Project 02 · Intermediate · about 45 minutes
Blinky: the 555 timer
An LED that flashes on its own, with no microcontroller and no code. The 555 timer chip, a capacitor and two resistors do all the timing. It is one of the most-built electronics circuits of all time, and it shows how capacitors turn into clocks.
Builds on: Capacitors, Voltage dividers, LEDs
What you need
| Part | Value | Notes |
|---|---|---|
| 555 timer IC | NE555 (or a CMOS version such as the TLC555) | Check your part’s supply range on its datasheet. |
| R1 | 1 kΩ | Between + and pin 7. |
| R2 | 10 kΩ | Between pin 7 and pin 6. |
| C | 47 µF, rated 16 V or more | Electrolytic: the + leg goes to pins 6 and 2. |
| LED and resistor | any colour, 470 Ω | From pin 3 to −. |
| Optional capacitor | 10 nF | Pin 5 to −. Often added to keep the timing steady against noise. |
| Power | 9 V battery and clip | Plus a breadboard and jumper wires. |
How it works
Inside the chip, three equal resistors divide the supply into thirds, and two comparators watch the capacitor’s voltage.
- Charging: the output (pin 3) is high and the LED is on. The capacitor charges through R1 and R2.
- When it reaches ⅔ of the supply, the output goes low (LED off) and pin 7 starts discharging the capacitor through R2.
- When it falls to ⅓ of the supply, the output goes high again, and the cycle repeats.
Because it charges through R1 + R2 but discharges through R2 alone, the LED is always on for a little longer than it is off.
Design your own timing
Each charge or discharge between ⅓ and ⅔ of the supply takes ln 2 ≈ 0.693 time constants, which gives these standard results:
ton = 0.693 (R1 + R2) C · toff = 0.693 R2 C · f ≈ 1.44 ÷ ((R1 + 2 R2) C)
Make C much smaller (around 10–100 nF) and the 555 runs at hundreds or thousands of hertz. Replace the LED with a small speaker (through a capacitor) or a passive piezo disc and it plays a tone. “Hear this frequency” plays what your values would sound like; anything below about 20 Hz is a blink, not a tone.
Build it
- Place the 555 across the centre gap of the breadboard, notch to the left or top, so each pin has its own row. Practise the whole layout first in the breadboard trainer.
- Power pins: pin 8 (VCC) and pin 4 (RESET) to the + rail; pin 1 (GND) to the − rail.
- Timing parts: R1 from + to pin 7; R2 from pin 7 to pin 6; a jumper from pin 6 to pin 2.
- Capacitor: + leg to pin 6 (and so pin 2), − leg to the − rail. Optionally, 10 nF from pin 5 to −.
- Output: pin 3 → 470 Ω → LED long leg; LED short leg to −.
- Check everything, then connect the battery. The LED should flash about 1.5 times a second.
| What you see | Likely cause |
|---|---|
| LED never lights | LED backwards, pin 4 not tied to +, or the chip is in the wrong way round (check the notch). |
| LED stays on | The capacitor is missing or not connected to pins 6 and 2, so the voltage never reaches ⅔ of the supply. |
| Flashes too fast or too slow | Check the values: 47 µF and 4.7 µF look alike, and so do 10 kΩ and 1 kΩ (brown-black-orange versus brown-black-red). |
| Chip gets hot | Disconnect at once. Usually + and − are swapped, or pin 3 or pin 7 is shorted to a supply rail. |
Use a 9 V battery or a low-voltage supply within your 555’s rating (about 16 V for the NE555). Fit the electrolytic capacitor the right way round; reversed electrolytics can overheat.