@▷ Firefly Lights Circuit | Diagram for 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 1year using the NXP 7555 CMOS timer IC & standard alkaline battery, or about 6months using the more common TI TLC555
  • Flash rate: approx 0.25hZ (unless using optional potentiometer)
  • Dimensions: 14.5mm²
  • Minimum voltage: approx 6V

Firefly flasher circuit schematic

firefly lights circuit schematic

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

  • PCB ExpressPCB.com
  • 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 100uA. 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 2years.

Happy experimenting!