed project. You can adjust where the lamps sit in the jar by simply bending the wires. You will want to have them at different heights and be more or less random about their position. This will give a bit more realism to the effect. The firefly lamp draws a very tiny amount of current doesn't generate any heat in operation. Since the lamps have a lifetime of 10,000 hours and do not operate continuously, these things last a long, long time. Decades. 

There it is, as promised: Bob's your uncle.*

*this may be news to Bob...

Step 8:

Here's a schematic of the relaxation oscillator.


lmperkins (author)2012-04-20

I built on your concept a bit and tried a wine bottle. Lots of attribution to this 'structable.
Thanks again for the original.

meenzal (author)lmperkins2012-04-20

Looks great! I like the idea of using a bottle. Very clever and a really nice job!

lmperkins (author)2012-03-24

Nice Instructable. Here is my implementation. I used 0.43 uF caps and 3.3 Mohm resistors (it was what I could find). Drilling the hole in the pickle jar was the hardest part.

meenzal (author)lmperkins2012-03-24

Excellent!! You did a great job!

And yes, grinding through the glass is a bit spooky, especially as you just get through the glass and then have to widen the hole. But these jars are more rugged than they appear.

Again, nice job!

askjerry (author)2012-03-18

I'm looking at this and thinking that it would be really nice to do with some green LEDs. Since they light at 2 to 3 volts the project could use much smaller capacitors, and the input could be from a wall transformer thus it would be a safer build.

Also... the LED could be the frosted green type which is closer to the color of real fireflies. You could also use wire-wrap (30 gauge) wire, so artificial plants could be used and the wire could be glued to them and nearly invisible.

Great idea... the randomness is good... slightly slower would be better. If the lower voltage and LED didn't work properly you could always use some 555 timers or a small microprocessor like a PIC or Atmel. This gives me some ideas... thanks! Jerry

meenzal (author)askjerry2012-03-19

Personally, I prefer the soft light and illumination of the neon lamps.

But, if you want to make this with LEDs, the easiest way is to just use blinking LEDS and attach them to a battery. You can find these all over, but here's a link:

subrotos (author)2012-03-16

What a delightful idea.
Any suggestions on how the circuit can be modified to work with 220
v mains (the supply where I live is 220v/50hz)?
Great instructable.

meenzal (author)subrotos2012-03-16

Yes! You can get a stepdown transformer to put inline on the power cord. The easiest one to find would be a "travel adapter" they sell for US travelers visiting countries with 220v. Plug it into the wall and then build the jar as shown.

corycar (author)2012-03-15

Okay - this may be a dumb question - but I'm immediately reacting to the video posted above...

Is there an easy way to S L O W down the rate of flashing? So instead of a rave party... it would be like a relaxing evening out on the porch?

switch62 (author)corycar2012-03-15

You can slow down the flash rate by increasing either the resistor or capacitor values or both. The flash rate is proportional to 1/RC seconds. Double the resistor value and it will flash at half the rate, similar with the capacitor. The exception with the capacitor is that the flash might be brighter.

If you do increase the values, you may need to experiment. I also suspect that as each neon fires it will also draw some extra current through the resistor and so affect the timing of the other neons. So raising the resistor may reduce the randomness.

meenzal (author)switch622012-03-15

Exactly. I set this up for a 1 second blink rate, but there are fluctuations that vary the actual rate --probably as much as 10% or so. I attribute that to neon discharge and tiny line variances in line voltage. That's why the lamps appear to be random flashes. In watching this one, I've only seen 3 lit up at the same time.

In building others I have used different values. But I find that this particular balance keeps enough activity to be interesting while not being too frenetic. A .7 meg resistor blinks too often and 1.5 meg too slow --at least for my tastes.

switch62 (author)meenzal2012-03-16

There will always be natural variations in timing as there is a tolerance in components. The resistors you are using can vary by 5% either way, the capacitors usually 20% or 10% depending on type. Even the neon strike voltage will vary slightly. So depending on combinations you could get up to 25% variation in flash rates without any other factors.

datacomm (author)2012-03-15

It appears that there are two diodes in your circuit and parts list. In the schematic, they're both going to be charging the cap negative but on both sides. This wouldn't charge the cap to light the light. removal of one or the other would make a half wave charging circuit and save money as well. Reversing one or the other would work but would not be needed for operation.

