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Design Notes: 555 Timer LED Blinker

Revision 1.0 · KiCad 10.0.x · Last updated: 2026-10-21

This document records every engineering decision behind the board: the electronics theory, the derivation of the timing equations, the calculations, the timing diagram, the KiCad configuration, the layout rules, the verification results and the manufacturing settings.

Contents

  1. Requirements
  2. Electronics background
  3. Calculations
  4. Timing diagram
  5. Schematic
  6. KiCad configuration
  7. Layout
  8. Verification
  9. Manufacturing outputs
  10. Bring-up and testing
  11. Lessons learned
  12. References

1. Requirements

# Requirement
R1 Powered from a 5 V DC source through a 2-pin header
R2 The LED flashes automatically, with no button or microcontroller
R3 Blink rate of about 1 Hz, with the LED on for roughly half of each cycle
R4 LED current between 5 mA and 10 mA in every tolerance case
R5 Stable operation: the timer must be decoupled and its control pin bypassed
R6 Hand-solderable parts (DIP socket, 0805 SMD, one radial capacitor)
R7 2-layer board that a low-cost fab can build without special options
R8 Zero ERC errors and zero DRC errors

2. Electronics background

2.1 The capacitor

A capacitor stores energy in an electric field between two plates. Three relationships describe it:

\[Q = C \times V \qquad I = C\,\frac{dV}{dt} \qquad E = \tfrac{1}{2}\,C\,V^2\]
Symbol Meaning Unit
$Q$ Stored charge coulomb (C)
$C$ Capacitance farad (F)
$V$ Voltage across the capacitor volt (V)
$I$ Current into the capacitor ampere (A)
$E$ Stored energy joule (J)

The middle equation is the important one: current only flows while the voltage is changing. A capacitor blocks steady DC and passes changes. A larger capacitor needs more charge to change its voltage by the same amount, so it changes more slowly for the same current.

Type Used here for Notes
Aluminium electrolytic (C1) Timing capacitor, 10 µF Large value, polarised (reverse voltage damages it), tolerance typically ±20 %
Ceramic X7R (C2, C3) Decoupling and control-pin bypass Small value, not polarised, low series inductance

2.2 Charging and discharging through a resistor

When a capacitor is charged through a resistor $R$ from a source $V_S$, starting at voltage $V_0$:

\[v_C(t) = V_S - (V_S - V_0)\,e^{-t/\tau} \qquad \tau = R \times C\]

When it discharges through a resistor to 0 V:

\[v_C(t) = V_0\,e^{-t/\tau}\]

$\tau$ is the time constant. After a given number of time constants the capacitor has covered this fraction of the distance to its final value:

Elapsed time Fraction of the way
1 τ 63.2 %
2 τ 86.5 %
3 τ 95.0 %
5 τ 99.3 %

2.3 Inside the 555 timer

The NE555 contains five blocks:

Block Function
Resistor divider (three equal resistors) Creates two reference voltages: two thirds and one third of VCC
Threshold comparator (pin 6, THRES) Trips when the input rises above two thirds of VCC
Trigger comparator (pin 2, TRIG) Trips when the input falls below one third of VCC
SR flip-flop Remembers which comparator tripped last
Output stage (pin 3) and discharge transistor (pin 7) Drives the output, and shorts pin 7 to ground while the output is low

Function table (paraphrased from the datasheet, voltages are nominal):

RESET TRIG voltage THRES voltage Output Discharge switch
Low any any Low On
High below 1/3 VCC any High Off
High above 1/3 VCC above 2/3 VCC Low On
High above 1/3 VCC below 2/3 VCC Unchanged (holds its previous state) Unchanged

Because both reference levels are fixed fractions of VCC, the timing is almost independent of the supply voltage (the datasheet gives about 0.3 % per volt for the NE555 in astable mode).

Pin functions (DIP-8):

Pin Name Function in this circuit
1 GND Ground
2 TRIG Tied to pin 6 and to the timing capacitor
3 OUT Sinks the LED current
4 RESET Tied to VCC so the timer is never held in reset
5 CONT Bypassed to ground with C3
6 THRES Tied to pin 2 and to the timing capacitor
7 DISCH Discharges the capacitor through R2
8 VCC +5 V

2.4 How the astable oscillator works

  1. At power-up the capacitor is empty (0 V). The trigger comparator sees a voltage below one third of VCC, so the output goes high and the discharge transistor is off.
  2. The capacitor charges through R1 and R2 toward VCC.
  3. When its voltage rises above one third of VCC, nothing changes: the flip-flop holds its state.
  4. When its voltage reaches two thirds of VCC, the threshold comparator trips. The output goes low and the discharge transistor switches on.
  5. The capacitor now discharges through R2 into pin 7. R1 only feeds current into pin 7 during this phase.
  6. When its voltage falls to one third of VCC, the trigger comparator trips. The output goes high, the discharge transistor switches off, and the cycle repeats from step 2.

