CMOS 555 LED Flasher – Long‑Duration, Low‑Power Red LED Pulsing Circuit

PARTS AND MATERIALS

• Two AAA batteries
• Battery clip (Radio Shack catalog #270-398B)
• One digital multimeter (DVM) or voltage–current meter (VOM)
• U1 – CMOS TLC555 timer IC (Radio Shack catalog #276-1718 or equivalent)
• D1 – Red LED (Radio Shack catalog #276-041 or equivalent)
• R1 – 1.5 MΩ, ¼ W, 5 % resistor
• R2 – 47 kΩ, ¼ W, 5 % resistor
• C1 – 1 µF tantalum capacitor (Radio Shack catalog 272-1025 or equivalent)
• C2 – 100 µF electrolytic capacitor (Radio Shack catalog 272-1028 or equivalent)

CROSS‑REFERENCES

Lessons In Electric Circuits, Vol. 1, Ch. 16: “Voltage and Current Calculations”

Lessons In Electric Circuits, Vol. 1, Ch. 16: “Solving for Unknown Time”

Lessons In Electric Circuits, Vol. 3, Ch. 9: “Electrostatic Discharge”

Lessons In Electric Circuits, Vol. 4, Ch. 10: “Multivibrators”

LEARNING OBJECTIVES

• Apply the RC time constant to a real‑world circuit
• Implement a CMOS 555 astable multivibrator configuration
• Understand and calculate duty cycle
• Handle electrostatic‑discharge (ESD) sensitive components safely

SCHEMATIC DIAGRAM

ILLUSTRATION

INSTRUCTIONS

Because the CMOS 555 is highly ESD‑sensitive, protect it according to Volume 3, Ch. 9 before connecting the batteries. The IC consumes very little current, yet it will deplete batteries rapidly if the LED is driven continuously. This design uses a slow flash rate and a high‑value timing network to keep the average current below 1 mA.

The TLC555 supports a supply range of 2 V to 18 V, allowing operation from a single 1½ V AAA pair. The output drives the LED for a 30 ms pulse at a 1 Hz flash rate, giving a duty cycle of about 3 %. Assuming a 20 mA LED rating, the average current is only 0.6 mA, so the LEDs can glow for several months on AAA cells.

Use the large resistors to limit current, and keep the electrolytic capacitor (C2) charged during the off periods to buffer the supply voltage. This simple trick extends battery life well beyond the nominal capacity.

After assembling the circuit, the LED should flash immediately. You can verify the low current draw by measuring the battery output while the TLC555 is powered; a typical reading falls between 9 mA and 24 mA depending on the chip batch.

THEORY OF OPERATION

At its core, the circuit is a standard astable multivibrator. Pin 7’s internal transistor charges and discharges the timing capacitor (C1) through R1 and R2. When the capacitor voltage reaches 2/3 V_CC, the flip‑flop resets, turning on the discharge transistor and pulling the voltage down to 1/3 V_CC. The cycle repeats, producing a square‑wave output that drives the LED.

The large C2 capacitor stores most of the battery energy during the 97 % “off” period, delivering the brief current pulse during the 3 % “on” period. This greatly reduces the average load on the batteries.

Because the TLC555’s output current decreases with supply voltage, the LED brightness dims as the batteries discharge. Nevertheless, the circuit continues to flash, demonstrating the effectiveness of the low‑power design.