I was invited to help a local charity put on an event to raise some money, The theme was sunset beach, and this summoned images of beautiful skys and lighting bugs / firefly's. I know several people have created firefly jars before but I wanted something simple. I needed to make 8 of these jars, and I only wanted the jars to activate when it got dark enough. Cost was also an important factor, as I didn't want to charge anything back to the charity.

This is how I made it.

Step 1: Collecting All the Bits

I decided to order all of the components on Ebay. Quite often I ended up ordering packs of 50 or 100 as this made it cheaper, I have a fair few components left over.

The circuit is basic, the variable resistor allows you to adjust the sensitivity of the LDR so that the LEDs switch on when it gets to the desired level of darkness. The 100k resistor is to protect the transistors just in-case you turn it all the way down to 0 Ohms.

The transistors are arranged as a Darlington transistor pair to make the switch much more sensitive.

To cut down the cost I am using Flashing LEDs in series with normal LEDs. This means I get two flashing LEDs for every one. I chose green, but you could choose yellow or a combination of both. Once they start flashing they look almost random after a few seconds.

The Ultra Violet LED is for another effect I will describe later on.

The whole circuit runs off a 9V PP3 battery.

Extra items not shown in the parts list in the picture are as follows:

Jam Jar with Lid
Black Insulating Tape
White Insulating Tape
Glue Gun
Drinking Straw
PP3 Battery Connector
9V PP3 battery
Glow in the dark Nail Varnish
A garland of leaves to cut up (or lots of individual leaves)

Step 2: Building the Circuit

I assembled the circuit into the smallest space possible as I needed to mount this in a jam-jar lid. In the picture you can see the blue variable resistor, the protective 100 Ohm resistor is actually under it. there is a battery connector clip on the left hand side, the wires going off the bottom connect to the LDR, and the red/black wires to the right are to connect to the LEDs.

Step 3: Making the Fire Flys

I soldered each set of 2 "flys" (LEDs) together, one flashing LED, one LED and one resistor in series.

I covered the connections using black insulating tape, being careful to use just enough. I then used white insulating tape over the LED to diffuse the light output. This prevents it from beaming out a spot.

Once I'd made 5 of these I assembled them together at random heights and tied them with a bit of green garden wire. I then joined the ends together so that I had two just connections. I also attached the UV LED and resistor to this.

Step 4: Leaves

I purchased a garland of fake plastic ivy leaves to cut up. I pulled off several leaves and attached them around the wires to conceal them a little and make it look like the LEDs were randomly arranged around them. This was then attached to the main circuit board.

Step 5: Putting It Together

The UV LED is a 5mm LED. I placed a drinking straw over the end of it, and used a hot-glue gun to stick it to the bottom of the jar.

I also purchased from Ebay a little put of glow-in-the-dark nail varnish which I dabbed as little dots around the inside of the jar. These will react with the UV LED and glow on their own adding to the effect.

I made a small hole in the lid of the jam jar and glued the LDR in place.

I then used the glue gun to stick the circuit board and battery clip to the lid

Finally I connected up the battery and adjusted the variable resistor until the LEDs went off during daylight, then at night, the magic appeared......