From then on the capacitor voltage swings between one third and two thirds of VCC.

2.5 Derivation of the timing equations

High time. The capacitor charges from $V_{CC}/3$ toward $V_{CC}$ through $R_1 + R_2$ and stops at $2V_{CC}/3$:

\[\tfrac{2}{3}V_{CC} = V_{CC} - \left(V_{CC} - \tfrac{1}{3}V_{CC}\right)e^{-t_H/\tau_c} \;\Rightarrow\; e^{-t_H/\tau_c} = \tfrac{1}{2} \;\Rightarrow\; t_H = \ln 2 \times (R_1 + R_2)\,C \approx 0.693\,(R_1 + R_2)\,C\]

Low time. The capacitor discharges from $2V_{CC}/3$ to $V_{CC}/3$ through $R_2$ only:

\[\tfrac{1}{3}V_{CC} = \tfrac{2}{3}V_{CC}\,e^{-t_L/\tau_d} \;\Rightarrow\; e^{-t_L/\tau_d} = \tfrac{1}{2} \;\Rightarrow\; t_L = \ln 2 \times R_2\,C \approx 0.693\,R_2\,C\]

Period, frequency and duty cycle:

\[T = t_H + t_L = 0.693\,(R_1 + 2R_2)\,C \qquad f = \frac{1}{T} \approx \frac{1.44}{(R_1 + 2R_2)\,C} \qquad \frac{t_H}{T} = \frac{R_1 + R_2}{R_1 + 2R_2}\]

$V_{CC}$ cancels out of every step, which is why the frequency does not depend on the supply voltage.

Power-up. The first charge starts from 0 V instead of $V_{CC}/3$, so it takes longer:

\[t_{first} = \ln 3 \times (R_1 + R_2)\,C = 1.099 \times 0.727\ \text{s} \approx 0.80\ \text{s}\]

3. Calculations

3.1 Targets and constraints

Item Value Source
Supply voltage $V_{CC}$ 5.0 V Requirement R1
Target frequency about 1 Hz Requirement R3
Target duty cycle close to 50 % Requirement R3
$R_1$ lower limit 1 kΩ or more Keeps the discharge current and the wasted current small
$R_1 + R_2$ upper limit far below 3.4 MΩ at 5 V Datasheet limit set by the threshold input current

3.2 Choosing the timing components

  1. Choose $C_1$ first. 10 µF is a common electrolytic value that gives convenient resistor values at 1 Hz.
  2. Total resistance needed for $f = 1$ Hz:
\[R_1 + 2R_2 = \frac{1.44}{f\,C_1} = \frac{1.44}{1 \times 10\ \mu\text{F}} = 144\ \text{k}\Omega\]
  1. Make $R_2$ much larger than $R_1$ so the duty cycle approaches 50 %. With $R_1$ = 4.7 kΩ:
\[R_2 = \frac{144\ \text{k}\Omega - 4.7\ \text{k}\Omega}{2} = 69.7\ \text{k}\Omega \;\rightarrow\; \text{68 k}\Omega\ \text{(standard E12 value)}\]

3.3 Timing results

Quantity Formula Result
$\tau_c = (R_1 + R_2)\,C_1$ 72.7 kΩ × 10 µF 0.727 s
$\tau_d = R_2\,C_1$ 68 kΩ × 10 µF 0.680 s
$t_H$ 0.693 × 0.727 s 0.504 s
$t_L$ 0.693 × 0.680 s 0.471 s
$T$ $t_H + t_L$ 0.975 s
$f$ 1.44 / (140.7 kΩ × 10 µF) 1.02 Hz (the exact form with ln 2 gives 1.025 Hz)
Output high share $t_H / T$ 51.7 %
LED on share (output low) $t_L / T$ 48.3 %

3.4 LED current

The LED sits between +5V (through R3) and the output pin, so the output pin sinks the LED current while it is low.

