555 Timer Astable LED Flasher: Circuit, Measurements and Online Simulator

To build this 555 Timer Astable LED Flasher circuit you need a bench power supply, an oscilloscope, a 555 timer, a 1 kΩ resistor, a 68 kΩ resistor, a 10 µF electrolytic capacitor, a 470 Ω resistor and a red LED. The classic astable: R1 = 1 kΩ, R2 = 68 kΩ, C = 10 µF gives about 1 Hz. The LED blinks and the scope shows the capacitor ramping between ⅓ and ⅔ of the supply. Make the 17 connections in the wiring table, press Run, and count the blinks, then read the frequency off the scope: f ≈ 1.44 / ((R1 + 2·R2)·C). Change C to 1 µ for 10× faster.

Open this circuit in the free Electronics bench simulator

Parts list

Qty	Part	What it does
1	Bench Power Supply	Set the voltage and a current limit. It holds the voltage (CV) until the load wants more than the limit, then holds the current (CC). + and − float; GND is earth.
1	Oscilloscope	Two channels against earth (GND). Turn TIME/DIV and VOLTS/DIV to fit the wave, set the trigger level to steady it, and read Vpp, RMS and frequency. X-Y mode draws Lissajous figures. Choose a digital (DSO) or phosphor (CRO) screen.
1	555 Timer	The classic timer. Astable: R1 from VCC to DIS, R2 from DIS to THR+TRIG, C to GND — f ≈ 1.44 / ((R1 + 2·R2)·C). Tie RESET to VCC.
1	1 kΩ resistor	Limits current. 220 Ω in series with an LED; 10 kΩ as a pull-up or pull-down.
1	68 kΩ resistor	Limits current. 220 Ω in series with an LED; 10 kΩ as a pull-up or pull-down.
1	10 µF electrolytic capacitor	A large, polarised capacitor for smoothing supplies: the stripe marks − . Reverse it, or exceed its voltage rating, and it fails.
1	470 Ω resistor	Limits current. 220 Ω in series with an LED; 10 kΩ as a pull-up or pull-down.
1	red LED	Lights when A (anode, the bent leg) is high and C (cathode) is low. Use a resistor.

Wiring

#	From	To
1	Bench Power Supply +	555 Timer VCC
2	Bench Power Supply +	555 Timer RESET
3	555 Timer GND	Bench Power Supply -
4	Bench Power Supply -	Bench Power Supply GND
5	Bench Power Supply +	1 kΩ resistor leg 1
6	1 kΩ resistor leg 2	555 Timer DIS
7	555 Timer DIS	68 kΩ resistor leg 1
8	68 kΩ resistor leg 2	555 Timer THR
9	555 Timer THR	555 Timer TRIG
10	555 Timer THR	10 µF electrolytic capacitor +
11	10 µF electrolytic capacitor -	Bench Power Supply -
12	555 Timer OUT	470 Ω resistor leg 1
13	470 Ω resistor leg 2	red LED anode (A, long leg)
14	red LED cathode (C, short leg)	Bench Power Supply -
15	555 Timer OUT	Oscilloscope CH1
16	555 Timer THR	Oscilloscope CH2
17	Bench Power Supply -	Oscilloscope GND

