Session 2 of 6

Blinking LED with a 555 Timer

Build an LED that blinks on and off by itself — no computer needed!

What You Need

Click each component to tick it off as you find it in your tray.

1×555 Timer ICBlack rectangle, 8 legs, notch or dot at one end

1×LED (any colour)Small dome — long leg = anode (+), short leg = cathode (−)

1×Resistor — 470Ω (R3)Bands: Yellow – Violet – Brown – Gold

1×Resistor — 1kΩ (R1)Bands: Brown – Black – Red – Gold

1×Resistor — 47kΩ (R2)Bands: Yellow – Violet – Orange – Gold

1×Electrolytic capacitor — 10µF (C1)Small cylinder, white stripe = NEGATIVE side

1×Ceramic capacitor — 100nF (C2)Tiny flat disc marked "104" — no polarity

1×BreadboardWhite/cream board with rows of holes

6+Jumper wiresAssorted colours and lengths

1×6V battery pack4× AA — red wire = +, black wire = −

Know Your Chip — 555 Pin-Out

The notch or dot marks the pin 1 end. Count anti-clockwise.

Pin	Name	What it does in our circuit
1	GND	Ground (0 V) — connect to the − rail
2	TRIGGER	Starts the timing cycle — connect to pin 6
3	OUTPUT	Goes HIGH / LOW — this drives our LED
4	RESET	Connect to +6 V to keep the chip running
5	CONTROL	Bypass with 100 nF cap to GND for stability
6	THRESHOLD	Senses when to flip the output — connect to pin 2
7	DISCHARGE	Lets the capacitor discharge through R2
8	VCC	Power supply — connect to the + rail (+6 V)

💡 How Does It Work?

The capacitor (C1) slowly charges up through R1 and R2.
When it's full enough, the chip flips the output OFF and C1 discharges through R2.
When it's empty enough, the chip flips the output back ON and C1 starts charging again.
This cycle repeats forever — making the LED blink!
Bigger R or C = slower charging = slower blink.

Build It — Step by Step

Place the 555 chip

Put the 555 IC so it straddles the centre channel of the breadboard.

The notch/dot should be on the LEFT — pin 1 is bottom-left, pin 8 is top-left.

Push gently but firmly until it sits flat.

⚠ Tip: Don't force it! If the legs bend, carefully straighten them before trying again.

Wire up the power

Connect pin 8 (VCC, top-left) to the + power rail.

Connect pin 1 (GND, bottom-left) to the − power rail.

Connect pin 4 (RESET) to the + power rail. This keeps the chip running.

⚠ Tip: Pin 4 MUST go to +6 V. If you forget this, the chip stays reset and nothing will happen.

Add R1 (1kΩ) — first timing resistor

Connect R1 between the + power rail and pin 7 (DISCHARGE).

One leg in the + rail, the other in the same row as pin 7.

⚠ Tip: Resistors have no polarity — either way round is fine.

Add R2 (47kΩ) — second timing resistor

Connect R2 between pin 7 (DISCHARGE) and pin 6 (THRESHOLD).

One leg in the same row as pin 7, the other in the same row as pin 6.

Link pin 6 to pin 2

Use a short jumper wire to connect pin 6 (THRESHOLD) to pin 2 (TRIGGER).

These two pins must be connected together for the circuit to oscillate.

⚠ Tip: This is easy to forget! If your LED just stays on or off, check this connection first.

Add C1 (10µF) — timing capacitor

Connect the 10µF electrolytic capacitor between pin 2 and the − power rail.

POSITIVE leg (long leg, no stripe) → pin 2 row.

NEGATIVE leg (short leg, white stripe) → − rail.

⚠ Tip: Getting polarity wrong can damage the capacitor. Double-check the stripe!

Add C2 (100nF) — decoupling capacitor

Connect the small 100nF ceramic capacitor between pin 5 (CONTROL) and the − rail.

This smooths out the signal and stops erratic behaviour.

⚠ Tip: This tiny capacitor has no polarity — either way round is fine.

Add the LED and R3 (470Ω)

Insert the LED. Note which row the LONG leg (anode, +) is in.

Connect a jumper from pin 3 (OUTPUT) → LED's long leg row.

Connect the 470Ω resistor from the LED's short leg (cathode) → − rail.

⚠ Tip: The 470Ω resistor protects the LED from too much current. Never skip it!

Connect the battery — moment of truth!

🔍 Ask your teacher to check your circuit.

Connect the red wire → + rail.

Connect the black wire → − rail.

Your LED should be blinking! 🎉

It's Not Working! — Troubleshooting

LED stays on and doesn't blink

Check pin 6 is connected to pin 2. Check C1 is connected and has correct polarity (positive leg to pin 2, negative to − rail).

LED stays off

Is the LED the right way round (long leg towards pin 3)? Is pin 4 connected to +6 V? Is the battery pack connected and switched on?

LED is very dim

Check the battery — are the cells flat? Is every connection pushed in firmly? Try wiggling the 555 chip gently to ensure it's seated properly.

Nothing happens at all

Check the 555 chip orientation — notch/dot on the correct side. Verify pin 8 is on the + rail and pin 1 is on the − rail. Check jumper wires are in the correct rows.

LED flickers randomly

Check C2 (100nF) is connected between pin 5 and GND. Ensure all wires are pushed in firmly — loose connections cause erratic behaviour.

Challenges

⭐

Make it blink FASTER — Swap R2 (47kΩ) for a 10kΩ resistor. What happened?

⭐

Make it blink SLOWER — Swap C1 (10µF) for a 100µF capacitor. What happened?

⭐⭐

Calculate the blink frequency using: f = 1.44 ÷ ((R1 + 2×R2) × C1)

⭐⭐

Make it blink exactly once per second. What R and C values do you need?

⭐⭐⭐

Add a SECOND LED that lights when the first one is off. How did you do it?

📝 Quick Reference — Circuit Summary

Pin 1 (GND) → − rail Pin 8 (VCC) → + rail Pin 4 (RESET) → + rail R1 (1kΩ): + rail → pin 7 R2 (47kΩ): pin 7 → pin 6 Jumper: pin 6 → pin 2 C1 (10µF): pin 2 (+) → − rail (−) C2 (100nF): pin 5 → − rail Pin 3 (OUTPUT) → LED anode (long leg) LED cathode → 470Ω → − rail