Session 3 of 6

Alternating LEDs

The Astable Multivibrator

Two LEDs that take turns blinking — built from transistors, capacitors, and resistors!

What You Need

Click each component to tick it off as you find it.

2×NPN Transistor — BC547 or 2N2222Small black part, 3 legs, flat side. Labelled on the body.

1×LED — Red (LED1)Long leg = anode (+), short leg / flat side = cathode (−)

1×LED — Green (LED2)Long leg = anode (+), short leg / flat side = cathode (−)

2×Resistor — 470Ω (R1 & R2)Bands: Yellow – Violet – Brown – Gold. Protect the LEDs.

2×Resistor — 47kΩ (R3 & R4)Bands: Yellow – Violet – Orange – Gold. Control the bases.

2×Electrolytic Capacitor — 10µF (C1 & C2)Small cylinder, white stripe = NEGATIVE. These set the blink speed.

1×BreadboardWhite/cream board with rows of holes

8+Jumper wiresUse red for + and black for − connections

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

Know Your Transistor — Pin-Out

Hold the transistor with the flat side facing you and legs pointing down. Left to right:

Pin	Name	What it does in our circuit
Left leg	Emitter (E)	Connects to GND (− rail). Current flows OUT here.
Middle leg	Base (B)	The "control" leg. A small current here switches the transistor ON.
Right leg	Collector (C)	Connects towards +6 V (through the LED and resistor). Current flows IN.

⚠ BC547 vs 2N2222 — Watch Out!

BC547 (flat side facing you): E – B – C (left to right). Most common in UK kits.
2N2222 (TO-92): Same layout — E – B – C.
If you have a metal-can 2N2222 (round, not flat), the pin-out is DIFFERENT — ask your teacher!

How Does It Work?

Two identical halves fight each other. When one side is ON, it forces the other OFF. Then they swap. Over and over:

🔴 Half-Cycle A — LED1 ON

Q1 is ON → current flows through LED1 → LED1 lights up.

C1 charges through R3, holding Q2's base LOW.

Q2 is OFF → LED2 is dark.

When C1 finishes charging, Q2's base goes HIGH…

🟢 Half-Cycle B — LED2 ON

Q2 switches ON → current flows through LED2 → LED2 lights up.

C2 charges through R4, holding Q1's base LOW.

Q1 is OFF → LED1 goes dark.

When C2 finishes charging, Q1's base goes HIGH…

💡 The Key Idea

The two capacitors take turns charging and discharging. Each holds the opposite transistor OFF.
Bigger capacitors or bigger base resistors = slower charging = slower alternation.
The circuit is perfectly symmetrical — both halves do the same job at opposite times.

Build It — Step by Step

Build the LEFT half first, MIRROR it for the RIGHT half, then cross-couple with capacitors.

🔴 Left Half — Q1 and LED1

Place transistor Q1

Insert Q1 with the flat side facing you.

Each of the three legs must be in a different row.

Leave plenty of space on the right for Q2.

⚠ Tip: Push gently — transistor legs are delicate. Straighten any bent legs carefully before retrying.

Wire Q1's emitter to GND

The LEFT leg (emitter) connects to the − power rail.

Use a short jumper wire from Q1's emitter row to the − rail.

Add LED1 (red) and R1 (470Ω) on Q1's collector

Insert LED1 (red) into the breadboard.

Connect LED1's ANODE (long leg) → + power rail.

Connect LED1's CATHODE (short leg) → one end of R1 (470Ω).

Connect the other end of R1 → Q1's COLLECTOR (right leg) row.

⚠ Tip: Current path: +6 V → LED1 → R1 → Q1 collector → Q1 emitter → GND.

Add R3 (47kΩ) to Q1's base

Connect one end of R3 (47kΩ) to the + power rail.

Connect the other end to Q1's BASE (middle leg) row.

This lets a small current reach the base to switch Q1 on.

🟢 Right Half — Q2 and LED2 (Mirror Image)

Place transistor Q2

Insert Q2 to the RIGHT of Q1, flat side facing you.

Same orientation as Q1 — each leg in a different row.

Wire Q2's emitter to GND

Connect Q2's LEFT leg (emitter) to the − power rail, just like Q1.

