Session 2 — Enhanced Build
Variable-Speed Blinking LED
555 Timer + Potentiometer
Turn a knob to control how fast your LED blinks — from a slow pulse to a quick flash!
⛔
Golden Rule
ALWAYS disconnect the battery pack before making any changes to your circuit. Only connect the battery when your teacher says it's OK.
In the basic circuit, the blink speed was fixed by R2 (47kΩ). Now we replace R2 with a potentiometer — a variable resistor you can twist to smoothly change the blink speed in real time.
❌ Basic 555 (Session 2)
R2 = fixed 47kΩ resistor
Blink speed is locked — one speed only.
To change speed you must swap the resistor.
FIXED
✅ This Build (Enhanced)
R2 = 10kΩ potentiometer + 1kΩ safety resistor
Twist the knob to go from slow pulse → quick flash.
Change speed instantly without touching the circuit!
VARIABLE
Click to tick off. Cyan items are new or changed compared to the basic build.
1×555 Timer ICBlack rectangle, 8 legs, notch/dot at one end
1×LED (any colour)Long leg = anode (+), short leg = cathode (−)
1×Resistor — 470Ω (R3)Yellow – Violet – Brown – Gold. Protects the LED.
1×Resistor — 1kΩ (R1)Brown – Black – Red – Gold. First timing resistor.
1×Resistor — 1kΩ (R-safety) ★ NEWBrown – Black – Red – Gold. Prevents zero-resistance when pot is at minimum.
1×Potentiometer — 10kΩ ★ NEW3 pins sticking out. Usually has a knob or a slot for a screwdriver.
1×Electrolytic capacitor — 100µF (C1) ★ CHANGEDCylinder with legs; white stripe = NEGATIVE. Larger than 10µF to give visible blink range.
1×Ceramic capacitor — 100nF (C2)Tiny disc marked "104" — no polarity
1×BreadboardWhite/cream board with rows of holes
7+Jumper wiresAssorted colours and lengths
1×6V battery pack4× AA — red wire = +, black wire = −
A potentiometer (pot) is a variable resistor. It has 3 pins. Turning the knob moves the wiper along a resistive strip, changing the resistance between the wiper and each outer pin.
Pin 1 — Outer
One end
One end of the resistive strip. We connect this to pin 7 of the 555.
Pin 2 — Wiper
Middle
The moving contact. We connect this (via R-safety) to pin 6 of the 555.
Pin 3 — Outer
Other end
The other end. We connect this to the wiper (pin 2) so it acts as a variable resistor.
🎛️ Why Connect Pin 3 to Pin 2?
- Connecting one outer pin to the wiper makes the pot act as a two-terminal variable resistor (also called a rheostat).
- Turning the knob fully one way = near zero ohms. Fully the other way = 10kΩ.
- The 1kΩ safety resistor (R-safety) is in series so the resistance never drops to zero — that would stop the circuit oscillating.
- Total R2 range: 1kΩ → 11kΩ. Paired with a 100µF capacitor, this gives a great visible blink range!
Same chip, same pins. The highlighted row shows where the potentiometer replaces R2.
| Pin | Name | Connection in this circuit |
|---|
| 1 | GND | → − rail |
| 2 | TRIGGER | → pin 6 (jumper wire) |
| 3 | OUTPUT | → LED anode (long leg) |
| 4 | RESET | → + rail (+6 V) |
| 5 | CONTROL | → 100nF cap → − rail |
| 6 | THRESHOLD | → R-safety (1kΩ) → pot wiper (pin 2) — NEW! |
| 7 | DISCHARGE | → pot outer pin (pin 1) — NEW! |
| 8 | VCC | → + rail (+6 V) |
💡 How Does Changing Resistance Change Blink Speed?
- The 555 blinks because C1 charges up through R1 + R2, then discharges through R2.
- When you increase R2 (turn the pot up) → C1 charges/discharges slower → slower blink.
- When you decrease R2 (turn the pot down) → C1 charges/discharges faster → faster blink.
- The formula: f = 1.44 ÷ ((R1 + 2×R2) × C1). R2 is now the pot + R-safety combined.
Steps 1–5 are the same as the basic 555. Steps 6–7 are where we add the potentiometer. Steps 8–10 finish the circuit.
1
Place the 555 chip
Straddle the centre channel. Notch/dot on the LEFT.
Pin 1 = bottom-left, pin 8 = top-left.
⚠ Tip: Gently straighten bent legs before inserting.
2
Wire up the power
Pin 8 (VCC) → + rail. Pin 1 (GND) → − rail. Pin 4 (RESET) → + rail.
⚠ Tip: Forgetting pin 4 is the #1 reason nothing happens!
3
Add R1 (1kΩ) — first timing resistor
Connect R1 between the + rail and pin 7 (DISCHARGE) row.
4
Link pin 6 to pin 2
Short jumper wire: pin 6 (THRESHOLD) → pin 2 (TRIGGER).
