kicad-learning-path

555 Timer LED Blinker

A 5 V NE555 astable oscillator that flashes a red LED about once per second. Project 2 of my KiCad learning path: capacitors, RC timing and the 555 timer.

3D render of the finished board

Overview
Key specifications
Gallery
Timing diagram
How the circuit works
Design calculations (summary)
Bill of materials
Netlist
Repository structure
Getting started
Testing
Verification status
Future improvements
License

Overview

Project 1 used only a resistor, a switch and an LED. This board adds time: a capacitor charges and discharges through resistors, and a 555 timer IC turns that slow voltage ramp into a clean on/off signal.

New ideas compared with Project 1:

Capacitors: charge, discharge and the RC time constant
An integrated circuit with eight pins, and how to power it correctly
Polarised parts (electrolytic capacitor, IC pin 1, LED)
Decoupling (bypass) capacitors and where to place them
Net labels in the schematic
A through-hole DIP footprint, ground pours on both layers and stitching vias
A timing diagram in the documentation

What it does: with 5 V on header J1, the capacitor C1 repeatedly charges and discharges between one third and two thirds of the supply voltage. The 555 flips its output each time a threshold is crossed. The LED is wired to light while the output is low, which gives about 0.47 s on and 0.50 s off.

The full engineering write-up (theory, derivations, KiCad configuration, layout rules, test procedure) is in docs/DESIGN_NOTES.md.

Key specifications

Parameter	Value
Supply voltage	5 V DC via `J1` (NE555 operating range 4.5 V to 16 V)
Mode	Astable (free-running) oscillator
Frequency	≈ 1.02 Hz (period ≈ 0.975 s)
LED on / off time	0.471 s / 0.504 s
Timing parts	`R1` 4.7 kΩ, `R2` 68 kΩ, `C1` 10 µF
Worst-case frequency range	≈ 0.84 Hz to 1.29 Hz (dominated by the ±20 % electrolytic capacitor)
LED current	≈ 8.6 mA (`R3` 330 Ω)
Supply current	≈ 2 mA (LED off), ≈ 12.7 mA (LED on), ≈ 7.2 mA average
Board layers	2
Board thickness	1.6 mm
Copper weight	1 oz (35 µm)
Board size	17.750 × 21.004 mm
Components	9 (`J1`, `U1`, `R1` to `R3`, `C1` to `C3`, `D1`)
Pads / nets	24 / 7
Design tool	KiCad 10.0.x
ERC / DRC	0 errors / 0 errors

Gallery

Schematic

Full-size PDF: docs/schematic.pdf

PCB layout

3D views

Top	Bottom

Timing diagram

Phase	Capacitor voltage	Output (pin 3)	Discharge (pin 7)	LED	Duration
Charging	Rises from 1.67 V to 3.33 V through `R1` + `R2`	High	Off (open)	Off	`tH` = 0.504 s
Discharging	Falls from 3.33 V to 1.67 V through `R2`	Low	On (shorted to GND)	On	`tL` = 0.471 s

After power-up the capacitor starts at 0 V, so the first charging phase is longer (about 0.80 s) before the steady pattern begins.

How the circuit works

                +5V
                 │
      ┌──────────┼───────────────┐
      │          R1 4.7k         R3 330
      │          │               │
      │ pin 8/4  ├── pin 7       D1 LED
      │          R2 68k          │
   NE555         │               │
      │ pin 6/2 ─┴── C1 10µF ─ GND
      │ pin 3 (OUT) ─────────────┘   (LED sinks current into the output)
      │ pin 1 ── GND

The capacitor charges through R1 and R2 while the output is high.
When its voltage reaches two thirds of the supply, the threshold comparator trips. The output goes low and the discharge transistor (pin 7) switches on.
The capacitor now discharges through R2 only.
When its voltage falls to one third of the supply, the trigger comparator trips. The output goes high and the discharge transistor switches off.
The cycle repeats forever.

The LED is connected between +5V (through R3) and the output, so it lights while the output is low. At a 5 V supply the 555’s low output level is much closer to the rail than its high level is, which makes the LED current predictable.