Why sink instead of source? At a 5 V supply the datasheet gives a low-level output voltage of typically 0.15 V (maximum 0.4 V) at 8 mA. The high-level output, in contrast, drops well below the supply: the datasheet lists only 3.3 V typical at 100 mA. A sinking LED therefore sees a predictable voltage across its resistor.

\[I_{LED} = \frac{V_{CC} - V_F - V_{OL}}{R_3} = \frac{5.0\ \text{V} - 2.0\ \text{V} - 0.15\ \text{V}}{330\ \Omega} = 8.64\ \text{mA}\]
Element Voltage Current Power
R3 2.85 V 8.64 mA 24.6 mW (0805 rating about 125 mW)
D1 2.0 V 8.64 mA 17.3 mW
Output pin (low) 0.15 V 8.64 mA 1.3 mW

3.5 Worst-case analysis

LED current. Assumptions: supply 5 V ± 5 %, R3 ± 1 %, LED forward voltage 1.8 V to 2.2 V, output low voltage 0.10 V to 0.40 V.

Case $V_{CC}$ $V_F$ $V_{OL}$ $R_3$ Current R3 power
Nominal 5.00 V 2.0 V 0.15 V 330.0 Ω 8.64 mA 24.6 mW
Maximum 5.25 V 1.8 V 0.10 V 326.7 Ω 10.25 mA 34.4 mW
Minimum 4.75 V 2.2 V 0.40 V 333.3 Ω 6.45 mA 13.9 mW

The current stays between 6.45 mA and 10.25 mA, inside requirement R4.

Frequency. Assumptions: R1 and R2 ± 1 %, C1 ± 20 %. The datasheet adds a typical initial timing error of 2.25 % for the NE555 in astable mode, which is small in comparison.

Case $R_1$ $R_2$ $C_1$ Period Frequency
Slowest 4.747 kΩ 68.68 kΩ 12 µF 1.18 s 0.84 Hz
Nominal 4.7 kΩ 68 kΩ 10 µF 0.975 s 1.02 Hz
Fastest 4.653 kΩ 67.32 kΩ 8 µF 0.77 s 1.29 Hz

The electrolytic capacitor’s tolerance dominates. For accurate timing, use a film or ceramic capacitor, or measure and trim R2.

3.6 Current and power budget

State Supply current Breakdown
Output high (LED off) about 2.0 mA 555 supply current (typical 2 mA) plus a few µA of capacitor charging current
Output low (LED on) about 12.7 mA 555 supply current (typical 3 mA) + LED 8.64 mA + R1 current into pin 7 (1.06 mA)
Time-weighted average about 7.2 mA Weighted by 51.7 % high and 48.3 % low
Average power at 5 V about 36 mW  

The 555 supply-current figures are typical values at 5 V with no load; the datasheet maximums are 5 mA (output high) and 6 mA (output low).

R1 power while the output is low: $P = I^2 R = (1.06\ \text{mA})^2 \times 4.7\ \text{k}\Omega = 5.3\ \text{mW}$.

3.7 Component ratings

Part Rating needed Chosen
C1 Voltage at least 2 to 3 times the working voltage (about 3.3 V peak, up to 4.2 V at the datasheet limit) 16 V
C2, C3 Voltage above 5 V with margin for DC-bias capacitance loss 50 V, X7R
R1 to R3 Power well above 5 mW to 35 mW 125 mW (0805)
U1 VCC 4.5 V to 16 V, output current up to ±200 mA NE555P at 5 V

3.8 Why the two extra capacitors

Part Value Reason
C2 (VCC to GND) 100 nF The 555 output stage draws sharp current spikes when it switches. A capacitor right at pins 8 and 1 supplies them locally so the supply does not glitch. An ideal 100 nF capacitor has an impedance of $1/(2\pi \times 10\ \text{MHz} \times 100\ \text{nF}) \approx 0.16\ \Omega$ at 10 MHz
C3 (CONT to GND) 10 nF The datasheet shows this connection and notes that decoupling the control pin can improve operation. It keeps noise off the internal reference levels

Using $f \approx 1.44 / ((R_1 + 2R_2)\,C_1)$ with $R_1$ = 4.7 kΩ:

$R_2$ $C_1$ Frequency
68 kΩ 10 µF 1.02 Hz (this design)
33 kΩ 10 µF 2.04 Hz
68 kΩ 100 µF 0.10 Hz
68 kΩ 1 µF 10.2 Hz (the LED appears to flicker)

4. Timing diagram

Timing diagram

Reading the diagram. The blue curve is the voltage on pins 2 and 6 (the timing capacitor). The black line is the output on pin 3. The orange areas show when the LED is lit.

