555 LED Blinker
Astable 555 oscillator blinking an LED at roughly 1 Hz.
Difficulty: Breadboard. Estimated build time: about 25 minutes. Estimated parts cost: about US$1.25. 8-line bill of materials. Compare supplier offers when available. A free-running oscillator built around a single NE555 timer IC that toggles an LED on and off at approximately 1 Hz.
Power rails
Connect Vcc (pin 8) to the positive rail and GND (pin 1) to the negative rail. Place a 100 nF ceramic capacitor directly across the supply pins on the breadboard, close to the IC. This keeps the supply quiet during output transitions.
Place the timer
Insert the NE555P across the center divider of the breadboard. Tie the reset pin (pin 4) to Vcc—leaving it floating will cause random resets. Leave pins 2, 3, 5, 6, and 7 unconnected for now.
Wire the timing network
Connect a 10 kΩ resistor from Vcc to pin 7 (discharge), then a second 10 kΩ resistor from pin 7 to pin 6 (threshold) and pin 2 (trigger). Bridge pins 2 and 6. Connect a 47 µF electrolytic capacitor from pin 6 to GND, observing polarity. Place a 10 nF ceramic capacitor from pin 5 (control voltage) to GND—this suppresses noise on the internal voltage divider.
Add the LED
Connect a 470 Ω current-limit resistor from pin 3 (output) to the anode of the LED; cathode to GND. The 555 output can sink or source up to 200 mA, so almost any LED works, but keep the resistor in place. Red LEDs are efficient at 5 V; other colors need slightly higher forward voltage—the resistor value is not critical.
Verify the blink rate
Apply power. The LED should blink at roughly 1 Hz. If it blinks faster or slower, check the capacitor value—electrolytic tolerances are wide. The formula is f ≈ 1.44 / ((R1 + 2×R2) × C). With both resistors at 10 kΩ and C at 47 µF, you get approximately 1 Hz. Adjust R2 or C to change the rate.