Project system

555 LED flasher: two LEDs that take turns, with every value worked out

By Bulan Sarkar, founder of ElectronicsInfoline (since 2000) · About a 10-minute read

The 555 flasher is the classic first chip project: eight pins, five parts and a battery, and the LEDs blink with no code at all. Wire two LEDs the right way and they take turns, which suits a diya, a rangoli or a model.

This page builds it step by step, works out the blink rate and the LED currents from the TI datasheet, and lists what goes wrong when it does not blink.

Wire an NE555 as an astable with RA = 10 kΩ, RB = 68 kΩ and C = 10 µF. It flashes at 0.99 Hz, high for 0.54 s and low for 0.47 s. Put one LED from pin 3 to ground and another from +9 V to pin 3, each through 470 Ω, and they take turns. The LEDs take about 11.3 and 14.6 mA, and a PP3 battery lasts roughly a day of continuous flashing.

Build it, buy it, or get help

Build it yourself

You want one to four LEDs blinking or taking turns, from a battery or a USB charger, with no programming. One evening on a breadboard, one more to solder it.

Buy a module instead

You only need one blinking LED: a self-flashing LED has the flasher built in and needs just a resistor. For many LEDs and patterns, a ready USB LED string or the Arduino system below is less work.

Stop and get help

Anything powered from 230 V mains, such as a mains serial-light set or a lamp. Do not wire a 555 into mains equipment. This page stays at 5 V to 12 V from batteries and certified adapters.

The steps, in order

1. Place the 555 and its power
Have ready: NE555 (DIP-8), breadboard, 9 V battery with clip, 100 nF capacitor
Push the 555 across the middle channel of the breadboard with the notch to the left. Pin 1 is bottom left, and the pins count anticlockwise. Run pin 8 (VCC) and pin 4 (RESET) to +9 V and pin 1 to ground. Put the 100 nF capacitor from pin 8 to pin 1, right at the chip.
Check: 9 V between pin 8 and pin 1
Figure 1. Step 1: the 555 seen from above. The notch is at the top here, so pin 1 is top left; the pins count anticlockwise.
2. Add the timing parts
Have ready: 10 kΩ (brown, black, orange), 68 kΩ (blue, grey, orange), 10 µF electrolytic
RA (10 kΩ) goes from +9 V to pin 7. RB (68 kΩ) goes from pin 7 to pin 6. Link pin 6 to pin 2. The 10 µF capacitor goes from pin 2 to ground, with its − stripe to ground. That gives f = 1.44 ÷ ((10 kΩ + 2 × 68 kΩ) × 10 µF) = 0.99 Hz.
Check: Pin 2 and pin 6 joined; capacitor stripe to ground
Figure 2. Step 2: the timing circuit, RA 10 kΩ, RB 68 kΩ, C 10 µF, about 0.99 Hz at 53.4 % duty.
Figure 3. Step 2: C charges through RA and RB and discharges through RB alone, so the high time is a little longer than the low time.
3. Fit the two LEDs on pin 3
Have ready: 2 red LEDs, 2 × 470 Ω (yellow, violet, brown)
LED A: pin 3 → 470 Ω → LED anode, cathode to ground. It lights while the output is high, at about (9 − 1.7 − 2.0) ÷ 470 = 11.3 mA. LED B: +9 V → 470 Ω → LED anode, cathode to pin 3. It lights while the output is low, at about (9 − 2.0 − 0.15) ÷ 470 = 14.6 mA. Together they take turns.
Check: Both LEDs light when you touch their leads to the supply by hand
Figure 4. Step 3: LED A from pin 3 to ground, LED B from +9 V to pin 3. They light in turn, about 11.3 mA and 14.6 mA.
4. Power up and time it
Have ready: Watch or phone stopwatch
Connect the battery. Count the flashes of LED A for 30 seconds: expect about 30. The first flash is longer than the rest, because the capacitor starts from 0 V instead of a third of the supply. A 10 µF electrolytic can be 20 % off, so anything from about 25 to 37 flashes is normal.
Check: About 30 flashes in 30 s
Figure 5. Step 4: the capacitor swings between 3 V and 6 V; the output is high for 0.54 s and low for 0.47 s.
5. Change the speed
Have ready: 1 µF to 100 µF capacitors
The capacitor sets the speed. Keep the two resistors and swap the capacitor: 4.7 µF roughly doubles the rate, 22 µF roughly halves it. To check the code on a small capacitor, use our capacitor code calculator. For a different duty cycle, use the 555 timer calculator.
Check: Rate changes in the direction you expected
Figure 6. Step 5: blink rate against the timing capacitor, with RA 10 kΩ and RB 68 kΩ fixed.
6. Solder it and fit it
Have ready: Stripboard, 8-pin IC socket, soldering iron, enclosure or diya holder
Copy the breadboard layout onto stripboard, with the 555 in a socket so a wrong solder joint never cooks the chip. Cut the copper strips under the chip between the two rows of pins. Keep the battery clip leads short and fix the battery so it cannot pull on the joints.
Check: Flashes on stripboard exactly as on the breadboard

Figure 7. Current from the 9 V battery over one cycle: the chip plus whichever LED is lit, about 18.8 mA on average.