Time Event Output Discharge pin 7 LED
0 Capacitor at 1/3 VCC (1.67 V), charging begins High Off Off
0 to 0.504 s Capacitor charges through R1 + R2 High Off Off
0.504 s Capacitor reaches 2/3 VCC (3.33 V): THRES comparator trips Goes low Switches on Turns on
0.504 s to 0.975 s Capacitor discharges through R2 Low On On
0.975 s Capacitor reaches 1/3 VCC (1.67 V): TRIG comparator trips Goes high Switches off Turns off

Signal summary:

Signal Level during $t_H$ (0.504 s) Level during $t_L$ (0.471 s)
Pin 2 / pin 6 voltage Rising, 1.67 V to 3.33 V Falling, 3.33 V to 1.67 V
Pin 3 (OUT) High (TTL-compatible) Low (about 0.15 V)
Pin 7 (DISCH) High impedance Shorted to GND
LED current 0 about 8.6 mA

The curve segments are exponentials, not straight lines: the capacitor changes fastest at the start of each phase and slows as it approaches its target. Charging is slightly slower than discharging because it goes through the larger resistance (R1 + R2 versus R2).

5. Schematic

5.1 Symbols and footprints

Ref Library symbol Value Footprint
J1 Connector:Conn_01x02_Pin Conn_01x02_Pin Connector_PinHeader_2.54mm:PinHeader_1x02_P2.54mm_Vertical
U1 Timer:NE555P NE555P Package_DIP:DIP-8_W7.62mm
R1 Device:R 4.7k Resistor_SMD:R_0805_2012Metric
R2 Device:R 68k Resistor_SMD:R_0805_2012Metric
R3 Device:R 330 Resistor_SMD:R_0805_2012Metric
C1 Device:CP 10uF Capacitor_THT:CP_Radial_D5.0mm_P2.00mm
C2 Device:C 100nF Capacitor_SMD:C_0805_2012Metric
C3 Device:C 10nF Capacitor_SMD:C_0805_2012Metric
D1 Device:LED RED LED_SMD:LED_0805_2012Metric
Power power:+5V, power:GND, power:PWR_FLAG ×2 n/a n/a

5.2 Nets

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

5.3 Design decisions

5.4 Schematic conventions

6. KiCad configuration

6.1 Project creation

  1. Create the folder 02-555-blinker inside the repository.
  2. In KiCad, choose File → New Project, browse to that folder, name the project 02_555_blinker, and untick “Create a new folder for the project” so the files land directly in 02-555-blinker.

6.2 Board setup

Board settings are imported from Project 1 so both boards share the same rules: in the PCB editor, open File → Board Setup, click Import Settings from Another Board…, select 01_led_button.kicad_pcb, tick the stackup, design rules, net classes and text defaults, and confirm.

Setting Value
Copper layers 2, board thickness 1.6 mm, 1 oz copper
Minimum clearance 0.2 mm
Minimum track width 0.2 mm
Minimum connection width 0.2 mm
Minimum annular width 0.15 mm
Minimum via diameter 0.6 mm
Copper to hole clearance 0.3 mm
Copper to edge clearance 0.5 mm
Minimum drill size 0.3 mm
Hole to hole clearance 0.5 mm
Silkscreen: minimum clearance / text height / text thickness 0.15 mm / 1.0 mm / 0.15 mm
Net class Clearance Track width Via diameter / drill Nets
Default 0.2 mm 0.25 mm 0.6 mm / 0.3 mm all others
Power 0.2 mm 0.5 mm 0.8 mm / 0.4 mm +5V, GND

The values are the same ones used for Project 1 and are based on typical low-cost 2-layer fab capabilities (see References). Re-check your fab’s current limits before ordering.

6.3 Grid

PCB grid: 0.5 mm for placement, 0.25 mm or 0.1 mm for routing.