Add LED2 (green) and R2 (470Ω) on Q2's collector

Insert LED2 (green) into the breadboard.

Connect LED2's ANODE (long leg) → + rail.

Connect LED2's CATHODE (short leg) → one end of R2 (470Ω).

Connect the other end of R2 → Q2's COLLECTOR (right leg) row.

⚠ Tip: This is an exact mirror of Step 3. If LED1 works, just copy the pattern!

Add R4 (47kΩ) to Q2's base

Connect one end of R4 (47kΩ) to the + power rail.

Connect the other end to Q2's BASE (middle leg) row.

This mirrors Step 4 exactly.

🔗 Cross-Coupling — The Magic Step!

This is what makes the circuit oscillate. Each capacitor connects one transistor's COLLECTOR to the other's BASE.

Add C1 — Q1's collector → Q2's base

Take the first 10µF electrolytic capacitor (C1).

POSITIVE leg (long leg, no stripe) → Q1's COLLECTOR row.

NEGATIVE leg (short leg, white stripe) → Q2's BASE row.

⚠ Tip: This is the most important connection! It's what makes the two halves talk to each other.

Add C2 — Q2's collector → Q1's base

Take the second 10µF electrolytic capacitor (C2).

POSITIVE leg (long leg, no stripe) → Q2's COLLECTOR row.

NEGATIVE leg (short leg, white stripe) → Q1's BASE row.

⚠ Tip: The two capacitors criss-cross between the halves. Collector → opposite Base.

⚡ Power Up!

Connect the battery — moment of truth!

🔍 Ask your teacher to check your circuit.

Connect the red wire → + rail.

Connect the black wire → − rail.

Your two LEDs should be alternating: 🔴 ON 🟢 OFF… then 🟢 ON 🔴 OFF! 🎉

It's Not Working! — Troubleshooting

Both LEDs are ON and not blinking

The capacitors may not be cross-coupled correctly. Check: C1 goes from Q1's COLLECTOR to Q2's BASE. C2 goes from Q2's COLLECTOR to Q1's BASE. Not collector-to-collector!

Both LEDs are OFF

Check both emitters are connected to GND. Check the + rail has power. Check neither LED is reversed — long leg should go towards +6 V via the + rail.

Only one LED blinks

One half is probably wired incorrectly. Compare both sides — they should be mirror images. Check both base resistors (R3 and R4) are connected to +6 V and to the correct base.

LEDs alternate but very fast / just a glow

This is actually working! The capacitors are small so it switches very quickly. Try swapping both for 100µF capacitors to slow it down to a visible blink.

One LED is much dimmer than the other

Check the resistor values — both collector resistors should be 470Ω (Yellow–Violet–Brown). A mismatched resistor will cause uneven brightness. Also, red and green LEDs have slightly different brightnesses naturally.

A transistor feels hot! 🔥

Disconnect immediately! A hot transistor usually means a wiring error. Check the pin-out: flat side facing you → E–B–C from left to right. Also check no wires are shorting to the wrong row.

Challenges

⭐

Make it alternate SLOWER — Swap both capacitors for 100µF. What happened?

⭐

Make it alternate FASTER — Swap both capacitors for 1µF (if available) or change base resistors to 10kΩ. What happened?

⭐⭐

Make one LED stay on LONGER — Use mismatched capacitors (e.g. C1 = 10µF, C2 = 100µF). Which LED stayed on longer? Why?

⭐⭐

Calculate the frequency: f ≈ 1 ÷ (1.4 × R × C) using equal R and C values.

⭐⭐⭐

Compare this to the 555 timer from Session 2. What's similar? What's different? Which do you prefer and why?

📝 Quick Reference — Circuit Summary

LEFT: +6V → LED1 (anode→cathode) → R1 470Ω → Q1 collector. Q1 emitter → GND. +6V → R3 47kΩ → Q1 base. RIGHT: +6V → LED2 (anode→cathode) → R2 470Ω → Q2 collector. Q2 emitter → GND. +6V → R4 47kΩ → Q2 base. CROSS: C1 (+) Q1 collector → (−) Q2 base. C2 (+) Q2 collector → (−) Q1 base.