You could do this with batteries but you would need a bunch of 9v batteries in series and you would still be dealing with a high voltage in the circuit. You would not have the cord to worry about, though. If you use the batteries you don't need the diode just the resistor and cap for each ne2. Vary the resistor to change off times of the ne2 and vary the cap to change on times of the ne2.

You could use LED's or filament lamps but you could not use this kind of circuit. This one depends on the firing voltage of the ne2 being high compared to the cutoff voltage.

I like this little light!...Thanks!

switch62 (author)datacomm2012-03-15

The diodes will charge only on each half cycle as they both point in line with current flow. When the AC is positive on the left (vertical) diode, it is negative on the right(horizintal) diode. So current will flow through left diode, resistor, capacitor, and through right diode. When the cycle reverses both diodes block the current. Agreed that only one diode is needed.

meenzal (author)datacomm2012-03-15

The circuit really only needs one diode. I chose to put two in more for aesthetics than any requirement. Being a DC circuit, it could be battery powered, but I was going for minimum parts and simplicity. This thing will work for ages and no batteries to replace or take up room. But you're right, they could be used.

There are many ways to come up with blinking lights, I made one of these with an Arduino Pro Mini driving mini LEDS. Others have used RC circuits to saturate and unsaturate a transistor to blink incandescent mini lamps. Relaxation oscillators like this one are just one of the ways to go.

caliandris (author)2012-03-15

Would it be OK for me to pin on of the photographs from your project to Pinterest? I want to set up a Riven and Uru related makes category there. I recently took down my original Pinterest and I am trying to ask permission before pinning anything in future.

meenzal (author)caliandris2012-03-15

Sure. It's here for everyone to share. Go for it!

misa_chan (author)2012-03-15

Would this be able to be made to be solar powered instead of having to plug it in a wall?

I go to Larps and that would be a really neat prop to have!

meenzal (author)misa_chan2012-03-15

It's uses direct current so yes, you could make it solar powered. But keep in mind the voltage required for this particular circuit. It's pretty high. For solar power it might be more realistic to build a low voltage relaxation oscillator and use LEDs instead of neon lamps. There are lots of examples for relaxation oscillators (or LED blinkers) on the web, so I'm sure you could find an example to base your project on. Make sure to post it here if you build one so we can all see it!

Wepwopper (author)2012-03-15

LOL When I saw this I stopped by to make sure you put holes in the lid. Now I see it isn't necessary for these 'fireflies"! :)

mobiledeb (author)2012-03-15

Absolutely awesome idea! I am going to make mine this weekend. Thank you!

hepnerj (author)2012-03-15

Don't worry! You have a 1 Meg resistor in series with each NE-2. If the leads are shorted, that particular lamp will not light, but it will do no other harm. The current through the short will only be about 0.00015 Ampere (0.15 mA) which would be small enough to not trip even a Ground Fault breaker.

mrdovie (author)2012-03-15

I used to have some argon lamps with a florescent coating that glowed white. They would be great also. I did this with white LEDs.

Digitalker (author)2012-03-13

Nice project. If you want to make it look more realistic you can use phosphor
coated neon lamps that glow green in place of the clear lamps that you used.
They are also available in white, blue, and yellow in several sizes. Go to They can also be found on eBay as well. They do not
have the some electrical characteristics as NE2 lamps so some adjustment
in resisror and capacitor values will need to be made to get the desired flash

meenzal (author)Digitalker2012-03-13

A good suggestion and thanks for posting the sources. You're right about using the coated lamps and changing values. The benefit here is the absolute simplicity, with only a cap, resistor and lamp required for each oscillator.

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