7. Layout

7.1 Placement

Rule Reason
Place U1 first, near the centre, with pin 1 clearly marked Everything else connects to it
Place C2 as close as possible to pins 8 and 1 (within about 5 mm to 10 mm), on the same side Decoupling only works if the loop through the capacitor is short
Keep R1, R2 and C1 together, close to pins 2, 6 and 7 Short timing nodes pick up less noise and leakage
Place C3 next to pin 5 Short connection to the control pin
Place J1 at a board edge Easy connection of the supply
Orient C1 so its + marking is easy to see Reduces the chance of reversed assembly
Place D1 and R3 where the LED is visible Practical use

7.2 Routing

Rule Value
Signal track width 0.25 mm
Power track width (+5V, GND) 0.5 mm
Corner angles 45° or curved, never 90°
Copper to board edge at least 0.5 mm
Via size 0.6 mm pad / 0.3 mm drill

Between DIP pads. The DIP-8 footprint has 1.6 mm pads on a 2.54 mm pitch, which leaves a gap of about 0.94 mm. One 0.25 mm track plus a 0.2 mm clearance on each side needs 0.65 mm, so a single signal track fits between two pads. A 0.5 mm power track needs 0.9 mm and only just fits, so avoid routing it there. (Check the pad size in the footprint editor.)

7.3 Ground pour and stitching vias

7.4 Silkscreen

7.5 Board outline

A rectangle on Edge.Cuts with rounded corners, size XX × XX mm. The outline is a single closed shape with no gaps.

8. Verification

Check How Result
ERC Schematic editor → Inspect → Electrical Rules Checker 0 errors
DRC PCB editor → Inspect → Design Rules Checker 0 errors, 0 unrouted
Board statistics PCB editor status bar 24 pads, 7 nets
3D view PCB editor → View → 3D Viewer (Alt+3) Orientation of U1, C1 and D1 correct
Gerber check Third-party viewer (KiCad Gerber Viewer, Gerbv, tracespace) Outline closed, holes aligned, silkscreen clear of pads

Saved reports are in reports/.

9. Manufacturing outputs

9.1 Gerber plot settings (File → Fabrication Outputs → Gerbers)

Setting Value
Output directory fabrication/gerbers
Layers F.Cu, B.Cu, F.Paste, B.Paste, F.Silkscreen, B.Silkscreen, F.Mask, B.Mask, Edge.Cuts
Use Protel filename extensions On
Subtract soldermask from silkscreen On
Check zone fills before plotting On
Coordinate format 4.6, unit mm
Use extended X2 format On
Include netlist attributes On
Disable aperture macros Off
Use drill/place file origin Off
Plot drawing sheet Off

9.2 Drill file settings

Setting Value
Drill file format Excellon
Oval holes drill mode Use alternate drill mode
Drill origin Absolute
Drill units Millimetres
Zeros format Decimal format
Map file format (optional) Gerber

9.3 Suggested fab order options

Option Value
Layers 2
Thickness 1.6 mm
Outer copper weight 1 oz
Surface finish HASL (lead-free if available)
Quantity Fab minimum

Verify the generated files in a Gerber viewer, zip the gerbers folder, and upload the zip.

10. Bring-up and testing

10.1 Before powering

  1. Solder the 8-pin socket first, then the other parts, then insert U1 with its notch matching the pin 1 mark.
  2. Check the orientation of C1 (stripe side to GND) and D1 (cathode towards OUT).
  3. With a multimeter in continuity mode, confirm there is no short between +5V and GND.

10.2 Powered test

  1. Apply 5 V to J1 (pin 1 = +5 V, pin 2 = GND).
  2. The LED stays off for roughly 0.8 s, then flashes about once per second.
  3. Count the flashes over 30 seconds. About 31 is expected, and 25 to 39 is within tolerance.
  4. Measure the values in the table below.
Measurement Expected Measured
Supply voltage at J1 5.0 V  
Flashes in 30 s about 31 (25 to 39)  
Voltage at pin 2/6 (multimeter, average) about 2.5 V  
Voltage at pins 2/6 (oscilloscope) Swings between 1.67 V and 3.33 V  
Voltage at pin 3 while the LED is on 0.1 V to 0.4 V  
Voltage across R3 while the LED is on about 2.85 V (current about 8.6 mA)  
Supply current about 2 mA (LED off), about 12.7 mA (LED on)  

10.3 Troubleshooting

Symptom Likely cause
LED never lights D1 reversed, U1 inserted backwards, or R3 not connected to +5V
LED always on Pins 2 and 6 not connected to the capacitor, or C1 reversed or open
LED always off Pin 4 not tied to +5V, or U1 unpowered
Blink rate far from 1 Hz Wrong resistor or capacitor value (check the markings), or a worn electrolytic
Erratic or double flashes Missing or poorly placed C2 or C3, or a poor connection on the timing node
Capacitor gets warm or smells C1 is reversed: switch off immediately and replace it

11. Lessons learned

12. References

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