What goes wrong, and how to find it

Symptom	Likely cause	Check and fix
Nothing lights at all	Chip in backwards, pin 4 not tied to +9 V, or a flat battery.	Notch to the left, pin 1 bottom left. Measure pin 4: it must sit at the supply voltage. Measure the battery under load; below about 7 V a PP3 is near its end.
One LED stays on and never blinks	Pin 2 and pin 6 are not joined, or the capacitor is missing or the wrong way round.	Check the pin 2–pin 6 link. Check the capacitor stripe is to ground. With a multimeter on pin 2 you should see the voltage rise and fall between 3 V and 6 V.
Only LED A works, LED B never lights	LED B is in backwards. Its anode must face +9 V and its cathode pin 3.	Turn LED B round. Remember the two LEDs point in the same direction along the current: +9 V → B → pin 3 → A → ground.
It flashes far too fast or too slow	Wrong capacitor value (105 = 1 µF read as 10 µF), or 6.8 kΩ instead of 68 kΩ.	Read the bands and the code again. 68 kΩ is blue, grey, orange; 6.8 kΩ is blue, grey, red.
The chip gets hot	RA missing or shorted, so pin 7 puts the supply straight to ground every cycle; or pin 3 shorted to ground.	Disconnect at once. Check RA is 10 kΩ, not 10 Ω. Check for a solder bridge between pin 3 and pin 4 or pin 2.
It runs, then stops or stutters	No 100 nF across the supply pins, a loose breadboard contact, or a battery nearly flat.	Fit the 100 nF right at pins 8 and 1. Press the parts down firmly. Try a fresh battery.

Variants

9 V battery vs 5 V USB charger

From a USB charger at 5 V the timing stays the same, but the LED resistors change: 120 Ω for LED A (about 10.8 mA) and 220 Ω for LED B (about 13.0 mA). The NE555 needs at least 4.5 V, so a USB supply is fine and runs all night.

NE555 vs TLC555 (CMOS)

For two AA cells (3 V) use the TLC555, rated from 2 V. Same pinout, same formulas, and it draws far less supply current, so batteries last longer.

LEDs on pin 3 vs a 12 V strip

Pin 3 can source or sink up to 200 mA, but for a 12 V strip drive the gate of an IRLZ44N MOSFET from pin 3 exactly as in step 6 of the Arduino festival lights system, from a 12 V adapter. The 555 then runs on the same 12 V.

555 vs Arduino

A 555 gives one rhythm, set by parts. Chasers, fades and patterns need a microcontroller: see the Arduino festival lights system.

What we would do

Build it exactly as listed first: NE555, 10 kΩ, 68 kΩ, 10 µF and two 470 Ω resistors on a breadboard, from a 9 V battery. Once it flashes at about one per second, solder it onto stripboard with the chip in a socket and run it from a USB charger with the 5 V resistor values. If you then want patterns, move on to the Arduino system; a 555 does one rhythm well and nothing else.

Tips and tricks

Use a socket for the 555 on stripboard. If the chip is ever damaged, it swaps in seconds, and you never hold a hot iron on its pins.
Fix the capacitor first, then the resistors. Capacitors come in fewer values and wider tolerances than resistors.
Keep RA at 1 kΩ or more. Every time pin 7 switches on, RA sits straight across the supply.
Run it from a USB charger for an all-night display. A PP3 is fine for testing but lasts about a day.
Read the capacitor before you blame the circuit. 105 is 1 µF, 106 is 10 µF, and the blink rate changes tenfold between them.

Print the working copy

One A4 page for the bench: the parts list as a checklist, every connection with a tick box, and the tests with the readings to expect. It prints in black and white.

Open the printable card

Source and assumptions

Texas Instruments, xx555 Precision Timers datasheet, SLFS022K (revised March 2026). p. 4: recommended VCC 4.5–16 V, output current ±200 mA; p. 6: high-level output 13.3 V at VCC 15 V and 100 mA, low-level output 0.15 V at VCC 5 V and 8 mA, supply current 3 mA (5 V) and 10 mA (15 V) typical; p. 12: astable equations 1–6. Checked 6 October 2026.

Our own choices and estimates, not from the datasheet: the 2.0 V red LED drop, the 470 Ω resistors, the 1.7 V high-side drop taken from the 100 mA row (at 11 mA it is a little less, so LED A runs a touch brighter), 6 mA chip current at 9 V (between the 3 mA and 10 mA typicals) and 500 mAh for an alkaline PP3.

Questions people ask

Which capacitor makes a 555 blink once a second? With RA = 10 kΩ and RB = 68 kΩ, a 10 µF capacitor gives 0.99 Hz, very close to once a second. Swap it for 4.7 µF to blink about twice as fast, or 22 µF for about half as fast.

How do I make two LEDs flash alternately with a 555? Connect one LED from pin 3 to ground and the other from the positive supply to pin 3, each with its own resistor. When pin 3 is high the first one lights; when it is low the second one does.

What resistor do I need for an LED on a 555 at 9 V? About 470 Ω for a red LED. The output sits about 1.7 V below the supply when high, so an LED to ground gets about 11.3 mA; an LED from the supply to pin 3 gets about 14.6 mA.

How long will a 9 V battery run a 555 flasher? About a day. The chip and two LEDs average roughly 18.8 mA, and an alkaline PP3 gives a few hundred mAh at that current. For an all-night display, use a 5 V USB charger.

Why is my 555 LED always on? Usually pin 2 and pin 6 are not joined, or the timing capacitor is missing or reversed, so the circuit never toggles. Check those first, then that pin 4 is tied to the supply.

Tools and guides this system uses

All build systems: Build systems.