firefly lights circuit schematic

Firefly Lights Circuit

This firefly lights flasher is an easy, fun project.
It can be built on the tiny circuit board, or on perf board.


  • Battery life: approx 1 year using the NXP 7555 CMOS timer IC & standard alkaline battery, or about 6 months using the more common TI TLC555
  • Flash rate: approx 0.25 Hz (unless using optional potentiometer)
  • Dimensions: 14.5mm2
  • Minimum voltage: approx 6V

Schematic of the Firefly Flasher Circuit

firefly lights circuit schematic

Bill of Materials
(if on perf board, use standard through-hole rather than SMT components)

  • PCB
  • U1 NXP 7555 or TI TLC555 Timer IC, SOIC-8
  • R1 10K, 0805
  • R2 2M, 0805
  • R3 Potentiometer, 1M , 0.2” lead spacing (optional)
  • C1 10uF, 25V, X7R, Ceramic, 0805
  • D1 Diode, BAS16, or MMBD4148, SO23-3
  • D2 LED, Green, High Brightness, 0805
  • Battery Standard 9V

How the firefly lights works

It functions like almost any 555 astable timer, except that the LED is inserted into the capacitor reset path so that when pin 7 discharges C1 to common, this relatively high current must pass through the LED. In this way, the average battery current is only about 100µA. When battery voltage becomes very low, the LED threshold voltage prevents discharging C1 down to the lower threshold – when this happens, R2 finishes off the discharge because the output switches to ground potential the same time pin 7 turns on – you will find that this works on almost dead batteries that may be useless for other applications. D1 protects against accidental reverse battery voltage.


Fireflies are common through Northeastern US and also in many other parts of the world. This green LED has the same color, flashes at about the same duration and has a similar repetition rate just like a real firefly (lightning bug). Put a number or these around your grounds and they will truly resemble fireflies – there will be no synchronization of the flashes. The only thing this cannot do is to fly – fireflies ascend as they flash, and they do not flash when perched to protect from predators. Also, this is much larger than the tiny firefly beetle that measures about 3 x 20mm.


Visible on the photo is a wire jumper connecting R2 to IC pin 3. This was an afterthought when I found that low voltage operation was poor. The PCB file is updated to fix this. The optional resistor is not used in this case, but is easy to add if desired. You may adjust component values to obtain desired results.

For the Future

Make it extinguish during daylight hours thus doubling battery life to 2 years.

Happy experimenting!


Join the conversation!

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  • Rajeev

    The firefly circuit is modified using one NPN transistor, LDR and a resistance. The ‘firefly’ now glows only in dark and remains off in day light. See the picture and compare it with previous picture above.

    • Rajeev

      You may not use a transistor. Simply connect a 47K resistance between Pin 4 of IC and +ve supply also connect LDR between Pin 4 of IC and ground (-ve). The firefly will glow only when it is dark.

    • Mog

      Hi Rajeev, it is a little difficult to see in your photo how you have added the LDR, transistor and resistor. If you get some time, the modified schematic would be appreciated. Thanks.

  • Rajeev

    The circuit is working well. Here is the picture.

  • Rajeev

    An interesting circuit for beginners!

  • Sailcatter

    Let me add my thanks for an informative article.
    One question. Your comment, “You may adjust component values to obtain desired results”….Would you give a few examples of “What component affects what parameter”? Thanks again.

  • KD

    HI i a starter learning to use pcb
    i wanted to try this circuit
    bt couldn’t understand that where is the negative terminal attached
    and also pin 1 is connected to what?
    same goes for the other end of the capacitor

    • Stargzer


      I should have mentioned that it is frequently used as a way to “clean up” a schematic so you can concentrate on the signal path without a lot of other lines cluttering up the circuit diagram.

      Think of how an automobile is wired, with the negative terminal connected to the chassis (as ground) and only hot (positive) wires running throught the fuse panel to the various accessories.

      Or, think of the electrical wiring in a house (at least in the US; I’m not familiar with other countries), where the Neutral and Ground for each circuit is connected to separate Neutral and Ground buss bars that are connected to each other and are in turn connected to the panel, and only the “hot” wire runs through the fuse or circuit breaker.

    • Stargzer


      The Triangle with the point down is the symbol for Ground (Signal Ground in this case), which is a common point of reference in a circuit. All three points are connected to a common Ground. In some schematics you might see a voltage reference at certain points. You measure between that point and Ground.

      My guess is that “Ground” comes from early radio schematics, where a tuning coil would be connected to a antenna (aerial) and a ground (often a cold water pipe in a house) to pick up a stronger signal. See for a discussion and a diagram showing the symbols for signal ground, chassis ground, and earth ground.

    • P. Marian

      negative is at pin 1 and the capacitor

  • Nate

    I’m with Pedro. Than you very much Jim. Very well written, and explained. This is useful information for people and or students who are learning about circuits.

  • electron_revolution

    digital circuit is much easier to designed…

  • Pedro Gross

    thanks to you and to others like you that are making these items for us to enjoy. Not only are you making something we could use but in the process are teaching us. Again thank you for taking the time to post this.

  • Jim Keith

    How did I get such a small PCB? this is smaller than what can manufacture. In the process of purchasing custom circuit boards using their miniboard manufacturing service, there is sometimes wasted space because the miniboard has fixed dimensions. This wasted space need not be wasted, but can sometimes contain small stuff like this–in this case, there were (3) additional small circuit boards–you will learn about them in future weeks. After receiving the boards, I cut and smooth the edges–you can see that I did not smooth this one very well.

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