Audio Signal LED Indicator
Easy “signal present” LED for front panels. Works from line-level or power-amp tap (with resistor divider).
Schematic
See audio-signal-led.png. The circuit rectifies the audio, smooths it with an RC, and drives an LED via a transistor.
Parts list
| Item | Spec / Notes |
|---|---|
| Diodes | 2× 1N4148 (half-wave rectifier) |
| R-C filter | 100 k + 2.2 µF (adjust for LED hold time) |
| Transistor | BC547/2N3904 (NPN) |
| LED + R | e.g., 3.3 k with 24 V supply; 1 k with 5 V |
| Input resistor | 10 k–100 k (sets sensitivity) |
| Optional trimmer | 100 k to fine-tune threshold |
How it works
- Audio → rectifier → RC average ≈ signal level.
- When RC voltage exceeds ~0.6–0.7 V, the NPN conducts and lights the LED.
- Increase input resistor or reduce RC to make the LED less sensitive; do the opposite to make it more sensitive.
Power-amp tap: Use a divider (e.g., 100 k // 3.3 k) to bring speaker voltage down to line levels.
Build & wiring
- Keep the RC and transistor close together to reduce hum pickup.
- Use shielded cable from the audio source if the run is long.
- For 24 V LED rail, remember the LED series resistor must match your supply.
Testing
- Feed 100 mVrms @ 1 kHz; adjust trimmer so LED just lights.
- Play real music; ensure LED behavior looks natural (not constantly on).