Circuits / 555 and op-amps
555 astable blinking an LED
C1 charges through R1 + R2 and discharges through R2: f ≈ 1.44 / ((R1 + 2·R2) · C1)
Tap a part to see its voltages and currents. It is a model, not a measurement.
Predict, measure, explain
- Predict How many times a second will the LED blink?
- Measure Count the blinks. Then change R2 to 4.7 kΩ.
- Explain
Show the answerHide the answer
About 6.7 times: R1, R2 and the 10 µF capacitor set the time. With 4.7 kΩ the capacitor charges faster and it blinks about 13 times a second.
What do you need?
- Resistor 1 kΩSearch on Amazon
- Resistor 10 kΩSearch on Amazon
- Integrated circuit NE555Search on Amazon
- Capacitor 10 µF 25 VSearch on Amazon
- Capacitor 10 nFSearch on Amazon
- Resistor 330 ΩSearch on Amazon
- Red LEDSearch on Amazon
- Jumper wires × 7Search on Amazon
- BreadboardSearch on Amazon
How does a 555 timer make an LED blink?
In astable mode the 555 charges and discharges a capacitor between two thresholds and flips its output each time. C1 (10 µF) charges through R1 and R2 until it reaches two thirds of the supply, then the chip discharges it through R2 until it drops to one third. With 9 V, C1 swings between about 3 V and 6 V. The output is high while C1 charges, so the LED blinks at f ≈ 1.44 / ((R1 + 2·R2) · C1). With 1 kΩ and 10 kΩ that gives 6.9 Hz, and the simulation shows 6.7 Hz. Because R2 is much larger than R1, the LED is on for about half of each cycle, at about 18 mA through R3 (330 Ω). Change R2 to 4.7 kΩ and the blinking speeds up to about 13 Hz. C2 (10 nF) only keeps noise off the control pin.
Values from the Protoboard ISO simulation: a model, not a bench measurement.
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