LED Blinker Circuit Design
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Project Brief — 9V 555 LED Blinker
Objective
Create a hand-solderable circuit that blinks one red LED approximately once per second.
Requirements
- Power source: 9V block battery.
- Power control: on/off switch in series with battery positive.
- Indicator: one red LED.
- Blink rate: approximately 1 Hz.
- Construction: easy to hand-solder; use through-hole parts where available.
Implemented Architecture
- 9V battery snap/connector BT1 feeds a board-mounted slide switch SW1.
- SW1 is SPDT but used as SPST on/off: common and one throw are wired, the unused throw is marked no-connect.
- U1 is a Texas Instruments NE555P in PDIP-8, powered from the switched 9V rail.
- U1 is wired as a standard astable oscillator.
- D1 is a 5mm red through-hole LED driven from U1 OUT through R3.
- C2 provides 100nF supply bypassing on U1 VCC to GND.
- C3 provides 10nF control-voltage bypassing from CONT to GND.
Final Component Values
Table
| Designator | Function | Value / Part | Notes |
|---|---|---|---|
| BT1 | Battery connector | Keystone 967 | 9V battery snap, through-hole PCB pins |
| SW1 | On/off switch | Slide Switch SW_SPDT | SPDT used as SPST |
| U1 | Timer | NE555P | TI 555 timer, PDIP-8, 4.5V–16V operating range |
| R1 | Timing RA | 10kΩ, 1/4W, axial THT | VSW_9V to DISCHARGE |
| R2 | Timing RB | 68kΩ, 1/4W, axial THT | DISCHARGE to TIMING |
| C1 | Timing capacitor | 10µF, 16V | TIMING to GND |
| C2 | Supply decoupling | 100nF, 50V | VSW_9V to GND |
| C3 | Control bypass | 10nF, 50V | CONTROL to GND |
| R3 | LED current limit | 1.5kΩ, 1/4W, axial THT | OUT to LED anode |
| D1 | Indicator LED | LTL-307EE red LED | 5mm through-hole |
Nets
- BAT_9V: BT1 positive to SW1 common.
- VSW_9V: switched 9V rail feeding U1 VCC, U1 RESET, R1, and C2.
- GND: battery negative, U1 GND, LED cathode, timing/control/decoupling capacitor returns.
- DISCHARGE: U1 DISCH, R1, R2.
- TIMING: U1 THRES, U1 TRIG, R2, C1.
- CONTROL: U1 CONT and C3.
- LED_OUT: U1 OUT to R3.
- LED_ANODE: R3 to D1 anode.
Timing Calculation
Using the standard 555 astable equations:
- High time: t_high = 0.693 × (R1 + R2) × C1
- Low time: t_low = 0.693 × R2 × C1
- Period: T = 0.693 × (R1 + 2 × R2) × C1
With R1 = 10kΩ, R2 = 68kΩ, C1 = 10µF:
- Theoretical high time: ~0.541s
- Theoretical low time: ~0.471s
- Theoretical period: ~1.012s
- Theoretical frequency: ~0.988Hz
- Duty cycle: ~53.4%
LED Current
Assuming a red LED forward voltage near 2.0V:
- Current ≈ (9V − 2.0V) / 1.5kΩ ≈ 4.7mA
- R3 dissipation ≈ 33mW, safely below 0.25W rating.
Simulation Result
Simulation used a CMOS555 substitute because a bipolar NE555P SPICE model was not available, so timing is representative and LED current is close but real NE555 output-high voltage may be slightly lower.
Measured simulation results:
- High time: 0.545s
- Low time: 0.484s
- Period: 1.029s
- Frequency: 0.971Hz
- Duty cycle: 53.0%
- LED current while on: 4.67mA
Conclusion: the circuit blinks approximately once per second.
Review Status
Schematic-level review checks for floating pins, power pins, single-pin nets, no-connect handling, missing footprints, and missing manufacturer part numbers were clean after remediation. Airwires remain only because PCB placement/routing has not been started; these will be resolved during the layout and routing phases after user approval.
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