Design calculations (summary)

Timing (from the NE555 datasheet, astable mode):

\[t_H = 0.693\,(R_1 + R_2)\,C_1 = 0.693 \times 72.7\ \text{k}\Omega \times 10\ \mu\text{F} = 0.504\ \text{s}\] \[t_L = 0.693\,R_2\,C_1 = 0.693 \times 68\ \text{k}\Omega \times 10\ \mu\text{F} = 0.471\ \text{s}\] \[T = t_H + t_L = 0.693\,(R_1 + 2R_2)\,C_1 = 0.975\ \text{s} \qquad f = \frac{1.44}{(R_1 + 2R_2)\,C_1} \approx 1.02\ \text{Hz}\]

LED current (output low level of about 0.15 V at 8 mA, from the datasheet):

\[I_{LED} = \frac{V_{CC} - V_F - V_{OL}}{R_3} = \frac{5.0 - 2.0 - 0.15}{330\ \Omega} \approx 8.6\ \text{mA}\]

Quantity	Value
Period / frequency	0.975 s / 1.02 Hz
LED on-time share	48.3 %
LED current (nominal / worst case)	8.64 mA / 6.45 mA to 10.25 mA
`R3` power	24.6 mW (maximum 34.4 mW)
`R1` current while the output is low	1.06 mA

The full derivation, worst-case analysis and component ratings are in docs/DESIGN_NOTES.md.

Bill of materials

Ref	Qty	Value	Footprint	Description
`J1`	1	Conn_01x02_Pin	`PinHeader_1x02_P2.54mm_Vertical`	2-pin header, 5 V in
`U1`	1	NE555P	`DIP-8_W7.62mm`	555 timer, DIP-8 (use an 8-pin socket)
`R1`	1	4.7 kΩ	`R_0805_2012Metric`	Resistor, 1 %
`R2`	1	68 kΩ	`R_0805_2012Metric`	Resistor, 1 %
`R3`	1	330 Ω	`R_0805_2012Metric`	LED current-limit resistor, 1 %
`C1`	1	10 µF, 16 V	`CP_Radial_D5.0mm_P2.00mm`	Electrolytic timing capacitor (polarised)
`C2`	1	100 nF	`C_0805_2012Metric`	Ceramic decoupling capacitor, X7R, 50 V
`C3`	1	10 nF	`C_0805_2012Metric`	Ceramic control-pin capacitor, X7R, 50 V
`D1`	1	RED	`LED_0805_2012Metric`	Red LED, 0805

The same list is available as CSV in fabrication/bom.csv.

Netlist

Net	Connections
`+5V`	`J1`-1, `U1`-8 (VCC), `U1`-4 (RESET), `R1`-1, `R3`-1, `C2`-1
`GND`	`J1`-2, `U1`-1 (GND), `C1`-2 (−), `C2`-2, `C3`-2
`DISCH`	`U1`-7, `R1`-2, `R2`-1
`TIMING`	`U1`-6 (THRES), `U1`-2 (TRIG), `R2`-2, `C1`-1 (+)
`CTRL`	`U1`-5, `C3`-1
`Net-(D1-A)`	`R3`-2, `D1` anode
`OUT`	`U1`-3, `D1` cathode

Repository structure

This project lives in the 02-555-blinker/ folder of the kicad-learning-path repository.

02-555-blinker/
├── README.md
├── 02_555_blinker.kicad_pro     KiCad project file
├── 02_555_blinker.kicad_sch     Schematic
├── 02_555_blinker.kicad_pcb     PCB layout
├── fabrication/
│   ├── gerbers/                 Gerber and drill files
│   ├── 02_555_blinker_gerbers.zip
│   └── bom.csv
└── docs/
    ├── DESIGN_NOTES.md          Full engineering documentation
    ├── schematic.pdf
    ├── images/                  Timing diagram, screenshots and 3D renders
    └── reports/                 ERC and DRC reports

Getting started

Open the project

Install KiCad 10.
Clone or download this repository.
Open 02-555-blinker/02_555_blinker.kicad_pro in KiCad. The project uses only the standard KiCad libraries.

Rebuild the manufacturing files

Open the PCB editor and run Inspect → Design Rules Checker. It should report 0 errors.
Open File → Fabrication Outputs → Gerbers and Generate Drill Files. The exact settings are listed in docs/DESIGN_NOTES.md.
Zip the output folder.

Order the PCB

Upload fabrication/02_555_blinker_gerbers.zip to a PCB fabricator. Suggested options for a low-cost fab: 2 layers, 1.6 mm, 1 oz copper, HASL finish.

Testing

Bring-up steps, expected measurements and a troubleshooting table are in docs/DESIGN_NOTES.md. In short: apply 5 V and the LED should flash about 31 times in 30 seconds (anything from 25 to 39 is within tolerance).

Verification status

Check	Result
Electrical Rules Check (ERC)	0 errors
Design Rules Check (DRC)	0 errors, 0 unrouted nets
Gerber viewer check	Passed
Physical board built and tested	Not yet

Reports: docs/reports/

Future improvements

Add a potentiometer in place of R2 to make the blink rate adjustable
Add reverse-polarity protection on J1 (see Project 5 in the roadmap)
Try a CMOS 555 (for example TLC555) for lower supply current
Add a second LED that flashes in the opposite phase
Add mounting holes

License

Released under the MIT License.

Author: Nitish Ghadage · GitHub

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