Step by step

Open the electronics bench. Open the BitForBytes Embedded Lab in your browser. A circuit with no microcontroller runs on the analog bench: no install or account is needed.
Add the parts. Add a bench power supply, an oscilloscope, a 555 timer, a 1 kΩ resistor, a 68 kΩ resistor, a 10 µF electrolytic capacitor, a 470 Ω resistor and a red LED from the parts palette (Instruments, Power, Passive and Semiconductors).
Wire 1. Connect Bench Power Supply + to 555 Timer VCC.
Wire 2. Connect Bench Power Supply + to 555 Timer RESET.
Wire 3. Connect 555 Timer GND to Bench Power Supply -.
Wire 4. Connect Bench Power Supply - to Bench Power Supply GND.
Wire 5. Connect Bench Power Supply + to 1 kΩ resistor leg 1.
Wire 6. Connect 1 kΩ resistor leg 2 to 555 Timer DIS.
Wire 7. Connect 555 Timer DIS to 68 kΩ resistor leg 1.
Wire 8. Connect 68 kΩ resistor leg 2 to 555 Timer THR.
Wire 9. Connect 555 Timer THR to 555 Timer TRIG.
Wire 10. Connect 555 Timer THR to 10 µF electrolytic capacitor +.
Wire 11. Connect 10 µF electrolytic capacitor - to Bench Power Supply -.
Wire 12. Connect 555 Timer OUT to 470 Ω resistor leg 1.
Wire 13. Connect 470 Ω resistor leg 2 to red LED anode (A, long leg).
Wire 14. Connect red LED cathode (C, short leg) to Bench Power Supply -.
Wire 15. Connect 555 Timer OUT to Oscilloscope CH1.
Wire 16. Connect 555 Timer THR to Oscilloscope CH2.
Wire 17. Connect Bench Power Supply - to Oscilloscope GND.
Run and measure. Press Run. Count the blinks, then read the frequency off the scope: f ≈ 1.44 / ((R1 + 2·R2)·C). Change C to 1 µ for 10× faster.

Lab sheet

// Electronics bench: NE555 astable
//
// VCC and RESET to +9 V, GND to 0 V. R1 from VCC to DIS, R2 from DIS to THR,
// THR joined to TRIG, the capacitor from THR to GND. OUT drives an LED.
//
// f = 1.44 / ((R1 + 2·R2) · C) = 1.44 / (137 kΩ × 10 µF) ≈ 1.05 Hz
// The capacitor charges through R1 + R2 and discharges through R2 alone,
// so the high time is a little longer than the low time.

What to try

Press Run. Count the blinks, then read the frequency off the scope: f ≈ 1.44 / ((R1 + 2·R2)·C). Change C to 1 µ for 10× faster.

Frequently asked questions

How do I wire the 555 Timer Astable LED Flasher circuit?

Make 17 connections: Bench Power Supply + to 555 Timer VCC; Bench Power Supply + to 555 Timer RESET; 555 Timer GND to Bench Power Supply -; Bench Power Supply - to Bench Power Supply GND; Bench Power Supply + to 1 kΩ resistor leg 1; 1 kΩ resistor leg 2 to 555 Timer DIS; 555 Timer DIS to 68 kΩ resistor leg 1; 68 kΩ resistor leg 2 to 555 Timer THR; 555 Timer THR to 555 Timer TRIG; 555 Timer THR to 10 µF electrolytic capacitor +; 10 µF electrolytic capacitor - to Bench Power Supply -; 555 Timer OUT to 470 Ω resistor leg 1; 470 Ω resistor leg 2 to red LED anode (A, long leg); red LED cathode (C, short leg) to Bench Power Supply -; 555 Timer OUT to Oscilloscope CH1; 555 Timer THR to Oscilloscope CH2; Bench Power Supply - to Oscilloscope GND.

What parts do I need for the 555 Timer Astable LED Flasher?

A bench power supply, an oscilloscope, a 555 timer, a 1 kΩ resistor, a 68 kΩ resistor, a 10 µF electrolytic capacitor, a 470 Ω resistor and a red LED.

Can I simulate the 555 Timer Astable LED Flasher online for free?

Yes. The BitForBytes Embedded Lab solves the circuit in time, the way SPICE does (nodal analysis with real diode, transistor and capacitor models), and gives you a bench power supply, function generator, oscilloscope and multimeter to measure it — free, in the browser.

What does this circuit show?

The classic astable: R1 = 1 kΩ, R2 = 68 kΩ, C = 10 µF gives about 1 Hz. The LED blinks and the scope shows the capacitor ramping between ⅓ and ⅔ of the supply.