Design Notes
555 LED blinker timing
The NE555 is configured in the standard astable topology:
- R1 = 47 kΩ from switched 9 V rail to DISCH
- R2 = 47 kΩ from DISCH to the shared THRES/TRIG timing node
- C1 = 10 uF from THRES/TRIG to GND
Approximate astable frequency:
f ≈ 1.44 / ((R1 + 2 × R2) × C1)
Substitution:
f ≈ 1.44 / ((47 kΩ + 2 × 47 kΩ) × 10 uF) ≈ 1.02 Hz
So the LED flashes approximately once per second.
LED current limit
The LED is driven from U1 OUT through R3 = 680 Ω into a red through-hole LED to ground.
Approximate LED current, assuming a red LED forward voltage near 2 V and a high output near the 9 V rail:
I ≈ (9 V - 2 V) / 680 Ω ≈ 10.3 mA
This is suitable for a simple indicator LED and keeps resistor dissipation low:
P ≈ I²R ≈ 0.072 W
A 0.25 W through-hole resistor has adequate margin.
Power and bypassing
- BT1 provides the 9 V battery input.
- SW1 switches battery positive before the circuit rail
VCC_SW_9V.
- U1 RESET is tied to
VCC_SW_9V so the 555 runs whenever the switch is on.
- C2 = 100 nF bypasses U1 VCC to GND.
- C3 = 10 nF bypasses U1 CONT to GND for noise immunity.
PCB layout setup
- Board outline: compact 50 mm × 50 mm rectangle with 2 mm corner radius.
- Stackup: Standard 2 Layer, appropriate for a simple low-speed 555 LED blinker.
- General routing rule: 0.5 mm trace width with 0.3 mm minimum trace width.
- Universal board keepout / inset: 1 mm.
PCB component placement
- All ten PCB components are placed on the top layer for straightforward hand assembly.
- BT1 is placed along the left edge for 9 V battery clip access.
- SW1 is placed near the top edge for accessible on/off control.
- LED1 is placed along the bottom/front edge for visibility.
- U1 is near the center of the board.
- R1, R2, and C1 are grouped near the NE555 timing pins.
- C2 is placed close to U1 VCC/GND for supply bypassing.
- C3 is placed close to U1 CONT/GND for control-pin bypassing.
- R3 is placed between U1 OUT and LED1.
Verification notes
Schematic ERC checks passed for floating pins, single-pin nets, undriven nets, missing power, and driver conflicts after wiring. Passive components were assigned through-hole package/MPN metadata for layout/manufacturing readiness.
PCB placement verification confirmed all components are inside the board and placed without component body conflicts. PCB checks passed for overlapping copper, floating copper, missing footprints, invalid layers, protected intrusions, and under-connected vias. Airwires remain because the physical trace routing step has not been completed yet.