⚠ Tip: If your LED stays permanently on or off later, check this wire first.
5
Add C1 (100µF) and C2 (100nF)
C1 (100µF): POSITIVE leg → pin 2 row. NEGATIVE leg (stripe) → − rail.
C2 (100nF): Pin 5 (CONTROL) → − rail. No polarity.
⚠ Tip: Double-check the electrolytic capacitor's stripe — wrong polarity can damage it. The 100µF is larger than the 100nF — don't mix them up!
6
Place the potentiometer ★ NEW
Insert the potentiometer's three pins into three separate rows on the breadboard.
If it has a knob, the knob should face outward for easy turning.
Note which pin is which: Pin 1 (outer), Pin 2 (wiper/middle), Pin 3 (outer).
★ New component! If your pot doesn't fit the breadboard (some have wide pins), use jumper wires to connect it off-board and hold it by hand.
7
Wire the potentiometer into the circuit ★ NEW
Pot pin 1 (outer) → jumper wire → 555 pin 7 (DISCHARGE) row.
Pot pin 3 (other outer) → jumper wire → pot pin 2 (wiper) row. This shorts one end to the wiper, making it act as a variable resistor.
R-safety (1kΩ): One leg in the pot pin 2 (wiper) row → other leg in the 555 pin 6 (THRESHOLD) row.
★ Key connections: The path is now: 555 pin 7 → pot → R-safety (1kΩ) → 555 pin 6. The pot replaces the old fixed 47kΩ R2.
⚠ Tip: The 1kΩ safety resistor is essential. Without it, turning the pot to zero would stop the circuit working and could damage the 555.
8
Add the LED and R3 (470Ω)
Insert the LED. Connect pin 3 (OUTPUT) → LED anode (long leg).
Connect 470Ω resistor from LED cathode (short leg) → − rail.
9
Check your work
Before powering on, trace the timing path in your head:
+ rail → R1 (1kΩ) → pin 7 → pot (0–10kΩ) → R-safety (1kΩ) → pin 6/pin 2 → C1 → − rail.
Ask your teacher to confirm your wiring.
10
Connect the battery and turn the knob!
Red wire → + rail. Black wire → − rail.
The LED should start blinking. Now slowly turn the potentiometer knob.
Watch the blink speed change smoothly from slow to fast! 🎉
Enter your component values below to calculate the blink frequency. Try different pot positions!
The pot may not be wired correctly. Check: pin 1 (outer) → 555 pin 7. Pin 2 (wiper) → R-safety → 555 pin 6. Pin 3 (outer) → shorted to pin 2. If pin 3 isn't connected to pin 2, the pot won't act as a variable resistor.
Check pin 6 is connected to pin 2. Check C1 polarity. If the pot is turned to maximum (10kΩ), the blink will be quite slow — about one blink every 1.5 seconds. Wait and watch carefully.
The pot might be turned to near-zero. At very low resistance with a 100µF cap, the blink is fast (around 5 Hz) but should still be visible as a flicker. If it looks completely steady, check the wiring. Try turning the pot towards maximum resistance to get a clear, slow blink.
Is the LED the right way round? Is pin 4 connected to +6 V? Is the battery connected? Check the timing path: + rail → R1 → pin 7 → pot → R-safety → pin 6.
Some potentiometers have pins that are too wide or spaced for standard breadboard holes. Use jumper wires to connect the pot pins to the breadboard — hold the pot by hand or use tape/blu-tack to secure it.
⭐
Find the extremes — Turn the pot fully each way. What's the fastest blink speed? The slowest? Time them with a stopwatch and calculate the frequency.
⭐
Swap C1 for 10µF — How does a smaller capacitor change the range of blink speeds? Is the fast end now too fast to see?
⭐⭐
Use the calculator above — Predict the frequency at three different pot positions, then test if your predictions match reality. Record your results.
⭐⭐
Make a heartbeat pattern — Can you find a pot position that creates roughly 1.2 Hz (72 blinks per minute = resting heart rate)?
⭐⭐⭐
Add a second LED between pin 3 and the + rail (with its own 470Ω resistor). What happens? Why does it light up when the first one is off?
⭐⭐⭐
Replace the pot with an LDR (light-dependent resistor) + a fixed resistor. Now the blink speed changes with light! What happens when you cover the LDR?
📝 Quick Reference — Circuit Summary
Pin 1 (GND) → − rail Pin 8 (VCC) → + rail Pin 4 (RESET) → + rail
R1 (1kΩ): + rail → pin 7
POT pin 1 → pin 7 POT pin 3 → pot pin 2 (short) R-safety (1kΩ): pot pin 2 → pin 6
Jumper: pin 6 → pin 2 C1 (100µF): pin 2 (+) → − rail (−) C2 (100nF): pin 5 → − rail
Pin 3 (OUTPUT) → LED anode LED cathode → 470Ω → − rail